Ophthalmic Anaesthesia

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Ophthalmic Anaesthesia

Patients who present for eye surgery are frequently at the extremes of age. Neonatal and geriatric anaesthesia both present special problems. Some eye surgery may last many hours and repeated anaesthetics at short intervals are often necessary. The anaesthetic technique may influence intraocular pressure (IOP), and skilled administration of either local or general anaesthesia contributes directly to the successful outcome of the surgery. Close co-operation and clear understanding between surgeon and anaesthetist are essential. Risks and benefits must be assessed carefully and the anaesthetic technique selected accordingly.

Ophthalmic surgery can be classified into subspecialties and intraocular or extraocular procedures may be performed (Table 30.1); each has different anaesthetic requirements.

TABLE 30.1

Categorization of Ophthalmic Surgery

Ophthalmology Subspecialities

Paediatric

Oculoplastic

Vitreoretinal

Anterior segment

Glaucoma

Neuro-ophthalmology

Extraocular Operations

Globe and orbit

Eyebrow and eyelid

Lacrimal system

Muscles

Conjunctiva

Cornea, surface

Intraocular Operations

Iris and anterior chamber

Lens and cataract

Vitreous

Retina

Cornea, full thickness

PHYSIOLOGY OF THE EYE

The perception of light requires function of both the eye and its central nervous system connections. The protective homeostatic mechanisms of the eye are interfered with by anaesthesia in a similar way to the effects of anaesthesia on the central nervous system. The sclera and its contents are analogous to the skull and its contents. There is a similar elastance curve, but for slightly different reasons. This is due to the sclera being an elastic but completely full container unlike the rigid, but slightly empty, cranium which has some room for expansion of its contents.

Control of Intraocular Pressure

The factors controlling IOP are very complex and include external pressure, volume of the arterial and venous vasculature (choroidal volume) and the volumes of the aqueous and vitreous humour.

Intraocular pressure depends on the rigidity of the sclera as well as any external pressure. Functionally, it is a balance between the production and removal of aqueous humour (approximately 2.5 μL min− 1). Factors which affect IOP are shown in Table 30.2. Chronic changes in IOP (normally 10–25 mmHg (mean 15)), either upwards or downwards cause structural effects and loss of function. There is a relationship between increasing axial length and increasing IOP. Low pressure results in blood–aqueous barrier breakdown, cataract, macular oedema and papilloedema. High pressure causes iris sphincter paralysis, iris atrophy, lens opacities and optic nerve atrophy.

TABLE 30.2

Factors Which Affect Intraocular Pressure (IOP)

IOP Increase IOP Decrease IOP
Systemic Age
Large increase in blood pressure
Increased carotid blood flow
Increased central venous pressure
Valsalva manoeuvre
Carotid-cavernous fistula
Plasma hypo-osmolality
Hypercapnia
Sympathetic stimulation		 Exercise
Large decrease in blood pressure
Decreased carotid blood flow
Decreased central venous pressure
Parasympathetic stimulation
Pregnancy
Hypothermia
Acidosis
Plasma hyperosmolality
Adrenalectomy
General anaesthesia
Local Increased episcleral venous pressure
Blockage of ophthalmic vein
Blockage of trabecular meshwork
Contraction of extraocular muscles
Restricted extraocular muscle
Acute external pressure
Forced blinking
Relaxation of accommodation
Prostaglandin release (biphasic)
Hypersecretion of aqueous		 Decreased episcleral venous pressure
Decreased ophthalmic artery blood flow
Prolonged external pressure
Retrobulbar anaesthesia
Ocular trauma
Intraocular surgery
Retinal detachment
Choroidal detachment
Inflammation
Prostaglandins (biphasic)
Accommodation
Increased aqueous outflow

Pressure is distributed evenly throughout the eye and the pressure is generally the same in the posterior vitreous body as it is in the aqueous humour, despite the fact that the pressure is generated in the anterior segment. Each eye may have a different pressure. The aqueous is produced by an active secretory process in the non-pigmented epithelium of the ciliary body. Large molecules are excluded by the so-called blood–aqueous barrier between the epithelium and iris capillaries. The Na+/K+ ATPase pump is involved in the active transport of sodium into the aqueous. Carbonic anhydrase catalyses the conversion of water and carbon dioxide to carbonic acid, which passes passively into the aqueous. Acetazolamide, an inhibitor of carbonic anhydrase used in the treatment of raised IOP, reduces bicarbonate and sodium transport into the aqueous to produce its therapeutic effect.

In addition to this active secretory production, there is a less important hydrostatic element dependent on ocular perfusion pressure. The ciliary body is highly vascular and supplied directly by the ciliary arteries. Aqueous production is related linearly to blood flow. Flow and vascular pressure are controlled by the autonomic nervous system and autoregulation exists, similar to cerebral blood flow. Aqueous removal is inhibited by pressure within the pars plana, and episcleral venules restrict the vascular outflow, as does the IOP.

The aqueous flows from the ciliary body through the trabecular meshwork into the anterior chamber before exiting through the angle of Schlemm (Fig. 30.1). The sum of the hydrostatic inflow and the active aqueous production minus the active resorption and passive filtration must equal zero to achieve balance. Alteration of any individual feature can lead to changes in IOP.

OCULOCARDIAC REFLEX

The oculocardiac reflex is a triad of bradycardia, nausea and syncope. Classically precipitated by muscle traction, it may also occur in association with stimulation of the eyelids or the orbital floor, and pressure on the eye itself. Apnoea may also occur. The ophthalmic division of the trigeminal nerve is the afferent limb, passing through the reticular formation to the visceral motor nuclei of the vagus nerve.

