Visual acuity

Visual acuity is most reliably tested at 6 m (20 ft) using a standard chart such as the Snellen chart (Fig. 19.1). Tests of acuity in near vision are portable but limited by age-related loss of accommodation (presbyopia), which necessitates refractive correction in older patients. These use test types of varying sizes, based on the point system of the printers (Fig. 19.2), and the smallest type that can be read comfortably at a distance of 33 cm is recorded (normally N4.5 or N5 type).

Figure 19.1 Snellen chart for testing distance vision.

Figure 19.2 Near vision chart based on the point system of the printer.

Snellen distance vision

On the Snellen chart, each line of letters is designated by a number that corresponds to the distance at which those letters can be read by someone with ‘normal’ distance vision. For example, the largest letter, at the top of the chart – designated 60 – would be read at 60 m by a person with ‘normal’ vision.

Technique

The patient sits or stands 6 m (20 ft) from the chart. Where space is limited, a mirror may be used 3 m from both patient and chart, with the patient facing away from the chart and reading the letters in the mirror, giving a total of 6 m. Distance glasses should be worn if necessary, and each eye tested separately. The patient should read line by line from the top of the chart. If a patient cannot see the largest letter, designated 60, then the test distance should be reduced. If at 1 m the 60 letter cannot be seen, assess the following:

Counting fingers held up at about 1 m (CFs).

Hand movements (HMs).

Perception of light (PL).

When testing low vision, ensure that the eye not being tested is completely covered. If a patient cannot read 6/6 or better in either eye, check the vision again using a pinhole occluder (Fig. 19.3). This test distinguishes patients with poor vision due to refractive error from those who have ocular or neurological conditions. In a myopic (short-sighted) eye, the rays of light are focused in front of the retina. In hypermetropia (long-sightedness), light is focused behind the eye, because the eye is abnormally short. In astigmatism, the cornea is not uniformly curved and light is not focused evenly on the retina. When using a pinhole, only the central rays of light pass through to the retina and the above refractive errors are significally reduced.

Figure 19.3 Testing vision with a pinhole. Top: light is not focused on the retina owing to refractive error (in this case in front of the retina, as in myopia; short-sightedness). Bottom: the use of a pinhole selects rays of light not needing to be focused, giving a better indication of a patient’s true best corrected visual acuity despite refractive error.

Recording visual acuity

The top figure, the numerator, records the distance of the subject from the test chart – usually 6 m. The bottom figure records the line read by the patient. The normal person can read the line designated 6 at 6 m (i.e. 6/6 (20/20) vision).

Record a mistake of one letter in a line as ‘ −1’ and a single letter read from the next smaller line as ‘+1’. Thus, if all except one of the letters on the line designated 6 are read correctly, the visual acuity is recorded 6/6 −1. If only one letter on that line is read correctly, the visual acuity is recorded 6/9 +1. Like all ophthalmic findings except visual fields, right eye visual acuity is traditionally written on the left side of the page (as the patient’s eye appears to you), and vice versa. Whether the patient was using glasses, pinhole (ph) or was unaided (ua) is recorded in the middle (Fig. 19.4).

Figure 19.4 Recording visual acuity: the numerator is the test distance. ph, pinhole.

Colour vision

Tests of colour vision are important because colour perception, especially for red, is affected in optic nerve disease before changes in visual acuity can be detected. Show the patient a red target one eye at a time (any bright red target can be used) and ask if there is a difference between the eyes. In the affected eye, red appears ‘washed out’ (desaturated). Acquired defects of colour vision may also occur in macular disease. The Ishihara test (Fig. 19.5) was devised to test for congenital colour anomalies (colour blindness), but is often used to assess acquired visual disorders. Most inherited colour blindness occurs in males (sex-linked recessive inheritance). It ranges from total colour blindness (monochromatopsia) to subtle confusion between colours, typically between red and green. About 8% of men and 0.5% of women in the UK have congenital colour perception defects. Blue/yellow deficiencies and total colour blindness are uncommon.

