8 Apr 2013
Ben Blacklock
Job TitleBEN BLACKLOCK describes a procedure for safely performing eye removal without general anaesthesia in horses
CONVENTIONAL wisdom recommends a general anaesthetic for equine enucleation – except in rare circumstances. However, with appropriate sedation and multimodal analgesia, this can be carried out with the horse standing, thus avoiding the complications and costs associated with general anaesthesia.
This article aims to describe how a standing enucleation may be performed successfully and safely.
The indications for enucleation are wide-ranging and casedependent. The signalment of the patient, the owners and the veterinary surgeon will all influence the decision to enucleate. Specialist ophthalmologists are able to perform advanced procedures to preserve vision and alleviate pain, but these techniques are not always appropriate. Therefore, an enucleation is often the quickest, cheapest, and least painful way to manage a variety of ocular pathology.
While it can be considered a routine procedure in small animals, it may be quite an undertaking in equine species. In brief, any blind and painful eye should be removed. More specific indications include:
• intraocular infections that have destroyed the globe and are potential sources of systemic infection;
• intraocular tumours not amenable to local treatment;
• proptosis of the globe with severing of several extraocular muscles, or the optic nerve, with no hope of preserving vision;
• intraocular inflammation that has destroyed the globe and caused blindness (endophthalmitis or panophthalmitis), for example, following equine recurrent uveitis resulting in phthisis bulbi ;
• trauma with loss of intraocular tissues, without the possibility of successful repair; and
• enlarged and blind glaucomatous eyes.
Patients undergoing standing enucleation at the Animal Health Trust have a jugular catheter placed and a NSAID, such as flunixin, given intravenously. They are then restrained in stocks, with a headrest. Sedation and analgesia are provided by a combination of an alpha-2 adrenergic agonist and opioid (for example, detomidine and butorphanol), and supplemented by a constant rate infusion of detomidine (added to a saline fluid bag and administered via an infusion pump), which allows quick and easy alteration in the depth of sedation.
If indicated, antibiotics should be given prior to surgery to allow adequate tissue concentrations to be present. The tetanus vaccine status should be determined beforehand and either tetanus anti-toxin or vaccine should be given as appropriate.
Once adequately sedated, the skin around the orbit is clipped and partially prepped (with 1: 10 povidone iodine solution). The local nerve blocks are then performed using a local anaesthetic, such as mepivacaine two per cent (approximately 4ml per site).
For a routine enucleation – not involving extraocular infection, inflammation or neoplasia – frontal (supraorbital), auriculopalpebral, lacrimal, zygomatic and infratrochlear nerve blocks are performed, along with a retrobulbar block (Figures 1 to 5). A circumferential subcutaneous infiltration of local anaesthetic may be used instead of identifying the lacrimal, zygomatic and infratrochlear nerves individually.
The retrobulbar nerve block should be performed in all horses undergoing enucleation. It is also useful when performing other standing procedures when local anaesthesia and immobility of the globe are required. A three-and-a-halfinch, 20-gauge spinal needle is placed in the dorsal orbital fossa, caudal to the orbital rim. The needle is advanced perpendicular to the orbit. As the needle comes into contact with the retractor bulbi muscles, the globe can be seen to move slightly. The needle is advanced about 1cm further and 10ml of local anaesthetic instilled (Figure 6). Postoperative discomfort should be mild and controlled with a course of oral NSAID.
Subconjunctival technique is preferred if the ocular disease is within the globe. It is easier and simpler, with better surgical visibility due to less intraoperative haemorrhage and less postoperative swelling.
Once the surgical site is prepared (clipping and preparation of the skin with 1 : 10 povidone iodine solution, and preparation of the eye and conjunctiva with 1 : 50 povidone iodine solution, rinsed with saline) an adhesive drape is applied to facilitate sterility (Figure 7). An eyelid speculum is required as there will be ptosis following the auriculopalpebral nerve block.
The limbus is grasped with forceps and a circumferential incision is made through the bulbar conjunctiva with scissors, approximately 5mm caudal to the limbus to leave enough conjunctiva to allow manipulation of the globe.
Blunt dissection caudally under the conjunctiva will identify the extraocular muscles, each of which is sectioned through its aponeurosis close to the globe. The globe will now be relatively free within the orbit and the retractor bulbi muscles and optic nerve are sectioned blindly (Figure 8). It is not recommended to clamp or ligate the optic nerve – haemostasis is instead achieved by applying firm pressure with swabs for five minutes.
The nictitans may be removed with the globe, or cut away afterwards. An incision is made circumferentially around the eyelids, approximately 5mm from the lid margins. The eyelids are then removed along with remaining conjunctiva. Ensure both the medial and lateral canthus are completely removed – carefully avoiding the large angularis oculi vein as it runs over the medial orbital rim – and check for any remaining conjunctiva. The lacrimal gland does not require removal.
