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Homocystinuria, MTHFR Deficiency
This is the second most common human disorder caused by a genetic error in amino acid metabolism. It is caused by mutations in one of two genes and several subtypes with variable clinical features have been described.
The most common eye problem in homocystinuria is dislocation of the lens. It is found in 90% of individuals with homocystinuria and in the vast majority it is present by the age of 20 years. Together with other ocular abnormalities such as retinal detachments and damage to the optic nerve, this often results in significant loss of vision.
The clinical range of signs and symptoms is wide, from severe infant disease to adults with no symptoms. Newborns may present as ‘floppy’ babies with muscle weakness, difficulty breathing, failure to thrive, and seizures. Approximately half of patients have significant mental deficits. Long, thin fingers, tall stature, and a high arched palate are seen in some individuals and the hair is often lightly pigmented. Patients with homocystinuria have a considerable risk for blood clots that can lead to heart attacks and strokes. Osteoporosis is often present in older individuals. General anesthesia may increase this risk but this can be reduced with meticulous fluid and hydration management.
This is an autosomal recessive disorder that requires the presence of two mutations, one inherited from each normal parent.
Diagnosis is often made from mandatory newborn screening programs since few signs and symptoms are present at birth. Special diets and supplements including restriction of protein and especially the amino acid methionine have been reported to prevent mental retardation and reduce lens dislocations. Administration of betaine, folic acid and cobalamin has been shown to prevent the neurological complications but treatment must be started before brain damage occurs. Surgical removal of dislocated lenses may be necessary to prevent or treat a special form of glaucoma known as pupillary block glaucoma. It is also sometimes required when the dislocation creates vision problems. Even so, the majority of patients have some degree of vision loss. The prognosis for health and longevity is dependent upon the impact of blood clots should they occur.