Anaesthesia for Opthalmic Surgery Part 1: Regional Techniques (page 1)

Issue 6 (1996) Article 3: Page 1 of 3

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Anaesthesia for Opthalmic Surgery
Part 1: Regional Techniques

Dr. Andrei M. Varvinski,
Anaesthetic Department, City Hospital, N 1 Arkhangelsk, Russia;

Dr. Roger Eltringham,
Consultant Anaesthetist, Gloucestershire Royal Hospital.

Anatomy		Complications
Peribulbar block		Advantages & disadvantages
Retrobulbar block

Ophthalmic surgery can be performed under either regional or general anaesthesia. This article describes regional anaesthesia. In the next issue general anaesthesia will be discussed.

Anatomy: Some basic knowledge of the anatomy of the orbit and its contents is necessary for the succesful performance of regional anaesthesia for ophthalmic surgery. If possible carefully examine the orbit in a skull whilst reading this article. This will make understanding the techniques described easier.

Each orbit is in the shape of an irregular pyramid with its base at the front of the skull and its axis pointing posteromedially towards the apex. At the apex is the optic foramen, transmitting the optic nerve and accompanying vessels and the superior and inferior orbital fissures transmitting the other nerves and the vessels.

The depth of the orbit measured from the rear surface of the eyeball to the apex is about 25 mm (range 12-35 mm). The axial length (AL) of the globe (eyeball) is the distance from the corneal surface to the retina and is often measured preoperatively. An axial length of 26mm or more denotes a large eye, indicating that great caution is necessary as globes longer than this are easier to perforate during regional anaesthesia.

The angle between the lateral walls of the two orbits is approximately 90° (and the angle between the lateral and medial walls of each orbit is nearly 45° (see figure 1). Thus the medial walls of the orbit are almost parallel to the sagittal plane. (The sagittal plane passes directly from front to back of the body).

The orbit contains the globe, orbital fat , extraocular muscles, nerves, blood vessels and part of the lacrimal apparatus.

The Globe (eyeball, see figure 1 & 2): is situated in the anterior part of the orbital cavity closer to the roof than the floor and nearer the lateral than the medial wall. The sclera is the fibrous layer of the eyeball completely surrounding the globe except the cornea.
It is relatively tough but can be pierced easily by needles. The optic nerve penetrates the sclera posteriorly 1 or 2 mm medial to, and above, the posterior pole. The central retinal artery and vein accompany the optic nerve. The cone refers to the cone shaped structure formed by the extraocular muscles of the eye.

The orbital fat is divided into central (retrobulbar, intracone) and peripheral (peribulbar, pericone) compartments by the cone of the recti muscles. The central space contains the optic, oculomotor, abducent and nasociliary nerves. The peripheral space contains the trochlear, lacrimal, frontal and infraorbital nerves. All the motor and sensory nerves can be blocked by an injection into the orbital fat.

The extraocular muscles: The combined actions of the four rectus and two oblique muscles on each eyeball allow elevation, depression, adduction and abduction. Under normal circumstances unmodified activity of one muscle is rare but testing individual muscle function becomes necessary after local anaesthetic block to identify the unblocked nerve when some movement is still present.

Nerve supply to the eyes: The motor nerve supply to the extraocular muscles is easy to remember using the pseudoformula LR₆(SO₄)₃ - lateral rectus by the sixth (abducent) cranial nerve, superior oblique by the fourth (trochlear) and the remainder by branches of the third (oculomotor) nerve.

The sensory supply is mainly from the ophthalmic division of the 5^th (trigeminal) cranial nerve ( table 1). The lacrimal branch innervates the conjunctiva and the nasociliary branch the cornea, sclera, iris and ciliary body. The second cranial nerve (optic) conveys vision.

The parasympathetic supply is from the Edinger Westphal nucleus accompanying the 3rd nerve to synapse with the short ciliary nerves in the ciliary ganglion. The sympathetic fibres are from T1 (the first thoracic sympathetic outflow) and synapse in the superior cervical ganglion before joining the long and short ciliary nerves.

Injection of local anaesthetic solution into the lateral adipose compartment from an inferotemporal needle insertion normally blocks the nasociliary, lacrimal, frontal, supraorbital and supratrochlear branches of the ophthalmic division of the trigeminal nerve and the infraorbital branch of the maxillary division.

Injection into the medial compartment through a needle placed between the caruncle and the medial canthal angle usually blocks the medial branches of the nasociliary nerve, the long ciliary nerves, the infratrochlear nerve and medial components of the supraorbital and supratrochlear nerves.

(Continued...)