The risk of development of the oculocardiac reflex is highest in children undergoing squint surgery and patients receiving explant surgery for retinal detachment. Treatment requires either a cessation of the stimulus or an appropriate dose of an anticholinergic drug such as atropine or glycopyrrolate. Some anaesthetists consider it mandatory to use prophylaxis against this reflex in susceptible patients, using the same agents.

CONDITIONS FOR INTRAOCULAR SURGERY

For most intraocular operations, the eye must be pain-free and preferably immobile. Except for glaucoma surgery, the pupil should be dilated and intraocular pressure reduced.

Effect of Anaesthetic Drugs on Intraocular Pressure

CHOICE OF ANAESTHESIA

Ophthalmic surgery can be carried out under either local or general anaesthesia provided that there is both consent and compliance. The type of surgery, its urgency and the age and fitness of the patient influence the choice (Table 30.3). Local anaesthesia is preferred for older and sicker patients, because the stress response to surgery is diminished and complications such as postoperative confusion, nausea, vomiting and urinary retention are mostly eliminated. Younger patients may sometimes be too anxious for local anaesthesia and are usually managed with general anaesthesia.

TABLE 30.3

Preferred Anaesthetic Technique for Common Surgical Procedures in Ophthalmology

Local Anaesthesia

Cataract

Glaucoma techniques

Minor extraocular plastic surgery

Laser dacrocystorhinostomy

Minor anterior segment procedures

Simple vitrectomies

General Anaesthesia

Paediatric surgery

Squint surgery

Major oculoplastic surgery

Dacrocystorhinostomy

Penetrating keratoplasty

Orbital trauma repair

Penetrating eye injuries

Complex vitreoretinal surgery

Risks and benefits of the available techniques must be assessed carefully and anaesthesia selected accordingly. There is a need to maintain homeostasis in the eye if intraocular surgery is planned. For the purposes of patient comfort, it may also be necessary to consider the duration of the procedure and the patient’s ability to stay immobile for a period longer than a short cataract operation. However all types of ophthalmic surgery have been carried out with local anaesthesia in compliant patients, including repair of ocular trauma. As a general rule, patients who require general anaesthesia are usually children and special needs adults, or adults scheduled to undergo potentially complex ophthalmic surgery. It is important to understand the basic physiology and anatomy of the eye before embarking on anaesthesia, irrespective of whether general or local anaesthesia is chosen.

General Anaesthesia

Contraindications to General Anaesthesia

Contraindications to general anaesthesia are related to risk/benefit analysis. Cardiovascular, respiratory and neurological diseases increase in frequency with age. Adverse cardiac outcome, respiratory failure and postoperative cognitive dysfunction leading to admission to a Critical Care Unit can occur after either local or general anaesthesia. If a simple and safer anaesthetic solution exists and the opinion of the anaesthetist is that there is a significant risk of death or serious neurological morbidity from general anaesthesia, the balance may shift towards local anaesthesia or cancelling surgery. There are no absolute contraindications and it is not uncommon for patients with serious comorbidities which cannot be improved preoperatively to say that the risk of death associated with proceeding with surgery and general anaesthesia is worth it when the desired outcome is maintenance or improvement of vision.

Assessment and Preparation

Standard preoperative assessment should be carried out for all patients irrespective of the chosen anaesthetic technique. Multiprofessional teamwork is the norm and the Joint Royal Colleges’ guidelines offer appropriate advice. Appropriately trained nursing staff undertake pre-assessment and preoperative preparation of most patients, under the guidance of a lead ophthalmic anaesthetist. A thorough history is required and, with input from the surgeon, a decision can be made about the most appropriate choice of anaesthetic to be offered to the patient. Investigations should be based on the examination findings and NICE guidance. Increasing age, comorbidity (such as cardiorespiratory disease) and chronic drug treatments make routine investigations such as ECG, full blood count and measurement of serum urea and electrolyte concentrations potentially useful tests. However, if local anaesthesia is planned, investigations are usually reserved for very specific indications. Particular thought needs to be given to management of patients with hypertension, ischaemic heart disease, diabetes mellitus or chronic obstructive pulmonary disease. It is important that the preoperative preparation includes consideration of whether the patient will be able to lie flat for up to an hour without becoming uncomfortable, claustrophobic, hypoxaemic or suffering ischaemic cardiac problems, or coughing.

Chronic anticoagulation presents potential complications which are more relevant to the surgeon or those practising local anaesthesia (see below).

It is imperative to make sure that the patient understands and consents to the choice of anaesthetic by taking part in an informed discussion. Patients (and surgeons) often request anaesthetic choices which appear contrary to the anaesthetic risk/benefit assessment.

Induction of Anaesthesia

A smooth induction is the goal of all anaesthetists and is particularly important in the ophthalmic setting. Avoidance of coughing, straining and accidental increases in intrathoracic pressure which cause venous congestion are important so that optimal eye conditions are maintained. The choice of induction drug is of much less importance than how it is used. However, propofol has a number of ideal qualities in this setting, especially related to the ease of insertion of the laryngeal mask airway. In equipotent doses, propofol has a greater depressant effect on IOP than thiopental, but also causes more hypotension. Succinylcholine, in isolation, causes an increase in IOP due to muscular contractions and intraocular vasodilatation but this effect is more than balanced out by the effect of the induction agent.

Short-acting opioids such as fentanyl act synergistically with the induction agent and obtund cardiovascular responses to airway manipulation.