Figure 19.5 Ishihara colour vision test.

Visual field testing

Visual field testing is described in Chapter 14. Field defects may affect one or both eyes. Symmetric bilateral (homonymous) field defects are characteristic of lesions posterior to the optic chiasm, and asymmetric field defects are usually due to lesions anterior to the chiasm (i.e. in the optic nerves or retinae). Characteristic field defects (scotoma) occur in glaucoma, when damage to nerve fibres occurs at the optic disc, typically at the inferior or superior aspect of the optic cup. Fundoscopy shows an increase in vertical length of the optic cup (see later in the chapter for details about how examine the fundi) and field loss is arc-shaped (arcuate scotoma). If both the inferior and superior fields are involved, a ring-shaped scotoma develops. Untreated glaucoma results in loss of the peripheral field so that only a small central island of vision remains (tunnel vision). Computerized perimetry is useful in identifying early visual field loss. The Humphrey field test analyser, for example, provides statistical information indicating the reliability of the test in comparison with a group of age-matched controls (Fig. 19.6).

Figure 19.6 Automated static perimetry: the Humphrey field test analyser.

Pupils

Examination of the pupils in neurology is discussed in Chapter 14. In ophthalmic practice, there are three key aspects to pupil examination: size, shape and reactions.

Pupil size: anisocoria

In 12% of normal individuals, the pupils are slightly unequal, particularly in bright light (anisocoria), but in these subjects they react normally. Abnormal pupils dilate and constrict abnormally, and the degree of anisocoria varies with the ambient illumination. However, it is often difficult to decide which pupil is abnormal. In the absence of local eye disease, a small pupil may be due to paralysis of the dilator pupillae muscle (sympathetic innervated), part of Horner’s syndrome (Fig. 19.7), in which anisocoria is more pronounced in low ambient light. An enlarged pupil suggests a parasympathetic lesion, which may be preganglionic in oculomotor lesions or postganglionic as in the tonic pupil of Adie’s syndrome. Adie’s tonic pupil tends to be dilated in bright light and is very slow to react. A feature of both parasympathetic and sympathetic lesions is denervation hypersensitivity caused by upregulation of receptors at the neuromuscular junction (adrenergic in sympathetic and cholinergic in parasympathetic). This is the basis of pharmacological pupil testing. In both pre- and postganglionic parasympathetic blockade, the pupil is supersensitive to weak cholinergic drops (e.g. pilocarpine 0.1%). In sympathetic block, dilute adrenergic agonists such as phenylephrine 1% are unreliable, so the uptake blocker, cocaine 4%, is used. This dilates normal pupils but has no effect in pre- or postganglionic lesions. Hydroxyamfetamine 1% causes noradrenaline (nor-epinephrine) release from normal or intact postganglionic neurones and allows pre- and postganglionic lesions to be distinguished (the synapse is located in the superior cervical ganglion). In complex pupil abnormalities, for example bilateral Horner’s syndrome, infrared pupil imaging can be valuable. Causes of anisocoria are highlighted in Box 19.1. In congenital Horner’s syndrome, the affected iris is depigmented and appears blue.

Figure 19.7 Horner’s syndrome. On the affected side there is ptosis, which may be very slight, and a small pupil (miosis) that reacts to both light and accommodation.

(Reproduced with permission from Mir, 2003 Atlas of Clinical Diagnosis, 2nd edn, Saunders, Edinburgh.)

Box 19.1 Causes of anisocoria

Idiopathic (12% population)

Parasympathetic:

– third nerve palsy (external ophthalmoplegia)

– tonic pupil (internal ophthalmoplegia, e.g. Holmes-Adie)

Sympathetic:

– Horner’s syndrome

Argyll Robertson

Pharmacological

Ocular:

– iris inflammation, e.g. iritis

– iris ischaemia

– trauma

– iatrogenic

Direct ophthalmoscopy

The direct ophthalmoscope is an indispensable clinical instrument that allows direct visualization of the retina – which is part of the CNS – and its circulation. It consists of a bright, focused light source that illuminates the retina to allow the observer to see the image, magnified about 15 times.

