Panuveitis Iris Prolapse Secondary To Trauma In Horse Biology Essay

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A four year old, Dutch Warmblood gelding was presented with a two day history of blepharospasm, diffuse corneal oedema, iris prolapse and panuveitis of the left eye secondary to trauma. There was no menace response and no consensual pupillary light reflex. Cases such as this have a poor prognosis for vision and immediate enucleation is a viable treatment option. In this case medical treatment was started with a view to clearing infection so that iridectomy could be performed; this would result in a more aesthetically pleasing outcome than enucleation. There was a significant response to therapy over two weeks and 6 weeks after presenting some vision had been regained. Traumatic lacerations with or without iris prolapse have the best prognosis when they are presented in the acute stages immediately after injury has occurred, when successful surgical repair is possible.

Introduction

Iris prolapse seems to occur in similar proportions as a result of a traumatic injury or due to a perforating ulcer. Traumatic injuries occur most commonly in younger horses, which may be due to their more fractious nature compared to older horses (Chmielewski and others 1997). Iris prolapse particularly occurs with sharply penetrating injuries - blunt trauma generally results in much more severe intraocular damage as there is a rapid increase in intraocular pressure, often leading to an explosive rupture from the inside outward with expulsion of the intraocular contents (Lavach and others 1984, Capaldo and Komáromy 2006).

Millichamp (1992) explains that with sharply penetrating injuries, at the moment the cornea is perforated the iris is carried into the wound by escaping aqueous humor. Tissue damage causes breakdown of the blood-aqueous barrier and massive influx of protein into the anterior chamber. Fibrin formation produces a temporary seal around the plug of iris, and the trapped iridal tissue has a dark brown colour.

Ocular survival in cases of iris prolapse due to traumatic injury following combined medical and surgical therapy is reported to be 80%, but retention of vision is only 33% (Chmielewski and others 1997).

Case history

A four year old Dutch Warmblood gelding was presented with a two day history of blepharospasm and mucopurulent ocular discharge in the left eye following a penetrating injury to the cornea.

On clinical examination the menace response was completely absent and there was no consensual response to the pupillary light reflex (PLR) in the right eye. The horse was sedated with detomidine (0.01mg/kg intravenously [Domosedan 10mg/ml solution, Janssen Animal Health]) and butorphanol (0.01mg/kg intravenously [Torbugesic 1% solution, Pfizer Animal Health]) and auriculopalpebral and supraorbital nerve blocks of the left eye were performed using mepivicaine (2ml at each site [Intra-Epicaine 2% solution, Dechra Veterinary Products]). Upon examination there was conjunctival oedema, diffuse corneal oedema and a mass with a darkly pigmented rim protruding from the cornea at the lateral canthus. Marked anterior chamber hypopyon was evident obscuring the iris and pupil; ophthalmic examination of the posterior segment of the eye was not possible due to these abnormalities. Ultrasonographic examination revealed the anterior chamber of the left eye to be significantly smaller than that of the right eye. The corneal mass was identified as a prolapse of iris tissue. There was a significant amount of debris in the posterior chamber with a mixed echogenic appearance consistent with fibrin formation. There was no evidence of retinal detachment. A diagnosis of panuveitis and iris prolapse secondary to trauma was made.

A sub-palpebral lavage system was placed to allow administration of chloramphenicol (0.2mls every two hours [Chloramphenicol 0.5%, Martindale Pharmaceuticals]) and 0.2mls of autologous serum every two hours. A course of enrofloxacin (8mg/kg orally once a day [Baytril 10% oral solution, Bayer PLC]) was begun. Flunixin meglumine (1.1mg/kg intravenously [Meflosyl 5% solution, Fort Dodge Animal Health]) was given as the horse had been sedated and could not be fed, but this was changed to flunixin meglumine (1.1mg/kg orally twice a day [Finadyne Granules, Intervet/Schering-Plough Animal Health]) the next day.

Ophthalmic and ultrasonographic examination two days later revealed there was no change in the condition of the eye. The decision was made to change the chloramphenicol for ciprofloxacin (0.2mls every 2 hours [Ciloxan 0.3%, Alcon]).

Over the following 8 days, the level of ocular comfort improved and the amount of hypopyon reduced. The amount of fibrin in the posterior chamber also reduced, but a large clot remained attached to the posterior capsule of the lens. Neovascularisation of the iris prolapse also began, and there was the development of some photophobic response. The horse was discharged 13 days after admittance with instructions to continue the topical ciprofloxacin therapy.

6 weeks after initial presentation, the horse was presented for re-examination. On clinical examination there was a significantly reduced menace response in the left eye but there was a consensual PLR in the right eye, showing that the retina was able to respond to light to some degree. The left eye was phthitic and there was a relative enophthalmus, but no associated blepharospasm, ptosis or apparent discomfort was present. Mild corneal oedema was identified but the iris prolapse had reduced in size. The owner was advised that no further treatment was required but that an improvement in vision was unlikely.

Discussion

Medical treatment or surgery?

When presented with an acute case of a perforating injury with iris prolapse, immediate surgery under general anaesthetic gives the best possible prognosis (Dallap Schaer 2007, Severin 1998). If the wound is less than one hour in duration and the iris is not severely prolapsed or damaged, it can be replaced into the anterior chamber. If the iris is severely prolapsed or damaged, it can be amputated. There are numerous methods of laceration closure, including corneal suturing, adhesives, conjunctival flaps and corneal transplants. The method chosen depends on the characteristics of the injury and skill of the surgeon (Severin 1998). Referral to a veterinary ophthalmologist is advised.