Airway Management

Management of the airway is particularly important in head and neck surgery. The airway may remain inaccessible throughout surgery and any need to adjust or reposition an airway device during surgery could cause disruption to surgery, with potentially sight-threatening consequences in ophthalmic surgery. Thus, the safest option was traditionally felt to be to intubate the trachea and maintain ventilation and neuromuscular blockade throughout the operation. Topical and intravenous lidocaine during laryngoscopy (and during emergence) can help to reduce stimulation of the trachea and larynx. A south-facing RAE tracheal tube which is well stabilized with hypo-allergenic tape (avoiding ties) is the best choice and, along with mechanical ventilation, provides ideal conditions for nearly all types of ophthalmic surgery. Guaranteed paralysis with the use of neuromuscular monitoring avoids the risks of movement during surgery. However, tracheal intubation can be associated with a risk of increasing IOP as a result of coughing and bucking during laryngoscopy, the pressor response to laryngoscopy and intubation, laryngospasm or coughing after extubation, and postoperative nausea and vomiting related to the use of neostigmine. All of these complications assume much greater importance in open eye surgery.

The use of propofol followed by insertion of a laryngeal mask airway (LMA) has therefore become popular, particularly for short ophthalmic procedures, reducing many of the risks associated with tracheal intubation but carrying an additional risk that maintenance of the airway is less certain if the LMA is poorly positioned or inadequately secured. The use of neuromuscular blockade with the LMA may aid mechanical ventilation and tighter control of ocular physiology but is considered by some anaesthetists as carrying a significantly increased risk of aspiration.

Therefore a risk/benefit assessment should be made by the anaesthetist, taking into account the relative importance of the following factors: body mass index, history of gastro-oesophageal reflux, hiatus hernia, predicted ease of insertion of tube or LMA, length of operation, open eye operation and fasting time.

Maintenance of Anaesthesia

The choice of technique for maintenance of anaesthesia is influenced by personal preference and the method of airway management. The use of a volatile anaesthetic agent is commonest because of familiarity, controllability and cost. Inhalational anaesthesia causes a dose-dependent reduction in IOP. However, virtually all sedative and hypnotic drugs reduce IOP. There are, in practice, few clinical differences between the effects of different volatile anaesthetic agents, or between inhalational and intravenous anaesthesia.

The use of nitrous oxide depends on local availability of medical air and personal preference. The benefits of nitrous oxide are well known but two particular risks must be considered in relation to ophthalmic anaesthesia: the increased risk of postoperative retching and vomiting, and the effect on IOP when intraocular gas mixtures are used for vitrectomy (see below).

Relative hypotension during anaesthesia combined with normoxia and normocapnia provide a soft, well-perfused eye. A 15° head-up tilt may improve conditions. However, excessive hypotension may prompt questions from the ophthalmologist because of absence of flow in the retinal arteries during some ocular procedures. Maintenance of an adequate blood pressure is a greater challenge in elderly patients in the absence of significant surgical stimulation. Avoidance of an increased IOP is necessary to avoid loss of ocular contents during open surgery.

The systemic physiological disturbance associated with most eye surgery is low. There is little, if any, alteration in body fluid status and care should be taken not to be too liberal with intravenous fluids to avoid overloading the myocardium or inducing urinary retention in the elderly. The elderly are also more susceptible to the adverse effects of hypothermia and attention should be given to maintaining body temperature during all but very short procedures. Appropriate measures should be taken to minimize the risk of venous thromboembolism. Ophthalmic surgery is performed commonly on patients with diabetes due to complications of the disease. If general anaesthesia is required, local euglycaemia protocols must be followed. Analgesia requirements are based on the intraoperative use of a short-acting opioid and paracetamol. Non-steroidal anti-inflammatory drugs (NSAIDs) may be useful if there are no contraindications. Local anaesthesia with a longer-duration local anaesthetic drug is particularly useful provided that eye protection is maintained for the duration of action. It is unusual to require potent long-acting opioids and a cause for severe postoperative pain should be sought because this can be a sign of ophthalmic complications. Ophthalmic patients are particularly prone to suffer from nausea and vomiting despite the absence of long-acting opioids. Dexamethasone and ondansetron are useful as prophylaxis.

NOMENCLATURE OF BLOCKS

The terminology used for ophthalmic block varies but the widely accepted nomenclature is based on the anatomical location of the needle tip. The injection of local anaesthetic agent into the muscle cone behind the globe formed by the four rectus muscles and the superior and inferior oblique muscles is known as intraconal (retrobulbar) block whereas in the extraconal (peribulbar) block, the needle tip remains outside the muscle cone. Multiple communications exist between the two compartments and it is difficult to differentiate whether the needle is intraconal or extraconal after insertion. Injected local anaesthetic agent diffuses easily across compartments and, depending on its spread, anaesthesia and akinesia may occur. A faster onset of akinesia suggests that the block is intraconal. A combination of intraconal and extraconal block is described as a combined retro–peribulbar block. In sub-Tenon’s block, local anaesthetic agent is injected under the Tenon’s capsule and this block is also known as parabulbar block, pinpoint anaesthesia or medial episcleral block.

Relevant Anatomy

The orbit is a four-sided irregular pyramid with its apex pointing posteromedially and its base anteriorly. The annulus of Zinn is a fibrous ring which arises from the superior orbital fissure. Eye movements are controlled by four rectus muscles (inferior, lateral, medial and superior), and the superior oblique and inferior oblique muscles (Fig. 30.3). These muscles arise from the annulus of Zinn and insert on the globe anterior to the equator to form an incomplete cone. The distance from annulus to inferior temporal orbital rim ranges from 42 to 54 mm. It is very important that the needle should not be inserted too far, close to the annulus, where the vital nerves and vessels are tightly packed.

