Vet clinic

Clever graft­ing tech­niques can save a dis­eased or dam­aged eye, and even re­store vi­sion. Fer­nando Malalana MRCVS dis­cusses the op­tions

Horse & Hound - - News Insider - H&H

Graft­ing tech­niques for dis­eased or dam­aged eyes

AGRAFT is de­fined as a piece of tis­sue that is sur­gi­cally trans­planted to a dif­fer­ent lo­ca­tion in the body.

This tech­nique is of­ten used in equine oph­thal­mol­ogy (the branch of medicine re­lat­ing to the eye) to treat a num­ber of con­di­tions af­fect­ing the cornea, which is the thin, clear win­dow at the front of the eye. The cornea is made of spe­cialised tis­sue and is around 1mm thick. Grafts are used to re­pair de­fects and pro­vide struc­tural or nu­tri­tional sup­port to the cornea, and, when pos­si­ble, to main­tain or re­store its clar­ity.

De­pend­ing on the ori­gin of the tis­sue used, grafts can be di­vided into three types...

An au­to­graft is taken from one part of the body and trans­planted onto an­other site in the same in­di­vid­ual.

An al­lo­graft is taken from one in­di­vid­ual and trans­planted onto an­other in­di­vid­ual of the same species.

A xenograft refers to the tech­nique of trans­plant­ing tis­sue into an in­di­vid­ual of an­other an­i­mal species.

SUR­GI­CAL TECH­NIQUES

IN equine oph­thal­mol­ogy, there are three tech­niques typ­i­cally used for corneal surgery. The first is a con­junc­ti­val graft; the con­junc­tiva is the pink tis­sue that sur­rounds part of the eye and eye­lids. This is prob­a­bly the most com­mon type of graft per­formed, used mostly for the treat­ment of deep or non­heal­ing corneal ul­cers.

This graft is rel­a­tively easy to per­form and pro­vides im­me­di­ate struc­tural sup­port. Plus, the blood ves­sels in the con­junc­tiva de­liver nu­tri­tion and heal­ing fac­tors to the dam­aged cornea.

The main dis­ad­van­tage is that the con­junc­tiva is not trans­par­ent, so a trans­plant from this area has the po­ten­tial to in­ter­fere with the horse’s vi­sion to some de­gree — es­pe­cially if the graft is large or the le­sion (the dis­eased or dam­aged area) is in the cen­tre of the cornea.

Con­junc­ti­val grafts can be di­vided into dif­fer­ent types, de­pend­ing on their size and shape:

Pedi­cle graft — a stalk-like graft from the edge of the eye to the dam­aged area.

Bridge or bi­pedi­cle graft — two pedi­cle grafts that orig­i­nate from op­po­site sides of the eye and meet in the mid­dle of the le­sion.

Free is­land graft — when a por­tion of con­junc­tiva is separated from its blood sup­ply and su­tured di­rectly to the le­sion.

Hood or ad­vance­ment graft — cov­ers a le­sion on the edge of the cornea.

Com­plete graft — per­formed for ex­ten­sive le­sions where the whole cornea needs to be cov­ered.

Some oph­thal­mol­o­gists like to cut back the trans­planted flap af­ter four to 12 weeks, although in most cases it will be left in place for the rest of the horse’s life.

The sec­ond tech­nique is a cor­neo­con­junc­ti­val/cor­neoscle­ral trans­po­si­tion. This type of graft is used in le­sions of the cen­tral cornea, where main­tain­ing as much trans­parency as pos­si­ble is de­sir­able. The graft is com­posed of the pe­riph­eral cornea, still at­tached to the con­junc­tiva or sclera (the “white of the eye”), which is ad­vanced over the eye sur­face to cover the de­fect.

The fi­nal tech­nique is a corneal trans­plant, which in­volves re­mov­ing the dam­aged area of the cornea and re­plac­ing it with a frag­ment of cornea from a donor horse.

The donor cornea can be fresh, from a re­cently de­ceased horse,

but is more typ­i­cally ob­tained when avail­able and stored frozen un­til needed.

Un­for­tu­nately, some de­gree of re­jec­tion must be ex­pected when deal­ing with corneal trans­plants in horses, which means that opaci­fi­ca­tion (cloud­ing) of the graft is al­most guar­an­teed. How­ever, this graft is prob­a­bly the best op­tion when deal­ing with le­sions af­fect­ing the deep lay­ers or the whole thick­ness of the cornea, such as per­fo­rated ul­cers or deep corneal ab­scesses that fail to re­spond to med­i­cal ther­apy.

