Ophthalmic Anaesthesia


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.




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.



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.



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.


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May 31, 2016 | Posted by in ANESTHESIA | Comments Off on Ophthalmic Anaesthesia

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