Unilateral choristoma of the nictitating membrane in a horse

Kara R. Gornik Department of Clinical Sciences, Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.

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Christopher G. Pirie Department of Clinical Sciences, Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.

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Gillian L. Beamer Department of Infectious Disease and Global Health, Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.

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Abstract

Case Description—A 2-year-old Morgan mare was evaluated because of a corneal ulceration.

Clinical Findings—An irregular, deep stromal corneal ulcer in an area of malacia was noted in the left eye. Hypopyon was present in the ventral portion of the anterior chamber with moderate aqueous flare. The nictitating membrane of the left eye had hairs originating from its leading edge that contacted the corneal surface.

Treatment and Outcome—General anesthesia was induced, and a bulbar pedicle conjunctival graft was performed. The conjunctiva at the leading edge of the nictitating membrane, including the aberrant hair follicles, was excised. Microscopically, a nonkeratinized stratified squamous epithelium, sebaceous glands, and hair shafts were present, confirming a choristoma of pilosebaceous origin at the leading edge of the nictitating membrane. Six weeks after surgery, the horse had no signs of discomfort, with no regrowth of the hairs; no loss of vision was evident.

Clinical Relevance—Ocular choristomas develop secondary to defective fetal cellular differentiation and are rarely reported in the equine literature. The choristoma in this horse contained ectopic hair follicles with hair growth as well as sebaceous glands. This finding emphasizes the importance of a thorough adnexal examination in horses with corneal disease.

Abstract

Case Description—A 2-year-old Morgan mare was evaluated because of a corneal ulceration.

Clinical Findings—An irregular, deep stromal corneal ulcer in an area of malacia was noted in the left eye. Hypopyon was present in the ventral portion of the anterior chamber with moderate aqueous flare. The nictitating membrane of the left eye had hairs originating from its leading edge that contacted the corneal surface.

Treatment and Outcome—General anesthesia was induced, and a bulbar pedicle conjunctival graft was performed. The conjunctiva at the leading edge of the nictitating membrane, including the aberrant hair follicles, was excised. Microscopically, a nonkeratinized stratified squamous epithelium, sebaceous glands, and hair shafts were present, confirming a choristoma of pilosebaceous origin at the leading edge of the nictitating membrane. Six weeks after surgery, the horse had no signs of discomfort, with no regrowth of the hairs; no loss of vision was evident.

Clinical Relevance—Ocular choristomas develop secondary to defective fetal cellular differentiation and are rarely reported in the equine literature. The choristoma in this horse contained ectopic hair follicles with hair growth as well as sebaceous glands. This finding emphasizes the importance of a thorough adnexal examination in horses with corneal disease.

A 2-year-old 379-kg (834-lb) female Morgan horse was evaluated because of a nonhealing corneal ulcer in the left eye. The mare had a history of possible trauma to the eye several weeks prior to referral; this had initially been treated by the referring veterinarian with topical administration of neomycin-polymyxin-dexamethasone ophthalmic ointmenta and atropine ophthalmic ointment,b along with systemic administration of flunixin megluminec (1.0 mg/kg [0.45 mg/lb], PO, q 12 h) for presumed anterior uveitis. On recheck examination 1 week later, a superficial corneal ulceration of the affected eye was noticed by the referring veterinarian. The horse was subsequently hospitalized, and a subpalpebral lavage catheter was placed in the superior conjunctival fornix. Topical treatment was changed to ofloxacin,d 1% voriconazole,e autogenous serum, and atropine ophthalmic solution.f Administration of flunixin megluminec was continued. The horse was referred to the veterinary teaching hospital several days later because of exacerbation of the corneal ulceration and development of grossly visible intraocular inflammation.

On examination, the horse had marked blepharospasm of the left eye. Results of an ophthalmic examination included normal menace responses and dazzle reflexes bilaterally. Palpebral reflexes were normal in both eyes. Direct and indirect pupillary light reflexes were normal in the right eye but absent in the left. Following visual testing, an auriculopalpebral nerve block was performed with 2 mL of 2% lidocaine hydrochloride.g An approximately 10 × 6-mm irregular stromal corneal ulcer (with approximately 75% stromal loss at its deepest aspect) in an area of malacia was observed in the dorsomedial aspect of the left cornea. Corneal neovascularization was present, extending from the dorsal and medial aspects of the limbus and approaching the nasal (ventromedial) edge of the ulcer. The cornea surrounding and lateral to the defect was edematous and malacic.