The optic nerve (II), oculomotor nerve (III, containing superior and inferior branches), abducent nerve (VI), nasociliary nerve (a branch of nerve V), ciliary ganglion and vessels lie in the cone (Fig. 30.4). The ophthalmic division of the oculomotor nerve divides into superior and inferior branches before emerging from the superior orbital fissure. The superior branch supplies superior rectus and levator palpebrae superioris muscles. The inferior branch divides into three to supply the medial rectus, the inferior rectus and the inferior oblique muscles. The abducent nerve emerges from the superior orbital fissure beneath the inferior branch of the oculomotor nerve to supply the lateral rectus muscle. The trochlear nerve (IV) courses outside the cone but then branches and enters the cone to supply the superior oblique muscle. An incomplete block of this nerve leads to retained activity of the superior oblique muscle and this occurs frequently. Squeezing and closing of the eyelids are controlled by the zygomatic branch of the facial nerve (VII), which supplies the motor innervation to the orbicularis oculi muscle. This nerve emerges from the foramen spinosum at the base of the skull, anterior to the mastoid and behind the earlobe. It passes through the parotid gland before crossing the condyle of the mandible, and then passes superficial to the zygoma and malar bone before its terminal fibres ramify to supply the deep surface of the orbicularis oculi. The facial nerve also supplies secreto-motor parasympathetic fibres to the lacrimal glands, and glands of the nasal and palatine mucosa.

Tenon’s capsule or bulbar fascia is a membrane which envelops the eyeball from the optic nerve to the sclerocorneal junction, separating it from the orbital fat and forming a socket in which it moves (Fig. 30.5). The capsule originates at the limbus and extends posteriorly to the optic nerve and as sleeves along the extraocular muscles. Tenon’s capsule is divided arbitrarily by the equator of the globe into anterior and posterior portions. Anterior Tenon’s capsule is adherent to episcleral tissue from the limbus posteriorly for about 5–10 mm and is fused with the intermuscular septum of the extraocular muscles and overlying bulbar conjunctiva. The conjunctiva fuses with Tenon’s capsule in this area and the sub-Tenon space can be accessed easily through an incision 5–10 mm behind the limbus. The posterior sub-Tenon’s capsule is thinner and passes round to the optic nerve, separating the globe from the contents of the retrobulbar space. Posteriorly, the sheath fuses with the openings around the optic nerve.

Sensation to the eyeball is supplied through the ophthalmic division of the trigeminal nerve (V). Just before entering the orbit, it divides into three branches: lacrimal, frontal and nasociliary. The nasociliary nerve is sensory to the entire eyeball. It emerges through the superior orbital fissure between the superior and inferior branches of the oculomotor nerve and passes through the common tendinous ring. Two long ciliary nerves give branches to the ciliary ganglion and, with the short ciliary nerves, transmit sensation from the cornea, iris and ciliary muscle. Some sensation from the lateral conjunctiva is transmitted through the lacrimal nerve and from the upper palpebral conjunctiva via the frontal nerve. Both nerves are outside the cone. Intraoperative pain may be experienced if these nerves are inadequately blocked.

The superomedial and superotemporal quadrants have abundant blood vessels but the inferotemporal and medial quadrants are relatively avascular and are safer places to insert a needle or cannula.

To achieve adequate anaesthesia and akinesia, the cranial and sensory nerves described above must be blocked. However, it is very difficult to target these nerves individually and an adequate volume of local anaesthetic should be injected safely either into the retrobulbar or peribulbar space; subsequent diffusion will ultimately block the relevant nerves.

Selection of Patients and Blocks

Numerous published studies confirm the preference of ophthalmologists, anaesthetists and patients for local anaesthetic techniques. However, the preferred technique varies from topical anaesthesia, through cannula-based block to needle-based blocks. There is conflicting evidence about whether there are real differences in effectiveness of blocks, suggesting that peribulbar and retrobulbar anaesthesia produce equally good akinesia and equivalent pain control. There is insufficient evidence in the literature to make a definitive statement concerning the relative effectiveness of sub-Tenon’s block in producing akinesia when compared with peribulbar or retrobulbar block. The technique chosen depends on a balance between the patient’s wishes, the operative needs of the surgeon, the skills of the anaesthetist and the type of surgery.

Preoperative assessment is generally limited to medical history, drug history and physical examination. According to the UK Joint Colleges Guidelines 2012, routine investigations are unnecessary if local anaesthesia is to be employed and these are performed only if it is thought that the results may lead to improved general health of the patient. Patients are not fasted and this is particularly helpful in managing patients with diabetes mellitus who can receive all their normal medications and achieve better glycaemic control in the perioperative period. The blood sugar concentration should still be checked. Patients receiving anticoagulants and antiplatelet agents are advised to continue their usual medications unless told otherwise. Warfarin therapy is not considered an absolute contraindication to local anaesthesia provided that the preoperative INR value is in the therapeutic target range; a sub-Tenon’s block or topical anaesthesia is preferred. The axial length of the eye is usually measured before cataract surgery and serious caution in the use of needle blocks is required if the axial length exceeds 26 mm (Fig. 30.6) or if the axial length is unknown, e.g. in surgery for glaucoma. Antibiotics are not necessary in patients with valvular heart disease. Premedication is not usually necessary but, if needed, may be given intravenously just before the local anaesthetic block is inserted.

