The WSAVA guidelines for the recognition and treatment of pain state that procedures of the eye,
eyelid and surrounding tissues are usually associated with mild to severe pain. The root cause of pain associated with surgery and acute conditions, such as corneal ulceration, uveitis and trauma, is inflammation. It therefore makes mechanistic sense to target inflammation with therapeutics such as NSAIDs where possible.
For surgical procedures, local techniques offer excellent benefits to patient comfort. The third peri-operative pillars of analgesia are the opioids. Concurrent topical therapy for the ocular condition, like steroids, should be considered.
Peri-operative analgesia
Pre-medication with an opioid such as methadone provides excellent analgesia. Methadone has been demonstrated to be a superior analgesic to buprenorphine in dogs, although in cats, efficacy appears to be similar. Neither methadone nor buprenorphine cause vomiting, which may increase intra-ocular pressure.
The benefit of acepromazine in pre-medication for ocular procedures is that its long duration of effect may help to calm patients in recovery where alterations in sight may prove disorienting. Acepromazine is not an analgesic. Alpha-2 agonists in pre-medication have the advantage of augmenting opioid analgesia during surgery.
These versatile agents can be incorporated into a recovery protocol at low doses (dex/medetomidine 1-5mcg/kg) to calm patients during the recovery period. Incorporating simple pain-scoring tools can assist in decisions regarding repeated opioids during the peri-operative period.
Corneal surgery
The cornea contains a high density of nociceptors producing a marked pain response to stimuli. Topical local anaesthesia provides excellent, rapid onset analgesia.
Tetracaine has a more rapid onset, but a shorter duration than proxymetacaine and results in less conjunctival irritation. Repeated single drops have been shown to provide maximal analgesia of longer duration in horses compared to flooding the cornea (Monclin, 2011).
Enucleation
In patients where enucleation is necessary, there is likely to be pre-existing pain, and consideration should be given to analgesics, such as NSAIDs, prior to surgery – preferably for several days before surgery. A retrobulbar block is a simple technique to learn and provides effective anaesthesia for enucleation.
Local anaesthetic techniques for ocular surgery
Choice of local anaesthetic depends on the onset of action required and duration of effect. Clearly a long duration of action is most desirable in surgical patients and the slower onset of action of bupivacaine may be acceptable when surgical preparation times are taken into account.
Lidocaine
• Formulation – preservative-free, single-use vial, 1 percent or 2 percent solution
• Onset – 5 minutes
• Duration – 1-2 hours
• Dose – 4-10mg/kg
Bupivacaine
• Formulation – preservative free, single use vial, 0.25 percent or 0.5 percent solution
• Onset – 15 minutes
• Duration – 6-10 hours
• Dose 1-2mg/kg
Other long-acting local anaesthetics with a similar profile to bupivacaine include ropivacaine and levobupivacaine.
Retrobulbar block
• Area desensitised – cranial nerves II, III, IV, V and VI
• Indications – enucleation
• Volume to inject 1-4ml (dogs), 0.2-0.5ml (cats/rabbits)
• Needle size 23G 1” (pre-curved needles are available from Visitec)
The block should be injected either through the eyelid or the conjunctiva. Needle insertion point is half way between the lateral canthus and mid lower lid.
The needle should be walked around the bony orbit with the aim to position it caudal to the globe, where the local anaesthetic solution is then deposited. This technique should be avoided in neoplasia where there is a risk that neoplastic cells may be seeded (see splash block technique).
Peribulbar technique
This is an alternative technique to retrobulbar block and may be more suited for providing anaesthesia for the globe in cats. A short needle (23G 1”) is passed through the bulbar conjunctiva (avoiding the 12, 3, 6 and 9 o’clock positions) along the bony orbit, but unlike the retrobulbar technique, is not curved to end caudal to the globe. Local anaesthetic solution (2-4ml) is injected after aspiration.
Gentle massage of the globe should be performed following this technique to encourage spread of local anaesthetic into the intraconal space. This technique requires a larger volume than retrobulbar block to ensure sufficient spread, but avoids the risk of penetrating the optic nerve sheath.
A study by Shilo-Benjamini (2014), titled Comparison of peribulbar and retrobulbar regional anesthesia with bupivacaine in cats, demonstrated better deposition of local anaesthetic using this technique compared to retrobulbar injection. Peribulbar injection may therefore be a more suitable technique in the cat.
Sub-Tenon capsule block
This technique provides a desensitised area as for retrobulbar block, but has improved safety over the retrobulbar technique (Ahn et al., 2013; Shilo-Benjamini et al., 2013). Utilising this technique produces good desensitisation for enucleation or for corneal surgery. Skin (eyelid) sensation may not be completely eliminated though, and additional analgesia may be required for skin closure.
This method requires additional equipment and is technically more challenging than the retrobulbar method. The patient should be positioned in dorsal recumbency and a sterile prep performed. After application of topical anaesthesia, the mediodorsal portion of the bulbar conjunctiva (approximately 5mm from the limbus) is incised with tenotomy scissors, and the conjunctiva and sub-Tenon capsule are bluntly dissected from the underlying sclera. Sub-Tenon injection is performed through the incision with a 19-gauge, curved, blunt spatulated cannula. The sub-Tenon cannula often requires gentle tissue dissection as it is passed to allow it to be positioned caudal to the globe.
Splash block
Where neoplasia or infection is suspected, it is undesirable to seed these cells into deeper tissues.
For enucleation, although not a pre-emptive technique, the application of local anaesthetic once the eye is removed is widely practised. My preferred technique is local anaesthetic (bupivacaine) soaked into an absorbable haemostatic material placed at the surgical site (Ploog et al., 2014). In ltration of the skin incision prior to or post-closure will improve comfort levels.
Peri-ocular blocks
This technique is used for desensitising the skin around the eyes. It is useful for mass removal, eyelid surgery or as part of enucleation. Eyelid infiltration of local anaesthetic post-surgical procedure may be considered for post operative analgesia. Care should be taken to minimise post-operative swelling, which may result from local anaesthetic injection. Use of carefully-placed cold packs may prove to be useful. The nerves blocked are infratrochlear, zygomaticotemporal, frontal and lacrimal.