Preparation

1 Decide the objective of the examination.

2 Consider dilating the pupils with 1% cyclopentolate (Mydrilate) or 1% tropicamide (Mydriacyl). This blurs the vision for at least 2 hours, so patients should be forewarned and instructed not to drive. Some patients have a predisposition to closed-angle glaucoma (Box 19.4) and dilatation may precipitate an attack. At-risk individuals must be warned to return if symptoms of acute angle closure occur.

3 Briefly explain the examination so that the patient can cooperate fully. Ask the patient to fixate on a distant target – he should continue to look in this direction even if the examiner’s head obscures the target.

4 Dim the room lights.

5 Set the ophthalmoscope lens wheel to zero dioptres (D), unless correcting for your own short- or long-sightedness.

6 Check the ophthalmoscope light is bright. Unless the pupil is small, select a large light spot size (Box 19.5, Fig. 19.8).

Box 19.4 Risk factors for acute angle-closure glaucoma

dilate pupils cautiously

Intermittent symptoms of angle-closure attack (see symptoms in Table 19.2)

High hypermetropia

Old age

Narrow anterior chamber angles (detected on slit-lamp gonioscopy)

Family history of acute angle-closure glaucoma

Box 19.5 Technique of ophthalmoscopy

Use your LEFT eye for the patient’s LEFT eye, holding the ophthalmoscope with your LEFT hand and using your RIGHT hand with thumb over the LEFT brow to steady the patient’s head (the opposite approach is used for the right eye) (Fig. 19.8).

First, at arm’s length, look for and examine the red reflex (like that in flash photographs). The red reflex is dimmed in elderly people with cataracts, and these may obscure the retina. In a child, congenital cataract and retinoblastoma may abolish the red reflex.

Looking at the red reflex through the ophthalmoscope, gradually close on the eye to examine the anterior ocular structures. Use the +20 D lens to view the cornea and the +15 D lens for the iris. Some ophthalmoscopes have a lever to switch to these lenses.

Then, with a zero lens setting, locate the optic disc. If it is difficult to find, locate any blood vessel and follow it towards the optic disc. Fine-tune the focus with 1-2 D lens steps; if the focus worsens, reverse the other way. If your eye is emmetropic and your accommodation is relaxed, the strength of the lens necessary to bring the fundus into focus gives an indication of the refractive error of the patient’s eye (plus or red lenses indicate hypermetropia and minus or black lenses myopia).

Compare the two eyes. There is a broad range of normal appearances, so if an unusual appearance is symmetrical, it could be a variant of normal.

Figure 19.8 Examining the right eye with the direct ophthalmoscope.

Examining the fundi

Look carefully at the optic disc/cup, the retinal vessels and the retina and macula (Fig. 19.9).

Figure 19.9 Normal fundus anatomy. OC, optic cup; OD, optic disc. Retinal vascular arcades: F, fovea; IN, inferonasal; IT, inferotemporal; M, macula; SN, superonasal; ST, superotemporal.

Optic disc

This marks the point where the retinal axons exit the back of the eye to form the optic nerve. It corresponds to the physiological blind spot. The normal disc is round or slightly oval. If there is astigmatism, the disc may appear more oval than normal. There are four key questions to consider when viewing the optic disc:

1 Is the colour normal? The normal disc is yellow-pink and its temporal side is paler. An abnormally pale disc is a sign of optic atrophy (Fig. 19.10). In addition, in optic atrophy there is a reduction in the number of capillaries crossing the edge of the disc, from the normal 10 to 7 or fewer (Kestenbaum’s sign). The causes of optic atrophy are listed in Box 19.6. In primary optic atrophy due to optic nerve lesions, the disc is flat and white, with clear-cut edges. Secondary optic atrophy follows swelling of the optic disc due to papilloedema: the disc is greyish-white, with indistinct edges.