OPHTHALMIC REGIONAL BLOCKS

Insertion of an intravenous cannula is good practice and must be established if a sharp-needle technique is planned. Full cardiopulmonary resuscitation equipment and trained staff should be immediately available. Appropriate cardiorespiratory monitoring should be used. Ophthalmic regional anaesthesia should provide conditions appropriate for the surgeon’s needs and planned surgery.

Needle-Based Blocks

Atkinson described the classical retrobulbar block in 1936. In this technique, the patient is asked to look upward and inward. A needle 38 mm in length is inserted at the junction of the medial 2⁄3 and lateral 1⁄3 of the inferior orbital margin after raising a wheal of skin with local anaesthetic. The needle is directed towards the apex and 2–3 mL of local anaesthetic is injected close to the optic nerve. Akinesia and analgesia result quickly but a facial nerve block is essential to block the orbicularis oculi muscle. Both classical retrobulbar and facial nerve blocks are associated with significant sight- and life-threatening complications and these techniques have been replaced by the modern retrobulbar block.

Inferotemporal Peribulbar Block: Surface anaesthesia and asepsis are obtained as above. The globe is kept in a neutral gaze position and a needle of less than 31 mm in length is inserted as far as possible in the extreme inferonasal quadrant through the conjunctiva or lower lid. A peribulbar block is essentially similar to a modern retrobulbar block but the needle is not directed upwards and inwards and the needle always remains tangential to the globe along the inferior orbital floor (Fig. 30.7). A volume of 5–6 mL of local anaesthetic agent is injected. However, more than 60% of patients require a supplementary injection in the form of a medial peribulbar block.

Medial Peribulbar Block: A supplementary injection is often required either in the same quadrant or through an injection in the medial compartment and is called a medial peribulbar block. A needle is inserted between the caruncle and the medial canthus to a depth of 1–1.5 cm and 3–5 mL of local anaesthetic is injected. A single medial peribulbar block with 6–8 mL of local anaesthetic has been advocated if akinesia is essential in patients with myopic eyes.

In practice, the differentiation between retrobulbar and peribulbar block is more semantic than actual. If the onset of anaesthesia is rapid with a peribulbar anaesthetic, then the chances are that it has found a direct pathway or been injected directly into the cone.

The gauge of needle should be the finest that can be used comfortably but this is usually limited to a 25- or 27-gauge needle. Finer needles are difficult to manipulate but larger needles may cause more pain and damage. Sharp needles are used because blunt needles are painful to insert and cause vasovagal syncope. The operator should consistently use the same volume syringe with the same gauge needle, because it is then easier to feel and judge the resistance to injection. A correctly placed injection has minimal resistance.

Gentle digital pressure and massage around the globe help to disperse the anaesthetic and reduce IOP. Alternatively, a pressure-reducing device such as Honan’s balloon can be used. The maximum pressure should be limited to 25 mmHg in order to avoid compromise to the globe’s blood supply.

Sub-Tenon’s Block: Sub-Tenon’s block involves a minor surgical procedure, and although it avoids some of the complications of the other two techniques, its use is associated with some specific problems.

Surface anaesthesia and asepsis are obtained as above. The lower eyelid is retracted or a speculum used. The patient is asked to look upwards and outwards. The conjunctiva and Tenon’s capsule are gripped together with a non-toothed forceps 5–10 mm from the limbus in the inferonasal quadrant. A small incision is made through these layers with Westcott scissors until the white sclera is seen. A sub-Tenon cannula (19-gauge, curved, 2.54-cm long, metal, opening at the end) is inserted gently along the curvature of the globe and should pass easily without resistance. In the posterior capsule, 3–5 mL of local anaesthetic of choice is injected slowly. The injected local anaesthetic agent diffuses around and into the intraconal space leading to anaesthesia and akinesia. Inferotemporal, superotemporal and medial quadrants may also be used to access the sub-Tenon’s space. A variety of cannulae, both flexible and shorter lengths, are available. This method reduces the risk of CNS spread, optic nerve damage and global puncture but may be more likely to cause superficial haemorrhage. Akinesia may take longer to achieve.

Local Anaesthetic Agents and Adjuncts

The ideal local anaesthetic agent should be safe and painless to inject. It should block motor and sensory nerves quickly. The duration of action should be long enough to perform the operation but not so long as to cause persistent postoperative diplopia.

Lidocaine 2% remains the gold standard. It is safe and produces effective motor and sensory blocks. Bupivacaine has largely been superseded by its isomer levobupivacaine, which has less propensity to cause cardiovascular side-effects. It may be used in concentrations of 0.5% or 0.75%. Its onset of action is slower than that of lidocaine but it has a longer duration of action. The more concentrated solution may cause prolonged diplopia or myopathy if accidentally injected directly into one of the extraocular muscles. Prilocaine 2–4% has a rapid onset of action, few side-effects and a duration of action comparable with that of bupivacaine. Ropivacaine 1% has also been shown to be effective.

Hyaluronidase is an enzyme which reversibly liquefies the interstitial barrier between cells by depolymerization of hyaluronic acid to a tetrasaccharide, thus enhancing diffusion of molecules through tissue planes. The amount of hyaluronidase powder mixed with the local anaesthetic varies from 5 to 150 IU mL− 1. The use of hyaluronidase for ophthalmic blocks is controversial and its use for sub-Tenon’s block is questioned for a short operation such as cataract surgery. Side-effects are rare but include allergic reactions, orbital cellulitis and formation of pseudotumours.

