Orbital blocks

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CHAPTER 12 Orbital blocks

Anatomy

Orbital cavity

The superior orbital margin is formed by the frontal bone and contains the supraorbital foramen or notch, through which the superior orbital nerve emerges (Fig. 12.1). The inferior orbital margin is formed laterally by the zygomatic bone and medially by the maxilla. The roof of the orbit is formed by the frontal bone, the lateral or temporal wall by the zygomatic bone. The posterior wall of the orbit consists of the greater and lesser wings of the sphenoid bone. The orbital floor is formed by the maxilla, and the medial (nasal) wall of the orbit by the lacrimal bone and the orbital lamina of the ethmoid bone. The superior orbital fissure lies between the greater and lesser wings of the sphenoid bones, and the inferior orbital fissure lies between the maxilla and the superior wing of the sphenoid.

The medial walls of each orbit are parallel to each other (Fig. 12.2). The lateral or temporal wall of each orbit forms an angle of 90° to the contralateral lateral wall. The medial and lateral walls of the orbit make a 45° angle to each other. The floor of the orbit inclines at an angle of 10° anteriorly to posteriorly.

The A–P length of the orbit is 40–45 mm (Fig. 12.3) and the orbital volume is approximately 30 mL. The A–P diameter of a normal globe is 24 mm and the horizontal diameter is 23.5 mm. The globe lies eccentrically in the orbit, being displaced medially and superiorly.

Motor nerve supply

The motor nerve supply to the eye (Fig. 12.4) and surrounding structures comes from the cranial nerves. The third, fourth, and sixth cranial nerves supply the motor innervation to the recti muscles and partial motor innervation to the levator palpebrae superioris muscle. The seventh cranial nerve supplies motor function to the orbicularis oculi muscles. The parasympathetic nervous system supplies the circular muscle of the iris, while the sympathetic nervous system supplies the radial muscles of the iris and partial innervation to the levator palpebrae superioris muscle. The third, fourth, and sixth cranial nerves enter the orbit via the superior orbital fissure. The third cranial nerve (oculomotor) supplies all recti muscles with the exception of the lateral rectus muscle. The superior oblique muscle is supplied by the fourth cranial nerve (trochlear) and the lateral rectus muscle is supplied by the sixth cranial nerve (abducent). With the exception of the trochlear nerve, the motor nerves lie within the muscle cone formed by the four recti muscles. The nerves enter the muscle cone surrounding the optic nerve via the tendinous ring (annulus of Zinn).

Sensory nerve supply

Sensory supply to the eye (Figs 12.4A and 12.5) and surrounding tissues is via the ophthalmic nerve, which is the first division of the trigeminal nerve. It supplies sensation to the eye, lacrimal gland, conjunctiva, part of the mucous membrane of the nose, skin of the nose, eyelids, forehead, and scalp. The ophthalmic nerve enters the orbit via the superior orbital fissure. It is the smallest division of the trigeminal nerve and arises from the anteromedial part of the trigeminal ganglion. Just before entering the superior orbital fissure it divides into three branches: the lacrimal, frontal, and nasociliary nerves.

The lacrimal nerve is the smallest branch of the ophthalmic nerve. It enters the orbit via the superior orbital fissure and runs along the upper border of the lateral rectus muscle. It enters the lacrimal gland and gives branches to the lacrimal gland and conjunctiva. Finally it pierces the orbital septum and ends in the skin of the upper eyelid.

The frontal nerve is the largest of the branches of the ophthalmic nerve. It enters the orbit via the superior orbital fissure outside the muscle cone. It runs superiorly to the levator palpebrae superioris. It divides into the supraorbital and supratrochlear nerves. The supraorbital nerve passes through the supraorbital foramen or notch and gives branches to the conjunctiva and eyelid. It then continues and supplies the skin of the scalp almost to the lambdoid suture. The supratrochlear nerve emerges from the orbit between the trochlea and the supraorbital foramen. It supplies sensation to the conjunctiva and skin of the upper eyelid and the skin of the lower forehead.

