Impaired development of the pigmentation of the iris can lead to a congenital difference in coloration between the left and right iris (heterochromia). One iris containing varying pigmentation is referred to as iris bicolor. Iso-lated heterochromia is not necessarily clinically significant (simple hetero-chromia), yet it can be a sign of abnormal changes. The following types are differentiated:
❖ Fuchs’ heterochromic cyclitis (etiology unclear): This refers torecurrentiridocyclitis (simultaneous inflammation of several portions of the uvealtract) in adults, with precipitates on the posterior surface of the cornea without formation of posterior synechiae (adhesions between the iris and lens). The eye is free of external irritation. This disorder is often associated with complicated cataract and increased intraocular pressure (glaucoma).
❖ Sympathetic heterochromia: Inunilateral impairment of the sympatheticnerve supply, the affected iris is significantly lighter. Heterochromia withunilaterally lighter pigmentation of the iris also occurs in iridocyclitis, acute glaucoma, and anterior chamber hemorrhage (hyphema).
❖ Melanosis of the iris: This refers todark pigmentationof one iris.
Aside from the difference in coloration between the two irises, neither sym-pathetic heterochromia nor melanosis leads to further symptoms. The only form of heterochromia that leads to abnormal changes is Fuchs’ hetero-chromic cyclitis. The possible complications involved require specific treat-ment.
Albinism (from the Latin albus = white) is a congenital metabolic diseasethat leads to hypopigmentation of the eye. The following types are differen-tiated:
❖ocular albinism (involving only the eyes) and
❖oculocutaneous albinism (involving the eyes, skin, and hair).
In albinism the iris is light blue because of the melanin deficiency resulting from impaired melanin synthesis. Under slit-lamp retroillumination, the irisappears reddish due to fundus reflex (Fig. 8.6). Ophthalmoscopy will detect choroidal vessels (Fig. 8.7). Associated foveal aplasia results in significant reduction in visual acuity and nystagmus. Most patients are also photophobic because of the missing filter function of the pigmented layer of the iris.