Introduction

• Cause: hereditary and congenital disorder in the Collie breeds, in particular the Shetland Sheepdog and Rough and Smooth Collie. The characteristic feature is choroidal hypoplasia which may be accompanied by other abnormalities including optic nerve or peripapillary colobomata, retinal detachment and intra-ocular hemorrhage .
• Signs: on a spectrum from no perceivable visual deficits to complete blindness dependent on severity of lesions.
• Diagnosis: the key feature of choroidal hypoplasia is diagnosed on ophthalmoscopy. A 'pale patch' is present temporal to the optic nerve head where abnormal choroidal vessels are seen against a white scleral background. Diagnosis is challenging in dogs with subalbinotic fundi and the choroidal abnormalities can be masked by postnatal tapetal development and pigmentation in some individuals.
• Treatment: none available.
• Prognosis: if choroidal hypoplasia alone is present then the prognosis is good and signs are rarely progressive. Dogs with colobomata are at risk of retinal detachment and intraocular hemorrhage which tend to occur in the first few years of life.

Pathogenesis

Etiology

• The mutation responsible for choroidal hypoplasia is inherited as an autosomal recessive trait and a DNA test is available Inherited diseases: genetic tests. The genetic basis of coloboma has not been identified.

Predisposing factors

General

• Breed.

Pathophysiology

• Faulty obliteration of optic vesicle and abnormal differentiation of retinal pigment epithelium leading to abnormal development of choroidal and scleral tissues.

Masking of choroidal hypoplasia

• Melanin pigmentation of the RPE or postnatal tapetal development may mask small areas of choroidal hypoplasia in approximately 30% of affected puppies by 16 weeks of age ('go normal' phenomenon).
• It is therefore advised that puppies of affected breeds are screened at 6-7 weeks of age.
• Choroidal hypoplasia is also difficult to diagnose in dogs with subalbinotic fundi (ie those with merle coats).

Retinal non-attachment

• A small percentage of animals will present with retinal non-attachment at birth.

Retinal detachment and intra-ocular hemorrhage

• Significant colobomatous defects may cause retinal detachment within the first few years of life.
• Although colobomas are thought to be involved in the process of neuro-retinal detachment (RD), the mechanism by which this occurs is not fully understood. Liquified vitreous may accumulate in the subretinal space and/or cause retinal tears which lead to RD.
• Intra-ocular hemorrhage, seen in approximately 1% of affected dogs, may be the result of:
  • Rupture of retinal vessels during retinal detachment.
  • Rupture of unsupported vessels which develop at the equatorial parts of the globe in response to retinal hypoxia caused by the choroidal hypoxia.
  • Persistence of hyaloid vasculature.
  • Rupture of unsupported vasculature within papillary colobomata.
  • Neovascularization of retinal vessels.

Timecourse

• Choroidal hypoplasia may be masked by 12-16 weeks of age.
• Retinal detachment may occur within the first few years of life.

Diagnosis

Treatment

Prevention

Outcomes