Several hairs were observed to originate from the leading edge of the nictitating membrane of the left eye, contacting the medial surface of the cornea and touching the innermost aspect of the corneal ulcer (Figure 1). Hypopyon was present in the ventral portion of the anterior chamber. Moderate aqueous flare was detected. The left pupil was nearly maximally dilated. No remarkable anterior segment abnormalities were found in the right eye. A well-demarcated area (3 × 3 mm) of decreased pigmentation was present ventral to the optic nerve in the nontapetal fundus of the right eye. This finding was consistent with a posterior segment coloboma involving the retinal pigmented epithelium and underlying choroid. Fundic examination was not attainable in the left eye because of the severity of the corneal disease and intraocular inflammation.

Figure 1—
Figure 1—

Photograph of the left eye of a 2-year-old Morgan mare evaluated because of a corneal ulceration. A—A large, irregular corneal ulceration with edema and malacia is present in the dorsomedial region of the cornea, with blood vessels extending from the dorsal and medial aspects of the limbus. Intraocular inflammation and hypopyon are evident. B—Enlarged image of the outlined region in Panel A. Two hairs can be seen emanating from the leading edge of the nictitating membrane (arrows).

Citation: Journal of the American Veterinary Medical Association 246, 2; 10.2460/javma.246.2.231

Diagnostic testing included aerobic bacterial and fungal cultures, cytologic evaluation of a sample obtained by corneal scraping, and a CBC and serum biochemical analysis. Coagulation assessment, comprising measurement of fibrinogen concentration, prothrombin time, and activated partial thromboplastin time, was performed because the horse had a history of epistaxis following nasogastric tube placement while hospitalized under the care of the referring veterinarian. Aerobic bacterial culture results consisted of 1+ growth (scale, 1+ to 4+) of Streptococcus zooepidemicus, which was susceptible to all antimicrobials tested. Fungal culture yielded 1+ growth of Candida spp. Cytologic findings were consistent with severe neutrophilic inflammation. Degenerate and nondegenerate neutrophils were present, along with epithelial cells. No fungal organisms or intracellular bacteria were detected cytologically. The CBC results were within respective reference ranges. Serum biochemical abnormalities included low concentrations of magnesium (1.4 mEq/L; reference range, 1.8 to 3.0 mEq/L), total protein (3.7 g/dL; reference range, 5.6 to 7.0 g/dL), albumin (2.2 g/dL; reference range, 2.4 to 4.0 g/dL), and globulin (1.5 g/dL; reference range, 2.5 to 4.9 g/dL). Prothrombin time (10.4 seconds; reference range, 10.9 to 14.5 seconds) and activated partial thromboplastin time (50.2 seconds; reference range, 54.7 to 69.9 seconds) were both low, indicating more rapid clot formation times. A fast localized abdominal sonography evaluation was performed because of low serum protein concentrations and a history of reduced manure production. The horse had moderate thickening of the right dorsal colon, consistent with mild right dorsal colitis.

The horse was hospitalized for stabilization overnight. Treatments included topical application of autogenous serum (0.2 mL, q 2 h) and ophthalmic preparations of ofloxacind (0.2 mL, q 4 h), 3.3% cefazolinh (0.2 mL, q 4 h), 1% voriconazolee (0.2 mL, q 6 h), and atropinef (0.2 mL, q 12 h) via the subpalpebral lavage catheter. Systemic treatment with flunixin megluminei (1 mg/kg, IV, q 12 h), sulfamethoxazole-trimethoprimj (30 mg/kg [13.6 mg/lb], PO, q 12 h), fluconazolek (14 mg/kg [6.4 mg/lb], PO, once), omeprazolel (4.6 mg/kg [2.1 mg/lb], PO, q 24 h), misoprostolm (1 μg/kg, PO, q 12 h), ranitidinen (6.6 mg/kg [3 mg/lb], PO, q 8 h), and sucralfateo (22 g/kg [10 g/lb], PO, q 6 h) was initiated. Water was also administered frequently via a nasogastric tube.

The following day, the serum total protein concentration was considered to be stable at a value of 4.0 g/dL. General anesthesia was induced. An auriculopalpebral nerve block, supraorbital nerve block, and retrobulbar block were performed with 2% lidocaine hydrochloride.g The cornea was debrided, and the malacic tissue was removed and submitted for histologic evaluation. A bulbar pedicle conjunctival graft was harvested from the dorsomedial conjunctiva and sutured over the ulcerated region of the cornea with 8–0 polyglactin 910p in a simple interrupted pattern.