2 Are the margins distinct? The disc margin should be sharply defined. In optic disc swelling, this clear edge is obscured (Fig. 19.11) and venous pulsations are abolished. However, venous pulsations may be difficult to see in some normal subjects. Some important causes of disc swelling are listed in Box 19.7. In papilloedema, caused by raised intracranial pressure, the disc is abnormally red and its margins are blurred, especially at the upper and lower margins, and particularly in the upper nasal quadrant. The physiological cup becomes obliterated and the retinal veins are slightly distended. As the condition progresses, the disc becomes more definitely swollen (Fig. 19.12). In order to measure the degree of swelling, start with a high plus lens in the ophthalmoscope and reduce the power until the centre of the disc is just in focus. The retina, a short distance from the disc, is then brought into focus by further reduction of the lens power. This further reduction indicates the degree of swelling of the disc (3 D is equivalent to 1 mm of swelling). If papilloedema develops rapidly, there will be marked engorgement of the retinal veins with haemorrhages and exudates on and around the disc, but with papilloedema of slow onset there may be little or no vascular change, even though the disc may become very swollen. The retinal vessels will, however, bend sharply as they dip down from the swollen disc to the surrounding retina. The oedema may extend to the adjacent retina, producing greyish-white striations near the disc (Paton’s lines), and a macular fan of hard exudates temporal to the fovea may develop in some cases.

3 Is the central cup enlarged? The cup is a physiological central depression formed at the optic disc as nerve fibres leave the retina to form the optic nerve. It marks the point where the retinal vessels enter and leave the eye. It is paler than the surrounding rim of the disc. The optic cup : disc ratio is estimated by comparing their ratios vertically. In chronic open-angle glaucoma, the ratio is increased (>0.3) – optic disc cupping. When the cup is deep, in advanced glaucoma, retinal vessels disappear as they climb from the floor to the rim, and reappear as they bend sharply over the edge of the rim (bayonetting); in less advanced cases, the cup appears as a vertical oval extending to the edge of the disc (Fig. 19.13). In myopic individuals, the disc and cup appear large, and mimic glaucoma. Myopes often have a partial ring of pigmentation or white sclera surrounding the disc, which is easily mistaken for the edge of the cup. In severe myopia, degenerative chorioretinal changes may occur in the fundus, which can involve the macula and impair central vision.

4 Are there any other abnormal features? In proliferative diabetic retinopathy, new blood vessels (neovascularization) develop at the optic disc. Myelinated nerve fibres have a dramatic white appearance, but they are a unilateral, harmless and non-progressive congenital anomaly (Fig. 19.14). They have a characteristic feathered edge that may obscure the retinal vessels.

Figure 19.10 Primary optic atrophy. The disc is pale and whiter than normal, and its edges are unusually sharply demarcated from the retina. The retinal vessels are slightly attenuated.

Box 19.6 Causes of optic atrophy

Congenital:

– dominant and recessive

Optic neuritis

Chronic papilloedema

Toxic:

– tobacco, lead, alcohol

Optic nerve compression:

– thyroid

– tumour

Post-traumatic (direct optic nerve or indirect vascular damage)

Figure 19.11 Papilloedema: optic disc swelling due to raised intracranial pressure.

Box 19.7 Causes of the appearance of optic disc swelling

Raised intracranial pressure (papilloedema)

Infiltration:

– lymphoma

– sarcoid

Vascular:

– hypertension (grade 4)

– central retinal vein occlusion

– anterior ischaemic optic neuropathy

Pseudopapilloedema:

– congenital small discs

– high hypermetropia

Optic disc drusen

Figure 19.12 Severe papilloedema with retinal haemorrhages.

Figure 19.13 Glaucomatous disc cupping. The cup is oval in the vertical plane and appears pale. The retinal vessels are displaced nasally.

Figure 19.14 Myelinated nerve fibres: the white area obscures the disc; this is a normal variant.

