Ocular Local Anaesthetics

Local anaesthetics (LAs) are used in the initial assessment of minor trauma, the visualisation of the corneal epithelium, removal of foreign bodies, measurement of intraocular pressure using applanation tonometry and in ocular surgery. A more recent application of LAs in the correction of strabismus is being explored.¹

They should not be used for the long-term management of ocular symptoms and notably in management of pain: these agents are toxic to the corneal epithelium and abolish the corneal reflex so increasing the risk of corneal damage. Topical NSAIDs (e.g. diclofenac eye drops) may have a role in pain management although their efficiency is uncertain and they should not be used as a substitute for oral analgesia (paracetamol, oral NSAIDs).²

LAs work by blocking initiation and propagation of neuronal action potentials. Small diameter, myelinated nerves are most susceptible to LA action but not exclusively so and therefore patients will often report preservation of sensory modalities other than pain.³

You may find our records on Eye Trauma, Foreign Bodies in the Eye and Conjunctival Problems (including lacerations and foreign bodies) useful.

• Examples - lidocaine hydrochloride, oxybuprocaine hydrochloride, proxymetacaine hydrochloride, tetracaine hydrochloride.
• Use - largely for initial assessment of minor trauma and for removal of conjunctival and corneal foreign bodies. There is some debate about their use in surgery, particularly cataract surgery where some authors claim equal or better analgesia to injected LA.⁴ However in the UK, injected LA is very much the norm for ocular surgery in the absence of complicating factors (see below).
• Contraindications - known hypersensitivity reaction.
• Caution - hypertensive patients.⁵
• Administration - these come in single dose preparations. Warn the patient of brief stinging on application: proxymetacaine stings a little less (useful in highly anxious patients and children).⁶ Pain that is not relieved by topical LA suggests a more serious problem than superficial corneal or conjunctival injury.²
• Ocular side-effects - transient stinging; epithelial and stromal keratitis if overused.⁵ They are also known to inhibit corneal epithelial cell healing and so interfere with repair of corneal epithelial wounds,³ tetracaine being a particular offender. For these reasons, repeated administration of LA drops should be avoided and they should not be given to patients to take home for pain relief.
• Systemic side-effects - none noted with these topical drops.

• Examples - lidocaine hydrochloride is the most commonly used agent but individual surgeons may use other agents such as bupivacaine and cocaine.
• Use - minor operations, oculoplastic surgery, anterior segment and cataract surgery.
• Contraindications and cautions - see individual drug monographs. Other contra-indications for LA use in intraocular surgery include patient refusal and concurrent medical conditions preventing correct positioning of patient.⁷ Such patients require general anaesthesia.
• Administration - local subcutaneous injection to skin, subconjunctival injection, sub-Tenon injection (see below), peri- and retro-bulbar injections. The latter two carry a greater risk of adverse effects and are not commonly used. Maximal arterial plasma concentrations vary with each drug: using longer acting drugs is preferable to repeated injections. Do not inject into traumatised or inflamed tissues as this increases the likelihood of systemic absorption and adverse effects.
• Ocular side-effects - none from the drug if correctly administered. Peri- and retro-bulbar injections are trickier and can result in retrobulbar haemorrhage, globe puncture, optic nerve damage, muscle palsy and seventh cranial nerve complications.^7,8
• Systemic side-effects - rare but can occur if very large dose injected or if a normal dose is inadvertently injected intravenously.⁹ These include vasovagal reactions, confusion, respiratory depression, convulsions, hypotension and bradycardia.

The choice of anaesthetic in ocular surgery depends on the procedure, the patient and to a lesser extent, on the surgeon's preferences. There are office-based tests that can help evaluate the suitability of a given patient for local anaesthesia - these are best carried out by the operating surgeon.¹¹ There are wide variations in practice across the UK.

With any operation, the initial choice is between a general anaesthetic (GA) and LA. As a rule of thumb, trauma cases and children will all be given a GA. Young cataract patients are also frequently offered a GA as are patients who will have trouble keeping still because of a significant tremor or confusion. Some patients with a very high level of anxiety may request and be given a GA.

LA is used in 95% of cataract operations in the UK. When the decision is made to use an LA, the choice is between an injected anesthesia and drops.

Injected LA

Injected anaesthetics are routinely used for oculoplastic procedures, the LA being infiltrated directly into the skin around the operation site. For globe surgery (e.g. cataract operation), LA may be administered through the lower lid and under the globe (peri- / retrobulbar anaesthesia). Excellent anaesthesia and extraocular muscle block can be achieved but this method may be associated with serious complications.

The other injection method for globe surgery is the sub-Tenon's technique. This involves making a very small incision in the anaesthetised conjunctiva, the passage of a pre-curved blunted needle into the space between it and the globe (the sub-Tenon's space) and infiltration of the anaesthetic. Good anaesthesia is achieved with generally a good muscle block.

Risks of injected LA

These should be explained to the patient at the consent stage of proceedings. Generally, these are very safe techniques. Localised self-limiting haemorrhage is reasonably common but serious, sight complications are very rare, occurring in 0.06% of injected LAs. They are 2.5 times more commonly in peri/retrobulbar LA than with the sub-Tenon's technique and include retrobulbar haemorrhage, intravenous injection, globe perforation. It is certainly worth pointing out any anticoagulants or antiplatelet agents your patient is on at the point of referral as these patients are at significantly increased risk of minor complications associated with injected LAs.¹²

Drops

Locally applied anaesthetic drops also have a place in cataract operations, being used in about 22% of UK operations. They are easy to apply and are associated with minimal discomfort and side-effects. Their principle problem lies in not blocking muscular action which can be an issue where movements of a millimeter or less can have serious consequences. It has been suggested that post-operative discomfort is greater following topical anaesthesia but this point remains contentious.^4,13

• Fluorescein - combined with either lidocaine or proxymetacaine to enable visualisation of corneal epithelial defects and used for tonometry.
• Adrenaline - diminishes local blood flow so decreasing systemic absorption and prolonging local effect. Very low concentrations used (in the order of 1:80,000 to 1:200,000).
• Hyaluronidase - an enzyme added to increase tissue permeability to injected fluids. 15 units/mL is a favoured concentration.