The conjunctiva on the leading edge of the nictitating membrane, including associated hair follicles, was excised (Figure 2). Care was taken to avoid damage to the underlying cartilage of the nictitating membrane. Hemorrhage was controlled with cellulose spongesq and 2.5% phenylephrine solution.r The resultant wound was left to heal by second intention. The excised conjunctival tissue containing the hairs was submitted for histologic examination.

Figure 2—
Figure 2—

Photograph of the nictitating membrane of the left eye of the horse in Figure 1 at the time of surgery. Four aberrant hairs (arrows) are seen extending from the leading edge of the left nictitating membrane.

Citation: Journal of the American Veterinary Medical Association 246, 2; 10.2460/javma.246.2.231

Anesthetic recovery was uneventful, and the horse remained hospitalized for 7 days following surgery. The PCV and circulating total solids concentration were evaluated daily. Continued treatment included topical administration of autogenous serum (0.2 mL, q 6 h), ofloxacin (0.2 mL, q 6 h), cefazolin (0.2 mL, q 6 h), 1% voriconazole (0.2 mL, q 6 h), and atropine (0.2 mL, q 12 h) via the subpalpebral lavage catheter. Systemic administration of flunixin meglumine, sulfamethoxazole-trimethoprim, omeprazole, misoprostol, ranitidine, and sucralfate was continued at the previously described dosages, and the fluconazole dosage was reduced (5 mg/kg [2.3 mg/lb], PO, q 24 h). Flunixin meglumine administration was discontinued 5 days after admission, and administration of firocoxibs (0.2 mg/kg [0.09 mg/lb], PO, q 24 h for 2 days, then 0.1 mg/kg [0.04 mg/lb], PO, q 24 h) was initiated because of sustained low serum total protein concentration (4.8 g/dL) as well as increased serum creatinine concentration (from 1.8 mg/dL [reference range, 0.9 to 1.9 mg/dL] on admission to 2.2 mg/dL 5 days after admission).

Results of the histologic examination of the corneal biopsy sample were consistent with the cytologic findings and consisted of stromal infiltration by neutrophils, lymphocytes, plasma cells, and macrophages. No bacterial or fungal organisms were observed with H&E stain or with Gömöri methenamine silver stain. Histologically, the leading edge of the nictitating membrane contained a noncornified stratified squamous epithelium, sebaceous glandular tissue, follicular epithelium, and hair shafts (Figure 3).

Figure 3—
Figure 3—

Photomicrographs of conjunctival tissue obtained from the leading edge of the nictitating membrane. A—An ectopic hair follicle (asterisk) with surrounding sebaceous glandular tissue (dagger) can be seen within the submucosa. B—A hair shaft (arrow) is present within an ectopic hair follicle located on the leading edge of the nictitating membrane. H&E stain; bar = 60 μm.

Citation: Journal of the American Veterinary Medical Association 246, 2; 10.2460/javma.246.2.231

Ophthalmic examination 7 days after surgery indicated that the graft was well vascularized with focal areas of bruising (Figure 4). The surrounding cornea remained mildly edematous. The pupil was fully dilated, and the intraocular inflammation largely resolved. The total protein concentration remained moderately low at 5.0 g/dL. The serum creatinine concentration returned to a normal value of 1.4 mg/dL. The horse was discharged from the hospital, and the owner was given instructions to administer autogenous serum, ofloxacin, cefazolin, 1% voriconazole, and atropine topically via the subpalpebral lavage catheter at the same dosages used after surgery. Systemic administration of firocoxib (0.1 mg/kg, PO, q 24 h) omeprazole (dose reduced to 1.15 mg/kg [0.5 mg/lb], PO, q 24 h), fluconazole, misoprostol, and sucralfate (at previously described dosages) was continued. Systemically administered antimicrobials were changed from sulfamethoxazole-trimethoprim to minocyclinet (4.0 mg/kg [1.8 mg/lb], PO, q 12 h) owing to its anticollagenolytic and anti-inflammatory properties.

Figure 4—
Figure 4—

Postoperative clinical photograph obtained 7 days after surgery. The graft and sutures were in place at the dorsomedial aspect of the cornea. Corneal neovascularization was still present dorsally. There was moderate bruising in the center of the graft, and the edges remained well vascularized. Mild corneal edema was still visible lateral to the graft site. The intraocular inflammation had largely resolved, and the pupil was fully dilated. The nictitating membrane had healed with no visible hairs extending from its leading edge.