A vasoconstrictor such as adrenaline is commonly added to local anaesthetic solutions to increase the intensity and duration of block and minimize bleeding from small vessels. Absorption of local anaesthetic is reduced, which avoids any surge in plasma concentrations. Adrenaline may cause vasoconstriction of the ophthalmic artery, compromising the retinal circulation, and has also been implicated in complications in the elderly with cardiovascular and cerebrovascular comorbidities.

Commercial preparations of lidocaine and bupivacaine are acidic in solution and the basic local anaesthetic exists predominantly in the charged ionic form. The non-ionized form of the local anaesthetic agent which traverses the lipid membrane of the nerve produces the conduction block. At higher pH values, a greater proportion of local anaesthetic molecules exist in the non-ionized form and this allows more rapid influx into the neuronal cells. Adjustment of the pH of levobupivacaine and lidocaine by the addition of sodium bicarbonate allows more of the local anaesthetic solution to exist in the uncharged form. Alkalinization has been shown to decrease the onset time and prolong the duration of action after needle blocks but its use in clinical practice is probably unwarranted.

Complications of Ophthalmic Regional Blocks

Reported complications of needle blocks abound. They range from mild to serious, and may affect the eye or be systemic. Orbital complications include failure of the block, corneal abrasion, chemosis, subconjunctival haemorrhage, orbital haemorrhage, globe damage, optic nerve damage and extraocular muscle malfunction. Systemic complications such as local anaesthetic agent toxicity, brainstem anaesthesia and cardiorespiratory arrest may occur as a result of intravenous injection or spread or misplacement of drug in the orbit during or immediately after injection.

Sub-Tenon’s block is considered a safe alternative to needle block but a number of minor and major complications have been reported. Minor and frequent complications such as pain during injection, reflux of local anaesthetic, chemosis and subconjunctival haemorrhage occur with varying incidences. Visual analogue pain scores are typically low but even minor discomfort in the orbit may be interpreted as severe and unpleasant pain. Smaller cannulae may afford a marginal benefit. Anterograde reflux and loss of local anaesthetic on injection occurs if the dissection is oversized relative to the gauge of the cannula. Inadequate access into the sub-Tenon’s space can also promote chemosis. The incidence of chemosis varies with the volume of local anaesthetic, dissection technique and choice of cannula. Shorter cannulae are associated with an increased likelihood of conjunctival chemosis. Conjunctival haemorrhage is common. In one study of patients taking drugs with the potential to impair coagulation, conjunctival haemorrhage occurred in 19% of the control group, 40% of patients taking clopidogrel, 35% of those taking warfarin and 21% of patients taking aspirin. The incidence can be reduced with careful dissection, application of topical adrenaline or, controversially, the use of handheld cautery.

Orbital Haemorrhage

Orbital haemorrhage is a sight-threatening complication of intraconal and extraconal anaesthesia as well as, rarely, sub-Tenon’s block (Fig. 30.8). It occurs with a frequency of between 0.1 and 3% following needle-based blocks. The haemorrhage may be venous or arterial in origin and may be concealed or revealed. Venous bleeding is slow and usually stops. Venous haemorrhage usually presents as markedly bloodstained chemosis and raised IOP. It may be possible to reduce the IOP by digital massage and cautious application of an IOP-reducing device to such an extent that surgery can proceed safely. Before the decision is made to proceed with surgery or postpone it for a few days, it is advisable to measure and record IOP. Arterial bleeding is rapid, with blood filling the periorbital tissues, increasing tissue volume and pressure. This is transmitted to the globe, raising the IOP. Urgent measures must be taken to stop the haemorrhage and reduce IOP. Firm digital pressure usually stops the bleeding and, when it has been arrested, consideration must be given to reducing the IOP so that the blood supply to the retina is not compromised. Lateral canthotomy, acetazolamide or mannitol, or even paracentesis, may need to be considered in consultation with the ophthalmologist.

Central Spread of Local Anaesthetic Agent

Mechanism: The cerebral dura mater provides a tubular sheath for the optic nerve as it passes through the optic foramen. This sheath fuses to the epineurium of the optic nerve, providing a potential conduit for local anaesthetic to pass subdurally to the brain. Central spread can occur on injection if the needle tip has entered the optic nerve sheath. Central spread following sub-Tenon’s block has also been reported. Even an injection of a small volume of local anaesthetic may enter the central nervous system and/or cross the optic chiasma to the opposite eye and may cause life-threatening sequelae, e.g. catastrophic cardiorespiratory collapse. The time of onset of symptoms is variable but usually appears in the first 15 min after injection. Central spread may occur on rare occasions if an orbital artery is cannulated by the needle tip, resulting in retrograde spread up the artery until it meets a branch, where it can then flow in a cephalad direction; in addition to orbital haemorrhage, systemic collapse is almost instantaneous.

Prevention of Central Spread: Intraconal or extraconal injections should always be undertaken with the patient looking in the neutral or the primary gaze position. The optic nerve is a C-shaped structure and there is slackness in the primary gaze position so that it lies out of the way of the advancing needle (Fig. 30.9). If the needle encounters the optic nerve in this position, it is unlikely to damage or perforate its sheath because slackness in the structure allows the nerve to be pushed aside. The most dangerous position is when the patient looks upwards and inwards, as this presents the stretched nerve to a needle directed from the inferotemporal quadrant. The injection should not be made deep into the orbit, where the optic nerve is likely to be tethered.