Blood vessels

There are four pairs of arterioles and venules, which form the main retinal vascular arcades that emerge from the optic disc: superotemporal (above the macula); inferotemporal (below the macula); superonasal; and inferonasal. Study each in turn. Arterioles are thin, bright red in colour and with a longitudinal streak of light reflection. In branch arteriolar occlusion, a bright yellow (cholesterol) embolus may occasionally be seen (Fig. 19.15). In diabetes or venous occlusion, the venules are larger, darker and often dilated or tortuous. Look carefully at arteriolar/venous crossings: compression and localized dilatation of venules (arteriovenous (AV) nipping) with arteriolar narrowing (attenuation) is a sign of hypertension (Box 19.8, Fig. 19.16). Spontaneous arteriolar pulsation is an abnormal finding that may occur if the IOP is very high or the central retinal artery pressure very low. Spontaneous venous pulsation is frequently seen in normal eyes, but is reduced in papilloedema.

Figure 19.15 Retinal arteriolar emboli. Cholesterol emboli in the retinal arteries of a patient with atheromatous disease of the internal carotid artery in the neck.

Box 19.8 Appearance and classification of hypertensive retinopathy

Grade 1: arteriolar narrowing and vein concealment

Grade 2: severe arteriolar attenuation and venous deflections at crossings (AV nipping)

Grade 3: arteriolar copper wiring, haemorrhages, cotton wool spots and hard exudates

Grade 4: all of the above, plus silver wiring and optic disc swelling

Figure 19.16 Hypertensive retinopathy. The arteries are irregular in calibre and show ‘silver wiring’. Arteriovenous nipping is present. Characteristic ‘flame-shaped’ haemorrhages and ‘cotton wool’ spots (arrow) can be seen.

Retina and macula

As each main vascular arcade is followed and examined, the adjacent and peripheral retina can be systematically assessed. The macula is the central retinal area bounded by temporal vascular arcades. It mea-sures approximately five disc diameters across. The fovea at its centre is one disc diameter in size. The fovea, with its high density of cone photoreceptors, is responsible for fine discriminatory vision. To find the fovea, locate the optic disc and move the ophthalmoscope beam temporally (move yourself nasally). Alternatively, ask the patient to look directly into the light. However, if the pupil is undilated, it tends to constrict at this point, and the patient may recoil because of dazzle (you can dim the light beam to make it more comfortable). In young patients, the retina is very reflective and there is often a small yellow dot in the middle of the fovea (macula lutea or fovea centralis). Box 19.9 and Figures 19.17 and 19.18 identify common retinal abnormalities by their colour and appearance.

Box 19.9 Common retinal abnormalities

White

Cotton wool spots: white, fluffy, indistinct areas indicative of retinal ischaemia. This is the accumulation of axonal proteins in the nerve fibre layer. Causes include severe hypertension, diabetes and retinal vein occlusion

Chorioretinal atrophy: well-defined ‘punched-out’ lesions (the white is the sclera). May occur in conjunction with retinal pigment hypertrophy. Associated with previous retinal inflammation or injury (including retinal laser)

Yellow

Hard exudates: bright yellow with well-demarcated edges consisting of lipid deposits that have leaked out of abnormal blood vessels. Most commonly associated with microaneurysms in diabetes (Fig. 19.17)

Drusen: small multifocal round yellow features, usually located in the central macula. Generally smaller and less bright yellow than hard exudates. Typically bilateral and relatively symmetrical. Common in elderly people associated with ‘dry’ age-related macular degeneration

Red

Microaneurysms: the dots that typify diabetic retinopathy. They may leak to cause exudates or bleed to cause blot haemorrhages (Fig. 19.17)

Blot haemorrhages: rounded localized intraretinal blood, typically due to diabetic retinopathy, but other causes include severe hypertension and retinal vein occlusion

Deep large haemorrhages: associated with retinal ischaemia when numerous

Flame haemorrhages: have a characteristic feathery shape as the blood is in the nerve fibre layer; may be present in retinal vein occlusion (Fig. 19.18). Not typically associated with ischaemia

Black

Retinal pigment hypertrophy: well-defined black lesions, often in conjunction with chorioretinal atrophy. May occur with previous retinal inflammation or injury (including retinal laser therapy).