Citation: Journal of the American Veterinary Medical Association 246, 2; 10.2460/javma.246.2.231

Follow-up conversations with the referring veterinarian 6 weeks after discharge confirmed that the horse had no detectable visual defects and had no signs of discomfort in the left eye. The graft was not trimmed because of its medial location and presence outside of the visual field. No regrowth of hairs was noted on the leading edge of the nictitating membrane.

Discussion

The nictitating membrane or third eyelid is a structure that normally lies in the ventromedial conjunctival sac, between the lacrimal caruncle and the globe. The nictitating membrane is covered by conjunctiva on its bulbar and palpebral surfaces and is supported by a T-shaped fragment of hyaline cartilage.1 The nictitating membrane contains a lacrimal gland, which contributes substantially to the aqueous portion of the tear film. The primary function of the nictitating membrane is to provide added protection to the globe, sweeping across the cornea in a dorsolateral direction when the eye is passively retracted into the orbit.2 Histologically, a normal nictitating membrane is covered by a nonkeratinized stratified squamous epithelium with interspersed goblet cells on the bulbar and palpebral surfaces.3 The bulbar aspect also contains lymphoid tissue and deep glandular tissue, which adjoins the T-shaped cartilage.4

A choristoma is an accumulation of microscopically normal tissue located at an aberrant site resulting from abnormal cellular differentiation during fetal development.5 The most commonly recognized type of choristoma involving ocular tissue is a dermoid, which results from the abnormal proliferation of elements of skin and dermis and is composed of a cornified epithelium variably including hair, blood vessels, fibrous tissue, nerves, glands, smooth muscle, and cartilage components.6 Dermoids are most frequently located at the temporal limbus but may involve the conjunctiva, nictitating membrane, eyelid margin (termed hamartoma), limbus, or cornea, alone or in various combinations.7,8

Dermoids develop secondary to defective induction or differentiation of the surface ectoderm and are rare in horses.9 In a recent report, Greenberg et al10 described a lesion in a mature gelding that was similar in clinical appearance to that in the Morgan horse of this report and was located at the leading edge of the nictitating membrane. That lesion contributed to the formation of recurrent superficial corneal ulcerations.10 A previous clinical report11 has also described bilateral ocular colobomas affecting the dorsal limbus and sclera in a colt.

Histologically, dermoids typically contain a keratinized, variably pigmented, stratified squamous epithelium.9 They also include the regular presence of hair follicles, blood vessels, glandular tissue (sebaceous and sweat glands), adipose tissue, fibrous tissue, and nerves. On rare occasions, cartilage and bone can be identified among the contents of a dermoid.6 Generally, the edges of ocular dermoids are indistinct and merge with the neighboring corneal and conjunctival epithelium and stroma.12 Clinically, the choristoma reported in the horse of this report appeared to lack the typical skinlike appearance routinely associated with a dermoid. A lack of keratinized stratified squamous epithelium was confirmed histologically. Rather, in this mare, the nictitating membrane was observed to contain several ectopic hairs that emerged from the leading edge of the nictitating membrane along with associated sebaceous glandular tissue, making the clinical and histologic diagnoses less consistent with a true dermoid.

Other examples of choristomas affecting the ocular surface involve abnormal or misdirected hair growth that can lead to subsequent corneal and conjunctival irritation. These include distichiasis and ectopic cilia,2 both of which are rare in equine species. To the authors’ knowledge, distichiasis has only been reported once,13 and there have been 2 reports2,14 of ectopic cilia in horses. The first describes 1 horse with recurrent corneal ulcerations secondary to an aberrant eyelash located in the upper eyelid's palpebral conjunctiva.2 The other report14 was a case series of 7 horses with blepharospasm, ocular discharge, and keratitis secondary to the presence of ectopic cilia in the palpebral conjunctiva of the upper eyelid. The presence of the hair follicles arising from the leading edge of the nictitating membrane rather than the meibomian glands or the palpebral conjunctiva underneath the eyelid rules out the diagnoses of distichiasis or ectopic cilia in the horse of the present report.