Damage to the Globe

Global puncture is a serious complication of ophthalmic blocks. It has been reported following both intraconal and extraconal blocks and even following sub-Tenon’s and subconjunctival injection. Perforation of the globe has entry and exit wounds whereas penetration of the globe has only the wound of entry. With appropriate care, it should be a very rare complication because the sclera is a tough structure and, in most patients, is not perforated easily. Puncture of the eyeball is most likely to occur in patients with high myopia, previous retinal banding, posterior staphyloma or a deeply sunken eye in a narrow orbit. Not all globes are the same length and not all orbits are the same shape. In most patients who present for cataract surgery, axial length of the eyeball is measured with ultrasound (Fig. 30.10) to calculate the power of the intraocular lens. Normal globes have an axial length of 20–24 mm. Patients with high myopia have much longer axial lengths and extreme caution with needle blocks should be exercised in these patients.

Puncture of the globe is usually recognized at the time of surgery and presents as an exceptionally soft eye with a loss of red reflex. In cataract surgery, if the block is good, the surgeon should be encouraged to proceed with the lensectomy but to stitch up the eye with twice as many sutures as normal. Without lensectomy, it may not be possible to observe the damage to the posterior segment of the eye. It can be expected that the needle track through the vitreous will form a band of scar tissue. If this is not excised, it contracts and detaches the retina, sometimes causing sudden total blindness in the affected eye.

Extraocular Muscle Malfunction

The inadvertent injection of a long-acting local anaesthetic into any extraocular muscle mass may result in muscle damage manifesting as prolonged weakness, fibrosis or even necrosis of the muscle. The old-fashioned classical retrobulbar technique in which the needle was inserted between the lateral ⅓ and medial ⅔ junction of the inferior orbital rim predisposed to this complication. The safest site for inferotemporal injection is the extreme temporal area just below the lateral rectus. Recent evidence suggests that the addition of hyaluronidase to the local anaesthetic agent helps to disperse the agent before lasting damage can be done. Persistent diplopia following local anaesthesia should be investigated with a suitable scan because urgent surgical intervention to the affected muscle may be required.

OPHTHALMIC DRUGS RELEVANT TO THE ANAESTHETIST

β-Adrenergic blockade (e.g. timolol) decreases IOP by reduction of production of aqueous humour. Topical administration of drugs can cause clinically significant concentrations in the plasma via nasal drainage and the systemic side-effects of timolol including hypotension, bradycardia and bronchospasm are well reported. Phenylephrine applied to the eye intraoperatively to dilate the pupil can cause myocardial ischaemia and hypertension. Prostaglandin analogues (e.g. latanoprost) increase the uveoscleral outflow of aqueous, reducing intraocular pressure. α-Adrenergic drugs (e.g. clonidine) have the same effect.

Carbonic anhydrase inhibitors (e.g. acetazolamide) reduce aqueous formation and are used orally or intravenously to treat or prevent increases in IOP. These are sulphonamides without bacteriostatic actions and should not be used in patients with a relevant allergy. They can cause an acidosis (renal loss of bicarbonate) and a diuresis as a result of their effects on the renal tubules. The acidosis can be made worse in the perioperative period if the effect of opioids and anaesthesia reduce respiratory compensation.

Hypertonic mannitol increases aqueous outflow. Initial increases in blood pressure and systemic blood volume are followed by a diuresis. The use of intraoperative diuretics necessitates the use of urinary catheterization. Ecothiopate is of historical interest as a treatment for intraocular hypertension because it irreversibly bound to cholinesterase and could last for a week; the duration of action of succinylcholine was therefore prolonged significantly.

ANAESTHESIA FOR SPECIFIC OPHTHALMIC PROCEDURES REQUIRING GENERAL ANAESTHESIA

Penetrating Eye Injury

The penetrating open eye injury attracts first place in the list, if only because of its perceived importance in examinations undertaken by trainee anaesthetists. Eye injuries may be difficult to inspect in detail because of swelling and pain and exploration under general anaesthesia may be required at the earliest opportunity. The potential for loss of intraocular contents exists even if penetration is not obviously present preoperatively. Eye injury may also coexist with other major head injuries or polytrauma. The incidence of penetrating eye injury is highest in young adult males although the introduction of seatbelt legislation brought about a significant reduction. As with any trauma, there may be a short fasting time before the injury and subsequent delay in gastric emptying, especially if alcohol was consumed before the injury or if an opioid was administered in the Emergency Department. The situation may therefore exist of anaesthesia for a patient with a potentially full stomach.

In patients with penetrating eye injury alone or associated with other trauma, general anaesthesia is routine. Orbital regional anaesthesia has been used successfully in some centres. The classical dilemma of rapid tracheal intubation to prevent aspiration using succinylcholine and the subsequent risk of increased IOP causing loss of eye contents is a balance of anaesthesia risk versus surgical risk. The overwhelming importance is to choose the anaesthetic technique which minimizes the risk of pulmonary aspiration of gastric contents most effectively throughout the perioperative period, but consideration should be given to reducing the IOP until the eye is made safe. In principle, therefore, the use of succinylcholine as the muscle relaxant with the fastest onset of good or excellent intubating conditions, in association with cricoid pressure, is first choice. However, large retrospective studies of penetrating eye injury have not shown vitreous loss to be clinically significant.

The urgency of surgery has the greatest influence on the anaesthesia decision-making process. Ophthalmologists are currently more likely to choose to wait for 6 h after the last meal or often, due to the time of day, wait until morning before exploring the eye. This is dependent on the severity of the injury as well as the potential to produce a good ocular outcome. There is little incentive to risk aspiration and death if there is little likelihood of preserving vision as the benefit. If appropriate fasting delays have been followed, the anaesthetist is in a position to make whatever anaesthetic choice is suitable for any other intraocular surgery with similar airway risk factors.

