A 52-Year-Old Woman With Night Vision Loss: Osmosis USMLE Study Question of the Week

March 23, 2018

Answer: D. Retinitis pigmentosa

This patient's presentation with a loss of night vision, cerebellar ataxia, and steatorrhea in addition to the presence of acanthocytes ("thorny" red blood cells) and low serum triglycerides/cholesterol is suggestive of abetalipoproteinemia. Abetalipoproteinemia is an autosomal recessive disease characterized by low or absent serum apoB-100 and apoB-48.

This deficiency results in an absent synthesis and exportation of chylomicrons and very low-density lipoproteins (VLDL). Low-chylomicron concentrations cause fat to accumulate in the intestinal enterocytes, explaining the malabsorption of fat. Additionally, fat-soluble vitamins (A, D, E, and K) and essential fatty acids are also not well absorbed, resulting in fat-soluble vitamin deficiency (ie, loss of night vision, conjunctival xerosis, loss of visual acuity). If left untreated, patients classically progress to develop retinitis pigmentosa (progressive retinal degeneration).

The peripheral blood smear revealed acanthocytes, which are erythrocytes that have spiked cell membrane and abnormal thorny projections. Red blood cells are more susceptible to osmotic lysis due to the lack of antioxidant activity of vitamin E. Cerebellar ataxia results from damage to the dorsal column of the spinal cord, which interfere proprioception and vibration (ie, positive Romberg test result, wide-based gait). Laboratory studies from these patients result in low triglycerides and low cholesterol.

Major Takeaway: Retinitis pigmentosa can occur due to impaired vitamin E absorption, which results from the patient suffering from abetalipoproteinemia. This condition results in an absent synthesis and exportation of chylomicrons and VLDL, causing fat-soluble vitamins to have impaired absorption.

For more on retinitis pigmentosa, read here.

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