Ocular choristomas containing hair follicles may produce hairs that emanate from the surface and subsequently contact local ocular tissues. Irritation caused by these hairs can culminate in conjunctival and corneal inflammation, with secondary epiphora and corneal changes, including keratitis and corneal ulceration.15,16 Surgical removal of the abnormal tissue is recommended when aberrant hairs, if present, result in corneal or conjunctival irritation and pathological changes or, in cases of dermoids, if the lesion is large enough to obstruct vision.6 In this report, the aberrant hairs were observed to directly contact the cornea following movement of the nictitating membrane. It is possible these hairs caused the initial corneal ulceration, and it was considered likely that they contributed to its persistence and severity. Surgical removal of the hairs and underlying follicles, in conjunction with a bulbar pedicle conjunctival graft, was sufficient to repair the corneal ulceration and resolve the source of continued irritation.

Because choristomas result from abnormal tissue differentiation early in fetal development, they can often occur in conjunction with other ocular abnormalities.5 Baumgartner et al17 described a case in a Thoroughbred affected with bilateral corneal dermoids accompanied by multiple concurrent ophthalmic abnormalities, including microphthalmia, severe anterior segment dysgenesis, aphakia, retinal dysplasia, and optic nerve hypoplasia. Additionally, Joyce et al18 described a Quarter Horse sire and 4 offspring noted to have aniridia or severe iridial hypoplasia, limbal dermoids, as well as anterior and posterior nuclear cataracts. Although minor and considered to be an incidental finding, funduscopically, the mare of this report had a well-demarcated area of decreased pigmentation in the nontapetal fundus, consistent with a coloboma of the retinal pigmented epithelium and choroid in the right eye.19

Posterior segment coloboma locations can be categorized into typical and atypical colobomas. Typical colobomas are infrequently found in the retinas of horses and develop secondary to incomplete closure of the optic fissure during embryogenesis.20 Typical colobomas are located along a plane running vertically from the optic disc into the ventral region of the nontapetal fundus.19 Atypical colobomas are seen more frequently but are still relatively uncommon; these develop outside the plane of the optic fissure and are generally found within the nontapetal fundus, lateral or medial to the optic disc.19 Colobomas in the equine fundus can affect the tapetum, tapetum with the underlying retinal pigmented epithelium, retinal pigmented epithelium and underlying choroid, blood vessels, or optic papilla.20 The coloboma in the horse of the present report can be classified as typical, given that it was located ventral to the optic disc along a plane in the vertical meridian. The appearance of the coloboma indicated involvement of the retinal pigmented epithelium along with the choroid, exposing a small portion of the sclera.19 Histologic examination would be required to determine the full extent of retinal and choroidal involvement of the coloboma. In the mare of this report, a posterior segment coloboma occurred concurrently with the choristoma of the nictitating membrane. The finding of a choristoma that likely contributed to severe corneal ulceration highlights the importance of a thorough assessment of the adnexa in horses with corneal ulcerations.

a.

Neomycin-Polymyxin-Dexamethasone Ophthalmic Ointment, Falcon Pharmaceuticals Ltd, Fort Worth, Tex.

b.

Atropine Ophthalmic Ointment, Bausch & Lomb, Tampa, Fla.

c.

Banamine Paste, Merck, Whitehouse Station, NJ.

d.

Ofloxacin Ophthalmic Solution, Akorn Inc, Lake Forest, Ill.

e.

Voriconazole, Amerinet Inc, New York, NY.

f.

Atropine Ophthalmic Solution, Bausch & Lomb, Tampa, Fla.

g.

Lidocaine Hydrochloride, Hospira Inc, Lake Forest, Ill.

h.

Cefazolin, Hospira Inc, Lake Forest, Ill.

i.

Flunixin Meglumine, Schering Plough Animal Health, Summit, NJ.

j.

SMZ-TMP, Amneal Pharmaceuticals, Hauppage, NY.

k.

Fluconazole, Teva Pharmaceuticals, Sellersville, Pa.

l.

Gastrogard, Merial LLC, Duluth, Ga.

m.

Misoprostol, Greenstone LLC, Peapack, NJ.

n.

Ranitidine, Amneal Pharmaceuticals, Hauppage, NY.

o.

Sucralfate, sanofi-aventisus US LLC, Kansas City, Mo.

p.

Vicryl, Ethicon, Somerville, NJ.

q.

Eagle Labs, Rancho Cucamonga, Calif.

r.

AK Dilate, Akorn, Lake Forest, Ill.

s.

Equioxx, Merial LLC, Duluth, Ga.

t.

Minocycline, Teva Pharmaceuticals, Sellersville, Pa.

References

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  • Figure 1—

    Photograph of the left eye of a 2-year-old Morgan mare evaluated because of a corneal ulceration. A—A large, irregular corneal ulceration with edema and malacia is present in the dorsomedial region of the cornea, with blood vessels extending from the dorsal and medial aspects of the limbus. Intraocular inflammation and hypopyon are evident. B—Enlarged image of the outlined region in Panel A. Two hairs can be seen emanating from the leading edge of the nictitating membrane (arrows).

