There are two subtypes of Galactosemia – Classic Galactosemia and GALK Deficiency. Each of these types is caused by a different enzyme not working properly or missing in some amounts in the body. In healthy individuals, these enzymes are involved in breaking down galactose.

Classic Galactosemia and GALK Deficiency can greatly affect children’s development potential and quality of life, and may cause motor, speech and cognitive deficiencies, cataracts, tremors, seizures and (in females) primary ovarian insufficiency.

Classic Galactosemia is the most common subtype of the disorder. Patients with Classic Galactosemia have mutations or deletions in both copies (one from each parent) of their GALT genes, which means that the GALT enzyme is completely inactive. Patients with GALK Deficiency have mutations or deletions in the gene that codes for the GALK enzyme. GALT and GALK enzymes are necessary to process galactose. Without a place to go, excess galactose builds up. The enzyme Aldose Reductase, not normally involved in galactose metabolism, is activated to convert galactose into galactitol, an abnormal and toxic substance that is believed to cause the acute and long-term complications of Galactosemia. Galactitol is not produced or found in individuals who do not have Galactosemia.