Surgery may be bilateral and lengthy and subsequent return to theatre for repeated procedures is also common. Loss of vision in one or both eyes following accidental injury in the young population understandably heightens preoperative anxiety.

Vitreoretinal (VR) Surgery

VR surgery covers a range of intra- and extraocular procedures which may involve lengthy periods of time in the dark. Anaesthetic considerations relating to length of procedure and individual choices of technique and airway are described above. The use of a local block is common, with both needle- and cannula-based blocks in use. General anaesthesia is used in younger patients or if surgery is expected to exceed the patient’s ability to remain comfortable. Of particular interest is the relevance of the use of nitrous oxide in general anaesthesia. Vitrectomy removes all the vitreous from the eye with the purpose of clearing cloudy or bloody vitreous, as well as performing intraocular procedures on the retina. The integrity and pressure of the vitreal cavity is determined by the surgeon throughout the procedure whilst the structured jelly-like apparatus is removed. The cavity may then be filled with an air/gas mixture (commonly perfluoropropane or sulphur hexafluoride) or silicone. The surgeon may make a decision on which of these to use towards the end of surgery and therefore it is sensible to avoid the use of nitrous oxide for vitrectomy surgery because, if an air/gas mixture is used by the surgeon, nitrous oxide in equilibrium in the eye cavity may diffuse out quickly at the end of the procedure, leaving a lower pressure in the eye than surgically intended. This can cause detachment or re-detachment of the retina. If nitrous oxide has been used, it should be switched off well before the insertion of surgical gas into the vitreal cavity. Gases may persist in the eye for up to three months postoperatively and the non-ophthalmic anaesthetist needs to be aware of the relevance of ophthalmic gases. A wrist band is placed on the patient after surgery to alert any subsequent anaesthetist to avoid nitrous oxide; nitrous oxide would diffuse into the cavity faster than nitrogen would diffuse out and the IOP could increase, with serious consequences. Likewise, flying can cause the bubble to expand. The gas diffuses out of the eye slowly over time. Silicone oil used for the same purpose needs to be removed surgically at a later stage. Nitrous oxide use is of no relevance if silicone oil has been used.

Retinal surgery can also be performed from outside the sclera. Buckling, bands, cryotherapy and laser therapies are used to repair breaks. The eye requires a lot of surgical manipulation during these procedures and the oculocardiac reflex can be profound and recurrent. Anticholinergic prophylaxis may be helpful. The VR anaesthetist may find that the operating list is very flexible because some detachments (e.g. macular detachment) require urgent surgery and are therefore common additions to the list at the end of the day.

Subsequent repeated operations are very common. General anaesthetic considerations remain the same; however, local anaesthesia may become complicated by adhesions related to the original surgery.

Strabismus Surgery

This is the most commonly performed paediatric ophthalmic procedure and is usually undertaken as a day-case. Airway considerations of head and neck procedures apply but a laryngeal mask airway is the most commonly selected technique, particularly in older children. Long-acting opioids are not required and are likely to increase an already high risk of postoperative nausea and vomiting. Surgery itself requires tension to be applied to the extraocular muscles. Steady deep anaesthesia with or without muscle relaxation (to guarantee immobility) allows the surgeon to gauge how much muscle repositioning is required. However, it is the tension applied by the surgeon to the muscle which can cause severe bradycardia, especially in the vagally responsive child. Prophylaxis with glycopyrronium is recommended. Strabismus surgery in children has been linked to a first presentation of malignant hyperthermia and temperature measurement is mandatory. Standard measures to control postoperative pain and reduction of nausea and vomiting should be considered.

Dacrocystorhinostomy

Dacrocystorhinostomy (DCR) is a procedure performed for watery eyes. There is surgical exposure of the tear duct and a new opening is created into the nasal cavity. This is a relatively stimulating procedure. General anaesthesia is suitable although local anaesthesia (with or without sedation) has gained popularity. The operation may be performed with an open technique or through a nasal endoscope although, in relation to anaesthesia, the considerations are similar. All normal ophthalmic anaesthetic considerations apply. However, there is the additional risk of blood in the airway during and immediately after the procedure. Tracheal intubation and the safe use of a throat pack offer airway protection. Measures to prevent blood ooze at the site of surgery can aid the surgeon and these include hypotension, head-up position and the use of vasoconstriction in the surgical field. Xylometazoline or cocaine provides vasoconstriction in the nose. Endoscopic laser DCR is another surgical operation and the anaesthetist should have additional training in the practicalities of laser airway surgery. The laser safety officer will provide the correct eye protection for the anaesthetist.

Paediatric Procedures

In addition to strabismus surgery, children, including infants and neonates, may require other ophthalmic procedures. Although the majority of children are ASA I or II and may be managed as day cases, there are a number of patients with associated comorbidities who require detailed examinations or ocular surgery. Congenital cataracts, glaucoma, vascular and lens disorders can occur in diseases such as Down’s, mucopolysaccharidoses, craniofacial and connective tissue disorders. Retinopathy of prematurity may require treatment in sick neonatal patients outside the theatre suite. Anaesthetic considerations relevant to the condition balanced with the surgical requirements guide anaesthesia choices. An infant with airway anomalies may require a complex anaesthetic skill-set simply to undergo ophthalmoscopy.

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