MA­TE­RI­ALS AND METH­ODS

AN­OTHER op­tion is an am­nion graft. The am­nion is the thin yet very strong veil of tis­sue that forms the in­side layer of the mem­branes sur­round­ing the foe­tus while in the mare’s uterus. It can be col­lected af­ter birth and separated, washed and kept frozen un­til needed.

A com­mer­cial prod­uct com­posed of pig’s blad­der sub­mu­cosa (mem­brane lin­ing) is also avail­able for use in eye surgery. This has anti-scar­ring prop­er­ties and pro­vides numer­ous growth fac­tors that help re­pair the dam­aged cornea.

These sub­stances can be used in two ways. The am­nion can be su­tured to the sur­face of the eye tem­po­rar­ily to pro­tect dam­aged ar­eas of the cornea, such as the sur­gi­cal bed (un­der­ly­ing site) left be­hind when re­mov­ing a corneal tu­mour or an in­fected area known as a melt­ing ul­cer. In these cases, the am­nion is typ­i­cally re­moved af­ter five or six days.

Am­nion can also be stacked in lay­ers and used to fill de­fects in the cornea; the tis­sue is even­tu­ally in­cor­po­rated into the struc­ture and left there in­def­i­nitely.

Some grafts can be per­formed un­der stand­ing se­da­tion and lo­cal anaes­the­sia, while oth­ers re­quire gen­eral anaes­the­sia.

The su­ture ma­te­rial used to stitch grafts to the cornea can be as small as 0.04mm in di­am­e­ter (thin­ner than a hu­man hair), so good light­ing and some form of mag­ni­fi­ca­tion is es­sen­tial. As with most oc­u­lar pro­ce­dures in horses, the ex­per­tise and sur­gi­cal in­stru­men­ta­tion re­quired are highly spe­cialised and trans­porta­tion to a re­fer­ral hos­pi­tal is usu­ally nec­es­sary.

On oc­ca­sions, a tar­sor­ra­phy may be re­quired, which is a pro­ce­dure where the eye­lids are su­tured to­gether for a few days af­ter the graft is per­formed to pro­tect the sur­gi­cal site.

FIGHT­ING IN­FEC­TION

THE main com­pli­ca­tion fol­low­ing surgery is fail­ure of the su­tures that an­chor the trans­plant and sub­se­quently the graft it­self.

This usu­ally hap­pens due to in­fec­tion of the sur­gi­cal site, so in­tense ad­min­is­tra­tion of an­tibi­otics and an­ti­fun­gals is es­sen­tial. Mi­croor­gan­isms and white cells in the cornea can pro­duce en­zymes called col­la­ge­nases that can rapidly de­stroy the su­ture ma­te­rial and its an­chor points, so an­ti­col­la­ge­nase med­i­ca­tion is also an im­por­tant part of the post­op­er­a­tive treat­ment.

Uveitis, an in­flam­ma­tion in­side the eye as a con­se­quence of ei­ther the pri­mary dis­ease or the sur­gi­cal pro­ce­dure, is also com­mon. Like­wise, this re­quires med­i­ca­tion with an­ti­in­flam­ma­to­ries and anal­gesics.

These drugs must be ad­min­is­tered fre­quently, which re­quires com­pli­ance from the pa­tient. Where this proves prob­lem­atic the vet can use a sub­palpe­bral lavage sys­tem, which is where a small piece of tub­ing is in­serted un­der the horse’s eye­lid and ex­tends down his neck. This fa­cil­i­tates the ad­min­is­tra­tion of eye drops with­out hav­ing to ma­nip­u­late the eye.

The cornea is a com­mon site for in­jury and dis­ease. Suc­cess­ful treat­ment of corneal con­di­tions de­pends on prompt and ap­pro­pri­ate treat­ment, so seek vet­eri­nary ad­vice im­me­di­ately if you no­tice any changes or ap­par­ent dam­age to the nor­mal clear sur­face of the eye.

A con­junc­ti­val pedi­cle graft. Some of the con­junc­tiva has be­come pig­mented (black) but the eye is com­fort­able

A sub­palpe­bral lavage sys­temhas been placed in the lower eye­lid tofa­cil­i­tate the ad­min­is­tra­tion ofdrops to the eye

A layer of am­nion has been su­tured to the sur­face of the cornea to treat this melt­ing ul­cer

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