Nasal injection (figures 6,7).

The same needle is inserted through the conjunctiva on the nasal side, medial to the caruncule and directed straight back parallel to the medial orbital wall pointing slightly cephalad (20°) until the hub of the needle is at the same level as the iris.

The needle traverses the tough medial canthal ligament and may require firm gentle pressure. This may cause the the eye to be pulled medially briefly. After negative aspiration another 5ml of the anaesthetic mixture is injected.

The eye is then closed with adhesive tape. A piece of gauze is placed over the lids and pressure applied with a Macintyre oculopressor for 10 minutes at a pressure of 30 mmHg. If no oculopressor is available gently press on the eye with the fingers of one hand. This is to lower intraocular pressure (IOP) by reducing aqueos humour production and increasing its reabsorbtion.

The signs of a succesful block are:

• Ptosis (drooping of the upper lid with inability to open the eyes)

• Either no eye movement or minimal movement in any direction (akinesia)

• Inability to fully close the eye once opened.

Since the local anaesthetic is placed outside the muscle cone the concentration around the optic nerve may not be sufficient to abolish vision completely. Some light perception will therefore remain; however the patient is not able to see the operation.

If, after 10 minutes the block is inadequate a supplementary injection of 2-5 ml of the anaesthetic mixture may be required. If the residual eye movements are downward and lateral, the supplementary injection is given at the inferotemporal site and if upward and medial, at the nasal site. Pressure is then reapplied for a further 10 minutes.

Care of patient. The patient must be made comfortable in the operating theatre using pillows and pads as required. An assistant should remain with the patient monitoring their condition and giving reassurance. Patients should be asked to remain silent and to squeeze the assistant's hand before any movements are made in order to warn the surgeon. A right angle screen can be used to keep the drapes away from the patient's face and to support an oxygen delivery system. A high flow of oxygen (8l /min) can be used to increase the FiO₂ and prevent CO₂ accumulation. Sedation is rarely required and should be limited to small increments (1mg) of midazolam. Oxygen saturation, ECG and blood pressure should be monitored throughout. Avoid oversedation of patients who may then wake up and move during the operation.

 
Retrobulbar block. The conjunctiva is first anaesthetised as described under peribulbar block. A 3 cm needle is inserted half way between the lateral canthus and the lateral limbus in the lower conjunctiva. It is first directed backwards under the globe and then after the equator of the globe has been passed the needle direction is changed upwards and inwards to enter the space behind the globe between the inferior and lateral recti muscles. After aspiration 4 ml of local anaesthetic solution is injected slowly. Retrobulbar block has largerly been replaced by peribulbar block because of the higher incidence of complications (see below) and the occasional need for an additional facial nerve block.

 
Complications of regional blocks for ophthalmic surgery may result either from the agents used or the block technique itself.

Intravascular injection and anaphylaxis can occur, hence resuscitation facilities must always be readily available.

Haemorrhage:Retrobulbar haemorrhage is characterised by a sudden rise of IOP and usually requires postponent of surgery. It is very rare with shallow retrobulbar or peribulbar injections. Subconjunctival haemorrhage is less significant as it will eventually be absorbed. Surgery need not be postponed.

Subconjunctival oedema (chemosis): This is un-desirable as it may interfere with suturing. It can be minimised by slowing the rate of injection. It rapidly disappears when gentle pressure is applied to the closed eye.

Penetration or perforation of the globe: This is more likely to occur in myopic eyes which are longer but also thinner than normal. A diagnosis of per-foration may be made if there is pain at the time the block is performed, sudden loss of vision, hypotonia, a poor red reflex or vitreous haemorrhage. Perforation may be avoided by carefully inserting the needle tangentially and by not going "up and in" until the needle tip is clearly past the equator of the globe.

Central spread of local anaesthetic: This is due to either direct injection into the dural cuff which accompanies the optic nerve to the sclera or to retrograde arterial spread. A variety of symptoms may follow including drowsiness, vomiting, contra-lateral blindness caused by reflux of the drug to the optic chiasma, convulsions, respiratory depression or arrest, neurological deficit, and even cardiac arrest. These symptoms usually appear within about 5min.

Oculocardiac reflex is the bradycardia which may follow traction on the eye. An effective local block ablates the oculocardiac reflex by providing afferent block of the reflex pathway. However the institution of the block and especially rapid distension of the tissues by the solution or by haemorrhage might occasionally provoke it. Careful monitoring is essential for early detection.

Optic nerve atrophy. Optic nerve damage and retinal vascular occlusion may be caused by direct damage to the optic nerve or central retinal artery, injection into the optic nerve sheath or haemorrhage within the nerve sheath. These complications may lead to partial or complete visual loss.

 
Advantages of local blocks over general anaesthesia:

1. May be performed as day cases
2. Produce good akinesia and anaesthesia
3. Minimal influence on intraocular presssure
4. Require minimum of equipment

This article contained links to the following additional information:

Table 1: Summary of sensory nerve supply

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