Hereditary metabolic diseases usually have a progressive course, and produce various
complications if not diagnosed and treated at an early age Early and correct diagnosis is therefore important for patients who suffer from these disorders. Metabolic diseases often manifest themselves in tissues of the eye, some of which may cause abnormalities of the crystalline lens. These lens abnormalities may include alterations in transparency, position, and shape, and may subsequently result in alterations of vision, refraction, accommodation, intraocular pressure, etc. Mild abnormalities of the crystalline lens may reflect senous metabolic illness at an early age. In clinical practice, however, the aetiology of a lens abnormality, e.g., cataract, is often unknown, incorrect or incomplete. Menn and Crawford established a causative factor in 68% of 386 cases of congenital cataract. Others reporting smaller senes set the figure at 25% to 50%.
Metabolic diseases may be undiagnosed, misdiagnosed, or diagnosed at a late age when the
patient has irreversible damage. A number of metabolic diseases may be diagnosed from
routine screening programs. However, in the majority of metabolic diseases, an early diagnosis
is only possible if sufficiently characteristic clinical signs and symptoms have alerted the
clinician to the possibility of a metabolic disease Furthermore, a biochemical diagnosis is only
possible if appropriate biochemical examinations have been performed to establish the
diagnosis.
The crystalline lens has three properties that makes it an ideal tool to trace a hereditary
metabolic disease at a young age:
1. The crystalline lens appears unusually vulnerable to a variety of genetic and metabolic
defects. There have been many published reports of clinical lens abnormalities whose metabolic
bases have been demonstrated and of others whose metabolic bases, although not
demonstrated, are likely.
2. The lens is an exceptional organ in that the cells formed during its development are
normally not destroyed or replaced The earliest cells are in the centre, and the fibres formed
later are superimposed The time at which the different layers form is known from histological
and slit-lamp examinations Therefore, any opacity occurring within the lens from the earliest
weeks of the embryo to the last days of post-natal life can be localised in time.
3. Abnormal microscopic sections of the lens are visible "in vivo" with slit-lamp
examination as part of standard ophthalmologic examination in everyday practice.