Figure 19.17 Diabetic retinopathy. Microaneurysms (tiny red dots), blot haemorrhages, hard exudates and areas of new vessel formation (arrow) are characteristic of this condition. In many patients, hypertensive retinopathy is also present.

Figure 19.18 Branch retinal vein occlusion. There are flame-shaped retinal haemorrhages, but the disc is normal.

Slit lamp and intraocular pressure

The slit lamp (Fig. 19.19) provides a stereoscopic, magnified view of the eye and is the key examination tool for ophthalmologists. Many accident and emergency departments have a slit lamp, and it can be invaluable for assessing suspected foreign bodies and corneal abrasions. Some direct ophthalmoscopes are equipped with a slit-lamp beam, which can be useful. Alternatively, the anterior orbital structures and globe can be examined with a bright torch and basic magnification, and the same principles of systematic examination apply. The slit lamp comprises a table-mounted binocular microscope column with an adjustable illumination source that produces a narrow, slit beam of light.

Figure 19.19 Slit lamp.

The patient is seated with forehead and chin supported. The slit beam illumination and microscope have a common axis of rotation and coincident focal lengths, allowing the angle of illumination to be varied along with its width, length and intensity. Projected onto the globe, the slit beam illuminates an optical cross-section of the eye’s transparent structures, and this can be viewed with magnification varying from 10× to 40× power. An attachment allows the IOP to be measured (tonometry). The drainage angle can be seen with a special contact lens (gonioscopy) and, with the aid of a hand-held lens or a contact lens, the retina can also be viewed.

The following structures can be examined:

Figure 19.20 Everting the upper eyelid to expose the tarsal conjunctiva.

Figure 19.21 Fluorescein used to stain the cornea and tear film.

Measuring intraocular pressure: applanation tonometry

Intraocular pressures between 10 and 21 mmHg are considered normal. An increased IOP is a characteristic feature of glaucoma. A diminished IOP occurs in diabetic coma and in severe dehydration from any cause. The IOP may be assessed by palpating the eyeball, although only gross variations from normal can be appreciated. More accurate is applanation tonometry, in which the force required to flatten (applanate) an area of a sphere (the cornea) is proportional to the pressure within the sphere (Fig. 19.22). Topical anaesthestic and fluorescein are applied to the cornea and a bright cobalt blue filter is used to illuminate the sterile tonometer head. Contact between tonometer head and cornea creates a thin green circular outline of fluorescein, and a prism in the head splits this into two semicircles. The tonometer force is adjusted manually until the semicircles just overlap, and is read in millimeters of mercury (mmHg; Fig. 19.23).

Figure 19.22 Goldman tonometry.

Figure 19.23 Diagrammatic representation of tonometry.

Eyelids

People of Asian origin have a long, narrow palpebral aperture with an upward and outward obliquity and a characteristic fold of skin along the upper lid. The highest point of the aperture is typically at the junction of its middle and inner thirds. In Down’s syndrome, the palpebral fissure is also oblique. However, it is also short and wide, with its highest point at the centre of the lid.

Normally no sclera is visible above the limbus (the corneoscleral junction). The commonest cause of scleral show is eyelid retraction (Fig. 19.24) due to dysthyroid eye disease, accompanied by other signs such as lid lag, in which movement of the upper lid seems to lag behind that of the eyeball when the patient looks downwards. In parkinsonism, there may be reduced blink frequency. Look for reduced eyelid closure (lagophthalmos) and levator muscle function. Ptosis (drooping of the upper lid) may be congenital or acquired (check old photographs). In age-related ptosis, owing to levator disinsertion, levator function is retained and there is a high upper eyelid skin crease. In ptosis due to myogenic or neurogenic lesions, there is reduced levator function (see Ch. 14). In entropion, there is inversion of the lid margin with associated malpositioning of the lashes, which may rub on the cornea (Fig. 19.25); and in ectropion, eversion of the eyelid is often associated with watering. The lower lid is prone to skin tumours, particularly basal cell and squamous cell carcinomas (see Fig. 15.25). Xanthelasmas are fatty deposits that develop in the upper and lower eyelids in patients with longstanding hypercholesterolaemia.

