Ophthalmic Anaesthesia
Ophthalmic surgery can be classified into subspecialties and intraocular or extraocular procedures may be performed (Table 30.1); each has different anaesthetic requirements.
TABLE 30.1
Categorization of Ophthalmic Surgery
Ophthalmology Subspecialities
Paediatric
Oculoplastic
Vitreoretinal
Anterior segment
Glaucoma
Neuro-ophthalmology
Extraocular Operations
Globe and orbit
Eyebrow and eyelid
Lacrimal system
Muscles
Conjunctiva
Cornea, surface
Intraocular Operations
Iris and anterior chamber
Lens and cataract
Vitreous
Retina
Cornea, full thickness
PHYSIOLOGY OF THE EYE
Control of Intraocular Pressure
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.
TABLE 30.2
Factors Which Affect Intraocular Pressure (IOP)
IOP | Increase IOP | Decrease IOP |
Systemic | Age Large increase in blood pressure Increased carotid blood flow Increased central venous pressure Valsalva manoeuvre Carotid-cavernous fistula Plasma hypo-osmolality Hypercapnia Sympathetic stimulation | Exercise Large decrease in blood pressure Decreased carotid blood flow Decreased central venous pressure Parasympathetic stimulation Pregnancy Hypothermia Acidosis Plasma hyperosmolality Adrenalectomy General anaesthesia |
Local | Increased episcleral venous pressure Blockage of ophthalmic vein Blockage of trabecular meshwork Contraction of extraocular muscles Restricted extraocular muscle Acute external pressure Forced blinking Relaxation of accommodation Prostaglandin release (biphasic) Hypersecretion of aqueous | Decreased episcleral venous pressure Decreased ophthalmic artery blood flow Prolonged external pressure Retrobulbar anaesthesia Ocular trauma Intraocular surgery Retinal detachment Choroidal detachment Inflammation Prostaglandins (biphasic) Accommodation Increased aqueous outflow |
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.
CONDITIONS FOR INTRAOCULAR SURGERY
Effect of Anaesthetic Drugs on Intraocular Pressure
Muscle Relaxants
Succinylcholine increases intraocular pressure, with a maximal effect 2 min after i.v. administration, but the pressure returns to baseline values after 5 min. This effect is thought to be caused by the increase in tone of the extraocular muscles and intraocular vasodilatation. Pretreatment with a small dose of a non-depolarizing muscle relaxant does not obtund this response reliably. The problems involved with the use of succinylcholine in the patient with penetrating eye injury are discussed on page 616.
Non-depolarizing muscle relaxants have no significant direct effects on IOP.
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.
TABLE 30.3
Preferred Anaesthetic Technique for Common Surgical Procedures in Ophthalmology
Local Anaesthesia
Cataract
Glaucoma techniques
Minor extraocular plastic surgery
Laser dacrocystorhinostomy
Minor anterior segment procedures
Simple vitrectomies
General Anaesthesia
Paediatric surgery
Squint surgery
Major oculoplastic surgery
Dacrocystorhinostomy
Penetrating keratoplasty
Orbital trauma repair
Penetrating eye injuries
Complex vitreoretinal surgery