Due to the severe intraocular infection in this case and as it was already relatively chronic in nature, immediate surgical repair was not a sensible option. The two treatment options considered were therefore immediate enucleation or medical treatment with a view to clearing the infection enough to allow iridectomy. Immediate enucleation was a reasonable option as extensive ocular infection as a result of corneal penetration and disruption of intraocular contents are associated with a poor outcome for vision (Chmielewski and others 1997), and both were present in this case. Other negative prognostic indicators include lacerations greater than 15mm, lacerations with scleral involvement and extensive hyphaema (>50%) (Chmielewski and others 1997). When the lens and posterior chamber are not visible due to severe opacification, a negative dazzle response and consensual PLR can also be used as poor prognostic indictors for a return to vision. (Capaldo and Komáromy 2006). The decision was made to opt for medical treatment in this case though because the laceration was less than 15mm, and there was no evidence of retinal detachment on ultrasonographic examination, therefore it was a possibility that the quantities of debris in the anterior and posterior chamber were preventing a response to the menace test and PLR, i.e. there was a possibility that vision could be restored.

Antimicrobial delivery and choice

The normal blood-aqueous barrier prevents therapeutic levels of antimicrobial drugs occurring when they are given systemically (Severin 1998) and therefore, antimicrobials are usually given topically. However, when there are inflammatory processes occurring, such as with corneal perforation and iris prolapse, there is sufficient disruption to the blood-aqueous barrier that therapeutic levels can occur when antimicrobials are given systemically (Severin 1998). This is useful as topical administration is highly effective for anterior segment disease but may not establish therapeutic drug levels in posterior segment tissue; posterior segment disease is best treated with parenteral medications (Miller 1992). For horses with iris prolapse that are going straight to surgery, it is best to give systemic antimicrobials to avoid any potential further damage to the eye by topical solutions. Topical ointments should always be avoided as there is a risk that the petroleum base may cause uveitis (Dallap Schaer 2007, Severin 1998). Both systemic and topical antimicrobials were given in this case due to the severity of the intraocular infection and the frequency of application that was needed.

The normal equine eye has a predominantly Gram-positive bacterial population, with Staphylococcus species and Corynebacterium species being the most commonly isolated (Whitley and others 1983). However, this is significantly different in ocular disease, with Gram-negative Pseudomonas species and Escherichia coli being the most common isolates. The commensal bacteria can also act in an opportunistic manner, and Staphylococcus species and Streptococcus species are the most commonly isolated Gram-positive bacteria in ocular disease (Chmielewski and others 1997). Due to this variable population, antimicrobial choice would ideally be based on culture and sensitivity results. When these are not available, a broad-spectrum antibiotic should be chosen. Chloramphenicol is effective against most isolates of the equine eye and has high lipophilicty which allows ocular penetration (Moore and others 1995). However, it is not effective against Pseudomonas species (Moore and others 1995). Ciprofloxacin or ofloxacin have broad-spectrum activity including against Pseudomonas species and also have a high lipophilicity. When systemic antimicrobial therapy is required, enrofloxacin is useful due to the broad-spectrum activity and availability as an oral solution for ease of administration. Routine use of fluoroquinolones should be discouraged due to the potential to promote microbial resistance though, and thus chloramphenicol is a better first-line choice.

The solution tonicity is important - solutions that are outside physiologic ranges will cause local irritation and reflex tearing. Reflex tearing dilutes the solution and increases clearance from the ocular surface. Most animals tolerate tonicity ranging from 0.7% to 2% (Miller 1992).

Post-surgery, topical antimicrobial therapy should be continued until sutures are removed which typically occurs at 3 - 4 weeks (Severin 1998). Systemic antimicrobials should be given for 5 days or as long as deemed necessary.

Other therapeutics

Stromal collagenolysis can occur due to production of proteolytic enzymes by corneal epithelial cells, leukocytes and microbial organisms. There are a number of protease inhibitors that can be used to counteract this; two of the most common are 0.2% EDTA or autologous serum. Autologous serum is readily available, biologically non-toxic and has the widest spectrum of enzyme-inhibition (Capaldo and Komáromy 2006).

Presence of ocular discomfort requires analgesia to be given. Corticosteroids should not be used when there is a corneal defect as they have been shown to impair host defenses by reducing the migration of neutrophils and macrophages, delay corneal wound healing by decreasing keratocyte proliferation and collagen deposition, potentiate corneal collagenase activity and decrease epithelial healing rates (Giuliano 2004). Non-steroidal anti-inflammatory drugs (NSAIDs) have a more selective modality although there are still side-effects especially when given systemically (review the literature for details of pharmacodynamics). Flunixin meglumine is one of the most common NSAIDs to be prescribed traditionally and is anecdotally reported to be very good for ocular pain (Severin 1998). There also scientific evidence for its use; it has been shown to significantly limit miosis and reduce production of prostaglandin, and also to stabilise the blood-aqueous barrier (Giuliano 2004). If surgical repair is performed, the use of NSAIDs should be discontinued once the eye is comfortable as they slow vascularisation of the cornea, and complicated corneal lacerations heal by corneal vascularisation and scar formation (Millichamp 1992).

Atropine was not given in this case due to the risk of iris retraction from the corneal wound and deflation of the globe. However, there are reports that state this risk is very minimal (Capaldo and Komáromy 2006). Atropine causes mydriasis which therefore aids in the treatment of anterior uveitis and associated pain.

Conclusions

Due to the chronicity and extent of infection, this case of panuveitis and iris prolapse secondary to trauma was not amenable to immediate repair under general anaesthetic, which would have given the best prognosis. However, the medical treatment chosen resulted in significant improvement with some vision regained. If cases are to be managed medically, consider antibiosis and analgesia - topical chloramphenicol, systemic flunixin meglumine and topical atropine are suitable first-line treatments.

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