OM
Ocular Melanosis
Testing method
Slit-lamp biomicroscopy before pupil dilation, with particular attention to the iris root for thickening and the ventral sclera for pigment plaques. Following dilation, the remainder of the eye is examined in the usual way. Examination must be performed using slit-lamp biomicroscopy - standard indirect ophthalmoscopy alone is insufficient for early detection.
What is ocular melanosis?
Ocular melanosis is an inherited condition in which abnormal pigmented cells accumulate progressively within the structures of the eye. It results in thickening and pigmentation of the iris root, release of pigmented material into the aqueous humour, and deposition of pigment in the sclera and episclera — the white outer coat of the eye. As the condition advances, pigment may also accumulate in the drainage angle of the eye and in deeper structures including the retina and optic nerve.
The accumulation of pigment in the drainage angle can obstruct the outflow of aqueous fluid, leading to a rise in intraocular pressure and secondary glaucoma. This is the most serious complication of ocular melanosis and can cause pain and irreversible vision loss if not detected and managed promptly.
New to the BVA scheme from January 2026
Ocular melanosis has been added to the Known Inherited Ocular Disease (KIOD) schedule for Cairn Terriers from 1 January 2026. Cairn Terriers presented for eye examination under the scheme from that date are certified as clinically affected or clinically unaffected for ocular melanosis. This is a new addition to the scheme and reflects growing evidence of the inherited nature and clinical significance of the condition in the breed.
Staging
Ocular melanosis progresses through four stages, as described by Petersen-Jones:
Stage 1 involves a characteristic dark-coloured, donut-shaped thickening of the iris root. Scleral pigment deposits are not present at this stage, and intraocular pressures are normal.
Stage 2 is characterised by the presence of iris root thickening as in Stage 1, together with small spicule-shaped pigment plaques in the ventral sclera and episclera. In some dogs, gonioscopy reveals pigment deposits on the ventral pectinate fibres.
Stage 3 involves more extensive scleral and episcleral pigmentation, with patches typically several millimetres in size. The iris may have a lumpy appearance and the pupillary zone may appear thinned. Pigmented particles may be seen in the aqueous humour. Uveitis and episodes of raised intraocular pressure may be present.
Stage 4 is characterised by the development of glaucoma with further pigment deposition, globe enlargement, lens subluxation, optic disc cupping, vision loss, and in severe cases scleral staphyloma or phthisis bulbi.
Breeds affected
Ocular melanosis is currently certified under the BVA scheme in the Cairn Terrier only.
What the examination involves
Examination for ocular melanosis requires slit-lamp biomicroscopy and must be performed before pupil dilation. This is different from the routine eye examination sequence, and breeders should mention at booking that their Cairn Terrier requires ocular melanosis screening so that the correct examination protocol is followed.
The panellist examines the iris root carefully for thickening and the ventral sclera for pigment plaques. Following this assessment, the pupil is dilated and the remainder of the eye examined in the usual way.
Given the variable age of onset, annual examination of Cairn Terriers used for breeding is recommended. The certificate itself is valid for one year.