An intraorbital prosthesis may be placed in the orbit to prevent sinkage of the skin into the orbit postoperatively, which some owners find unsightly. It is a matter for debate as to whether the surgical site looks better after a prosthesis.
If no prosthesis is used, closure of the surgical site is in three layers. A polypropylene mesh (for example, Prolene 0 or 2-0) can be placed across the orbit, anchored to the periosteum (Figure 9). To access the periosteum, a lateral canthotomy is required, extending the incision by 1cm to 2cm. The subcutaneous tissue is then closed – taking care not to include orbital fascia – in a continuous pattern using absorbable suture material of the surgeon’s choice.
The skin may then be closed via the surgeon’s preferred technique. For example, simple interrupted or cruciate using a non-absorbable suture, intradermal absorbable sutures or stainless steel surgical staples.
The method of choice if any infectious or neoplastic process affects the globe is the transpal pebral technique. Skin preparation is the same. The eyelids are apposed using sutures in a continuous pattern or Allis tissue forceps. A skin incision is then made circumferentially around the eyelids, around 5mm from the lid margin (Figure 10).
Early transection of the medial and lateral canthal ligaments will facilitate dissection. The subcutaneous tissues of the lid are blunt – dissected caudally, beyond the conjunctival fornix, to emerge on to the globe (Figure 11). Care is required to avoid entering the conjunctival sac and contaminating the surgical site.
From this point, enucleation proceeds in a similar fashion to the previous technique, extraocular muscles are sectioned sequentially – freeing the globe – and the retractor bulbi muscles and optic nerve are cut blindly with curved scissors. Closure is as described above after careful inspection of the orbit (Figure 12).
All enucleated globes should be trimmed of extraocular tissue, fixed in formalin and sent for histopathology. If finances don’t allow histopathology, the globe should be stored in formalin. If any further disease process occurs in the orbit or fellow eye, the globe can be sent for analysis at that stage.
Mild to moderate haemorrhage always occurs. This is controlled with pressure and ensures a watertight seal is made when closing the surgical site. Tissue glue may be applied to the surgical wound after closing to help provide a seal and a swab may be sutured over the wound for 24 hours to absorb any seepage and apply pressure. If the patient will tolerate it, cold packing and gentle pressure at the surgical site may alleviate some swelling, but is rarely required.
Between days three and five postoperatively, blood clots within the orbit will start to break down. It is not unusual to see a bloody fluid appear at the nostril as drainage down the nasolacrimal duct occurs at this time. It is wise to warn owners this may happen.
The most common complication following enucleation without an intraorobital prosthesis is leaving behind some secretory tissue. It is important to remove the nictitans, conjunctiva and eyelids in their entirety. The medial canthus is often incompletely excised, creating a draining tract via the canthus. This should be revised surgically. Any nictitans and conjunctiva left behind can create a persistent mucoid accumulation with resultant draining tract or distension of the orbit, which may become septic. Surgical exploration, debridement and vigorous flushing are required. A drain placed during the surgery, exiting the orbit through the lower lid (ventral to the surgical wound) and left in place for three to five days can be useful.
Other described techniques to treat a septic orbit include temporarily placing a swab soaked in povidone iodine solution into the orbit following debridement and flushing. A small piece of the swab is left protruding through a stab incision ventral to the surgical site, and each day the swab is pulled out of the orbit a little bit (under sedation if necessary) until any discharge becomes serous. The stab incision is then left to heal.
Aggressive management is required to treat a septic orbit due to the risk of infection tracking up the meninges of the optic nerve to the brain. The risk of infection increases if an intraorbital prosthesis is used.
A retrospective study has showed the vast majority of horses (85 per cent) were able to return to work following unilateral enucleation (Utter et al, 2010).
If bilateral enucleation is required, horses can cope well if given time and appropriate management. It is, however, a major undertaking to look after a blind horse, and the personality of the horse, as well as the time and financial constraints of the owner, need to be taken into account. It is unlikely the animal will be able to work as it did previously, but may still be able to hack or perform dressage. Gilger (2011) provides some useful tips for managing blind horses, see panel above.
• Talk to the blind horse often, using a calm tone.
• Frequently touch the animal, especially when a new stimulus is pending.
• Teach it a few verbal commands : such as “whoa”, “step up”, “back” and so on.
• Choose a quiet companion for the blind horse and keep them as a pair.
• Fence paddocks and pastures with horse-safe boundaries.
• Police stalls and paddocks for hazards and sharp projections.
• Tape bucket handle hooks.
• Do not clip the muzzle whiskers or vibrissae.
• Demonstrate boundaries of any new enclosure.
• Practice loading the horse off and on a trailer.
• Keep food and water in a consistent location.
• Keep a consistent daily routine.
• Be aware that sudden noises or high winds may scare blind horses.
• Set limits for behaviour and reinforce them – spoiling a blind horse is not a good idea.
Ben Blacklock
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