Figure 19.24 Thyroid eye disease with upper eyelid retraction and mild exophthalmos (bilateral proptosis).

Figure 19.25 Lower eyelid entropion causing infective keratitis and corneal opacification.

Lacrimal

Examine the lacrimal gland by pulling up the outer part of the upper lid while the patient looks downwards and inwards. Acute inflammation (dacroadenitis) causes a tender swollen gland, with oedema of the upper lid and localized conjunctival injection. Chronic dacroadenitis, a painless enlargement of the lacrimal gland which is frequently bilateral, occurs in sarcoidosis and lymphoproliferative disorders. Tumours of the lacrimal gland produce a hard swelling of the gland associated with displacement of the globe. Involvement of the lacrimal gland by any disease process may cause a dry eye.

Assess the position and size of the puncta (see above). Painless watering is a feature of obstruction of the tear drainage pathway, but exclude reflex tearing and overflow from, for example, a dry eye. Schirmer’s test uses a standardized strip of filter paper to detect dry eyes by assessing the extent of wetting at 5 minutes (Fig. 19.26). Overt nasolacrimal duct blockage can be excluded if the patient reports fluid at the back of the throat on probing and syringing with normal saline (Fig. 19.27).

Figure 19.26 Schirmer’s test for dry eyes.

Figure 19.27 Syringe and probing to assess nasolacrimal duct function.

Orbit

The most common cause of forward displacement of the eyeball – proptosis when unilateral, or exophthalmos when bilateral – is thyroid eye disease (TED) (see Fig. 16.13). This can cause corneal exposure and ulceration. Optic nerve damage may occur despite minimal proptosis. Axial proptosis, in the primary direction of the eye in forward gaze, is typical of TED and of tumours in the extraocular muscle cone behind the eye (intraconal mass lesions) (Fig. 19.28). Non-axial proptosis occurs in association with space-occupying orbital lesions outside the muscle cone, for example lacrimal gland tumours; these displace the globe forward and inferomedially. Apparent (‘pseudo’) proptosis causes diagnostic confusion: for example in ipsilateral eyelid retraction or myopia (where the eye is longer than normal) or when there is contralateral ptosis or enophthalmos.

Figure 19.28 Axial CT orbits in TED. Left-sided thyroid eye disease with exophthalmos and marked hypertrophy (inflammation) of the medial and lateral rectus muscles on that side. These muscles in the other eye are also slightly enlarged. The optic nerve can clearly be seen between the enlarged muscles on the left side.

Proptosis and enophthalmos can be measured with the Hertel exophthalmometer (Fig. 19.29). A difference of >2 mm between sides is abnormal. A proptosis that increases while the patient performs a Valsalva manoeuvre is suggestive of a venous abnormality. Pulsatile proptosis with an orbital bruit is a feature of carotid cavernous fistula.

Figure 19.29 Hertel exophthalmometry to quantify proptosis or exophthalmos.

Blunt trauma to the orbit may cause a blowout fracture of the thin orbital floor. Orbital contents may prolapse through the fracture, restricting the movement of the inferior rectus muscle and limiting upgaze. A full orbital examination should include palpation, eye movement examination, optic nerve assessment and testing of the trigeminal nerve for altered sensation.

Plain X-rays

Plain X-rays have a limited role in the detection of foreign bodies, but have been largely superseded by computed tomography (CT) or magnetic resonance imaging (MRI). Ultrasound is used to assess the globe. A dacryocystogram uses a radiopaque dye introduced into the lacrimal drainage system to identify sites of lacrimal duct obstruction (Fig. 19.31). It is particularly useful in the watering eye, when carcinoma is suspected, when repeat surgery is planned or when trauma has occurred.

Figure 19.31 Dacryocystogram showing restricted flow of radiopaque dye in the right nasolacrimal duct.