The nasociliary nerve enters the orbit via the superior orbital fissure within the tendinous ring close to the oculomotor nerve and ophthalmic artery. It gives off the long posterior ciliary nerves and branches to the ciliary ganglion. It also gives off the anterior and posterior ethmoid nerves.

The optic nerve enters the orbit via the optic canal. It enters the muscle cone via the tendinous ring surrounded by the dura mater. It contains the central retinal artery and vein. The ophthalmic artery lies lateral to the optic nerve initially, and then crosses superiorly to lie medial to the optic nerve within the muscle cone. The ophthalmic venous plexus also lies in close proximity.

Ocular muscles

The muscles of the eyelids (Fig. 12.4B) include the levator palpebrae superioris and the orbicularis oculi muscles. The orbital muscles are the four recti muscles and the superior and inferior oblique muscles. The recti muscles arise from the semitendinous ring, which surrounds the optic canal. The muscles penetrate the fascial sheath of the eyeball (Tenon’s capsule) and are inserted in the sclera 5–6 mm posterior to the corneoscleral junction. The superior oblique muscle arises from the body of the sphenoid and, having traversed the trochlea, passes backward and is inserted into the sclera posterior to the equator. The inferior oblique muscle arises from the orbital surface of the maxilla medially and is inserted into the lateral part of the sclera behind the equator.

Tenon’s capsule (fascial sheath of the eyeball)

Tenon’s capsule (Fig. 12.6) is a thin membrane that envelops the eyeball from the optic nerve to the corneoscleral junction. It separates the eye from the orbital fat and forms a capsule within which the eye moves. It is separated from the sclera by the episcleral space. Tenon’s capsule is penetrated posteriorly by the ciliary vessels and nerves and fuses with the sheath of the optic nerve. Anteriorly, it fuses with the sclera just behind the corneoscleral junction. The tendons of the recti muscles penetrate the capsule anteriorly.

Surface anatomy

Extraconal retrobulbar and lateral peribulbar blocks

The needle insertion points for the extraconal retrobular (Fig. 12.7) and lateral peribulbar blocks are at the lowest margin of the inferior rim of the orbit. This approximates to the junction of the medial two-thirds and the lateral one-third of the lower lid. This will give the greatest distance from the globe and avoids the inferior rectus muscle.

Technique

As for all regional anesthetic procedures, after checking that the emergency equipment is complete and in working order, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.

Extraconal retrobulbar block

The side to be anesthetized and the axial length of the globe are verified. The axial length dictates which block will be used, because an axial length greater than 26 mm is a relative contraindication to retrobulbar block. The patient lies in the supine position.

The operator stands on the side to be blocked, below the patient’s shoulder. The local anesthetic cream, which is applied 1 h before surgery to the skin of the lower lid on the side to be anesthetized, is removed in the operating theater when the skin is prepared with antiseptic solution. The inferior orbital margin is identified at the junction of the medial two-thirds and lateral one-third of the lower eyelid. With the eye in the primary or neutral position, the inferior lid is indented, gently displacing the eye upward and medially within the orbit. A 32-mm 27-G Atkinson tipped needle on a 5-mL syringe is inserted between the indenting finger and the inferior orbital margin. The needle is directed initially vertically in the sagittal and coronal planes, using the indenting finger to maintain the direction of the needle. The needle is inserted to a depth of 16 mm, which ensures that the tip of the needle is past the greater diameter of the eye.

The needle is now angled parallel to the orbital floor and the lateral wall of the orbit, and is inserted to its full depth, or 25 mm for a lateral peribulbar technique. This technique reduces significantly the risk of the needle entering the muscle cone. Following aspiration, 4 mL of lidocaine 2% and 1 : 200 000 adrenaline (epinephrine) plus 75 IU/mL hyaluronidase (Hyalase) is injected slowly. The needle is removed and gentle pressure is applied to the closed eye. Assessing lateral and vertical eye movement verifies the efficacy of the block. Finally the eye is assessed for the presence of chemosis and hemorrhage. A Honan balloon is applied to the eye and inflated to 35 mmHg. This reduces the intraocular pressure prior to surgery.

Adverse effects