IMAGE OF THE MONTH: June 2023
Just another case of bilateral macular scars
A 5-year-old male presents with poor vision in both eyes. Visual acuity is 20/150 in the right eye and 20/400 in the left eye, which progressively declines to 20/400 and hand motions, respectively over the next five years. Fundus photos (A and B) show macular pathology, while wide-field fundus photography and autoflourescence imaging (C and D) are available for the right eye. On examination, similar findings are present in the left eye.
Diagnosis
Cobalamin C deficiency
Background: Cobalamin C (cblC) deficiency (OMIM #277400) is the most common error in intracellular vitamin B12 metabolism. CblC causes a wide range of severe ocular manifestations and systemic issues. Infants usually present early with macular and retinal degeneration, with profound vision loss. Systemic findings include failure to thrive, feeding issues, and microencphaly. This is an autosomal recessive disease caused by a mutation in the MMACHC gene on chromosome 1. When the MMACHC gene does not function correctly, the MMACHC protein that it codes for does not work properly, resulting in elevated concentrations of methylmalonic acid and homocysteine, while decreasing levels of methionine. If an error in B12 metabolism is suspected, blood tests can be used to evaluate if total plasma methylmalonic acid and homocysteine are elevated, and genetic testing can confirm the diagnosis.
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