Computed tomography and magnetic resonance imaging

These are used extensively in the diagnosis of orbital disease. CT is often considered superior because it defines the bony orbit, but the X-ray dose to the eye and lens is not inconsiderable. CT is the investigation of choice in blunt orbital trauma and blowout fractures (see above), where fine-cut coronal spiral images are desirable (Fig. 19.32).

Figure 19.32 Coronal CT orbits in blowout fracture. The right bony orbital floor is fractured and the orbital contents prolapsed.

A- and B-mode ultrasound

The A-mode scan is a one-dimensional time-amplitude study commonly used to assess axial length, which is an essential measurement for lens implant calculation prior to cataract surgery. The B-mode scan gives a two-dimensional cross-sectional view of the eye for the diagnosis of both intra-ocular and orbital tumours, retinal detachments and intraocular disorders when the fundal view is impaired (e.g. with vitreous haemorrhage; Fig. 19.33).

Figure 19.33 B-mode ultrasound scan showing lens opacity and vitreous opacities but no retinal detachment following penetrating trauma. C, cornea; L, lens; ON, optic nerve; V vitreous.

Retinal photography and fundus fluorescein angiography

Retinal photography in conjunction with the intravenous injection of sodium fluorescein gives a detailed assessment of the retinal and choroidal vasculature (Figs 19.34 and 19.35). A blue filtered light excites fluorescence (530 nm) as the dye circulates. Fundus fluorescein angiography is useful in investigating diabetic retinopathy, age-related macular degeneration and retinal ischaemia. Minor side-effects, including transient nausea and yellow discoloration of the skin and urine, are common. Severe anaphylaxis is, fortunately, very rare.

Figure 19.34 Fluorescein retinal angiogram of fundus in papilloedema. Note the late-phase leakage of the dye.

Figure 19.35 Fluorescein retinal angiogram of fundus in pseudopapilloedema.

Retinal and optic disc tomography

A variety of techniques have been developed to image and assess retinal structure and the shape of the optic disc that are particularly useful in the management of glaucoma. Optical coherence tomo-graphy (OCT) uses a laser to obtain high-resolution cross-sectional images of the retina at a resolution of approximately 10 µm. Scanning laser polarimetry (SLP) and the scanning laser ophthalmoscope (SLO) assess nerve fibre thickness indirectly.

Refraction and refractive assessment

A refraction test will ascertain the optical power of an eye, with a view to prescribing glasses or contact lenses. An objective refraction is performed using neutralizing lenses in conjunction with a retinoscope. In adults and cooperative children, this is then refined subjectively by placing neutralizing lenses in front of the eye and simultaneously assessing visual acuity. Increasingly, the retinoscope is being replaced by an automated technique (the autorefractor).

Amsler grid

The Amsler grid is a sensitive test of macular function. It comprises a series of vertical and horizontal lines with a central spot for fixation (Fig. 19.36). The patient is asked to look at this spot and describe any distortions or missing areas in the grid.

Figure 19.36 Amsler grid for testing macular function.

Indirect ophthalmoscopy

Binocular indirect ophthalmoscopy, using a light source supported on the examiner’s head and a hand-held lens in front of the patient’s eye, allows a much greater area of the fundus to be visualized (Fig. 19.37). The retinal periphery is more readily seen.

Figure 19.37 Indirect ophthalmoscopy, here used intraoperatively. Note the head light and hand-held 20 D lens.

Electrophysiological tests

Visual evoked potentials (VEP), recorded from the occipital cortex using scalp electrodes while the patient views an alternating black-and-white chequerboard stimulus, have a role in the diagnosis of disease of the visual pathway. The principal waveform recorded from the scalp is a positive deflection occurring about 100 ms after the stimulus (the P100 wave). This is attenuated in amplitude and increased in latency in disease of the optic nerve, for example optic neuritis or optic nerve compression. Electroretinograms (ERG) measure the electrical potential across the eye, recorded with a corneal electrode, with a reference electrode placed on the forehead. Flash, flicker or pattern stimuli are used to generate electrical responses from retinal activation. This is useful in assessing hereditary or acquired retinal degeneration.