Ophthalmic anaesthesia

Assessment and preparation

Standard preoperative assessment for patients undergoing general anaesthesia should be carried out for all patients (see Chapter 19 ). 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 having problems related to cognitive function; becoming uncomfortable, claustrophobic or hypoxaemic; developing myocardial ischaemia; or coughing. Concurrent upper respiratory infection should prompt a cancellation, as coughing and sneezing during the surgery can cause serious difficulties. Long-term anticoagulation presents potential complications that are more relevant to the surgeon or those practising local anaesthesia; however, there are a number of ophthalmic procedures that can be safely carried out without the need to interrupt anticoagulation or antiplatelet therapy. Warfarin therapy is not considered an absolute contraindication to local anaesthesia provided that the preoperative international normalised ratio (INR) is in the therapeutic target range; a sub-Tenon's block or topical anaesthesia is preferred in these cases.

Patients receiving only local anaesthesia are usually 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. The blood sugar concentration should still be checked.

Induction of anaesthesia

Smooth induction is particularly important in the ophthalmic setting. Coughing, straining and increases in intrathoracic pressure should be avoided as these cause venous congestion and elevate IOP. The choice of induction agent is of much less importance than how it is used (see Chapter 4 ). In equipotent doses, propofol has a greater depressant effect on IOP than thiopental but also causes more hypotension. Suxamethonium in isolation causes an immediate increase in IOP. Short-acting opioids (e.g. fentanyl or alfentanil) act synergistically with anaesthetic induction agents and obtund cardiovascular responses to airway manipulation.

Airway management

The airway may remain inaccessible throughout surgery, and any need to adjust or reposition an airway device during surgery could cause disruption to surgery. The use of supraglottic airway devices (SADs) has become popular, particularly for short ophthalmic procedures. The administration of an NMBA in conjunction with a SAD may aid mechanical ventilation and tighter control of ocular physiology but is considered by some anaesthetists to carry an increased risk of aspiration. If there is any doubt about maintaining the airway with a SAD, it should be changed to a tracheal tube before surgery commences. If tracheal intubation is used, it is important to avoid increases in IOP both at intubation (e.g. because of the pressor response to laryngoscopy) and extubation (laryngospasm, coughing/straining on tracheal tube); this is of much greater importance in open eye surgery. These responses can be attenuated with short-acting opioids (e.g. remifentanil) or topical/intravenous lidocaine. Tracheal tube ties are avoided, with tape used in preference.

Maintenance of anaesthesia

There are, in practice, few clinical differences between the effects of different volatile anaesthetic agents or between inhalational and intravenous anaesthesia (see Chapters 3 and 4 , respectively). The use of nitrous oxide depends on local availability of medical air and personal preference. Two particular risks of nitrous oxide must be considered in relation to ophthalmic anaesthesia: the increased risk of PONV, and the effect on IOP when intraocular gases are used for vitrectomy.

Relative hypotension during anaesthesia combined with normoxia and normocapnia provide a soft, well-perfused eye. A 15-degree head-up tilt may improve surgical conditions. However, excessive hypotension may prompt questions from the ophthalmologist because of the 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 increases in 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 i.v. fluids to avoid overloading the myocardium or inducing urinary retention in the older patient. Ophthalmic surgery is performed commonly on patients with diabetes because of complications of the disease. If general anaesthesia is required, local protocols must be followed (see Chapter 20 ). Analgesia requirements are based on the intraoperative use of a short-acting opioid and paracetamol. NSAIDs may be useful if there are no contraindications. Local anaesthesia with a longer-acting local anaesthetic drug is particularly useful intraoperatively. Ophthalmic patients are particularly at risk for PONV despite the absence of long-acting opioids. The combination of antiemetic agents from different classes is more effective than a single antiemetic agent (see Chapter 7 ).

Relevant anatomy for ophthalmic regional techniques

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 rectus muscles (inferior, lateral, medial and superior) and the superior oblique and inferior oblique muscles ( Fig. 38.2 ). 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 42–54 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 the trigeminal nerve), ciliary ganglion and vessels lie in the cone. 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, inferior rectus and 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. 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 secretomotor 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 posteriorly to the sclera anteriorly, separating it from the orbital fat and forming a socket in which it moves ( Fig. 38.3 ). The capsule extends 5–8 mm behind 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. The anterior Tenon's capsule is adherent to episcleral tissue from the limbus posteriorly for about 5–8 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 5–8 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 provides sensation 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. The globe occupies almost 50% of the orbital volume at birth and 33% at 4 years, whereas the adult globe only fills 22% of the orbital volume. As a result of these anatomical differences, sharp needle blocks carry more potential risks in children than in adults.

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 is known as an intraconal (retrobulbar) block ( Fig. 38.4 ), whereas in the extraconal (peribulbar) block, the needle tip remains outside the muscle cone ( Fig. 38.5 ). Multiple communications exist between the two compartments, and it is difficult to differentiate whether the needle is intraconal or extraconal during 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 intraconal block. A combination of intraconal and extraconal block is described as a combined retroperibulbar 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.

To achieve adequate anaesthesia and akinesia, the cranial and sensory nerves described earlier 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 most nerves.