  • Figure 2—

    Photograph of the nictitating membrane of the left eye of the horse in Figure 1 at the time of surgery. Four aberrant hairs (arrows) are seen extending from the leading edge of the left nictitating membrane.

  • Figure 3—

    Photomicrographs of conjunctival tissue obtained from the leading edge of the nictitating membrane. A—An ectopic hair follicle (asterisk) with surrounding sebaceous glandular tissue (dagger) can be seen within the submucosa. B—A hair shaft (arrow) is present within an ectopic hair follicle located on the leading edge of the nictitating membrane. H&E stain; bar = 60 μm.

  • Figure 4—

    Postoperative clinical photograph obtained 7 days after surgery. The graft and sutures were in place at the dorsomedial aspect of the cornea. Corneal neovascularization was still present dorsally. There was moderate bruising in the center of the graft, and the edges remained well vascularized. Mild corneal edema was still visible lateral to the graft site. The intraocular inflammation had largely resolved, and the pupil was fully dilated. The nictitating membrane had healed with no visible hairs extending from its leading edge.

  • 1. Dyce KM, Sack WO, Wensing CJG. The sense organs. In: Dyce KM, Sack WO, Wensing CJG, eds. Textbook of veterinary anatomy. 3rd ed. Philadelphia: Saunders, 2002; 323346.

    • Search Google Scholar
    • Export Citation
  • 2. Murphy CJ, Pollock RVH. The eye. In: Evans HE, ed. Miller's anatomy of the dog. 3rd ed. Philadelphia: Saunders, 1993; 10091057.

  • 3. Al-Ramadan SY, Ali AM. Morphologic studies on the third eyelid and its related structures in the one-humped camel (Camelus dromedaries). J Vet Anat 2012; 5: 7181.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 4. Samuelson DA. Ophthalmic anatomy. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary ophthalmology. 5th ed. Ames, Iowa: John Wiley & Sons Inc, 2013; 39170.

    • Search Google Scholar
    • Export Citation
  • 5. Cook CS. Ocular embryology and congenital malformations. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary ophthalmology. 5th ed. Ames, Iowa: John Wiley & Sons Inc, 2013; 338.

    • Search Google Scholar
    • Export Citation
  • 6. Ledbetter EC, Gilger BC. Diseases of the canine cornea and sclera. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary ophthalmology. 5th ed. Ames, Iowa: John Wiley & Sons Inc, 2013; 9761049.

    • Search Google Scholar
    • Export Citation
  • 7. Latimer CA, Wyman M. Neonatal ophthalmology. Vet Clin North Am Equine Pract 1985; 1: 235259.

  • 8. Maggs DJ. Cornea and sclera. In: Maggs DJ, Miller PE, Ofri R, eds. Slatter's fundamentals of veterinary ophthalmology. 4th ed. St Louis: Elsevier, 2008; 175202.

    • Search Google Scholar
    • Export Citation
  • 9. Grahn BH, Peiffer RL Jr. Veterinary ophthalmic pathology. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary ophthalmology. 5th ed. Ames, Iowa: John Wiley & Sons Inc, 2013; 435523.

    • Search Google Scholar
    • Export Citation
  • 10. Greenberg SM, Plummer CE, Brooks DE, et al. Third eyelid dermoid in a horse. Vet Ophthalmol 2012; 15: 351354.

  • 11. McLaughlin SA, Brightman AH. Bilateral ocular dermoids in a colt. Equine Pract 1983;5 (4): 1014.

  • 12. Andrew SE, Willis AM. Diseases of the cornea and sclera. In: Gilger BC, ed. Equine ophthalmology. St Louis: Elsevier, 2005; 157251.

    • Search Google Scholar
    • Export Citation
  • 13. Wilkinson JD. Distichiasis in the horse treated by partial tarsal plate excision. Vet Rec 1974; 94: 128129.

  • 14. Hurn S, Turner A, McCowan C. Ectopic cilium in seven horses. Vet Ophthalmol 2005; 8: 199202.

  • 15. Maggs DJ. Conjunctiva. In: Maggs DJ, Miller PE, Ofri R, eds. Slatter's fundamentals of veterinary ophthalmology. 4th ed. St Louis: Elsevier, 2008; 135150.

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