|
The clinical manifestations common to all forms of albinism pathognomonic for the diagnosis are: discolouration of the skin and hair and eye abnormalities. Anomalies of the eye and the visual system are specific and essential for the diagnosis of definition of albinism. These abnormalities are common in all types of albinism and appear to be related to the reduction of melanin during the embryonic development and postnatal life.
The reduction of melanin gives rise to a clear iris that transmits light through the iris tissue to the transillumination of the ocular globe.
In albinism, the melanin in the retinal pigment epithelium is greatly reduced or absent, making the retina transparent; as a result, the choroid vessels are visible beneath the retina at the ophthalmoscope.
The most distinct difference of the optical system in albinism is an abnormal decussation of the optic fibers in the chiasma.
In a mature eye pigmented during the development the nerve fibers of the nasal retina cross the chiasma and end in the contralateral geniculate nucleus, while the fibers of the temporal retina do not cross the chiasma and end in the ipsilateral geniculate nucleus.
In a human being the ratio between contralateral fibers (crossed) and ipsilateral fibers (not crossed) is about 55:45. This allows the overlap of the visual fields, bilateral visual impulses to the cortex in both eyes and stereoscopic vision.
This ratio is significantly altered in albinism. In all mammals with albinism the proportion of crossed fibers probably exceeds 90%.
The deviation of the optical fibers at the chasm can be clinically detected by recording the unilateral visual evoked responses. Nystagmus is present in all patients. The mechanism of nystagmus has not been fully understood.
|
