Refsum disease
Overview
Refsum disease is an extremely rare and complex disorder that affects many parts of the body. A form of the retinal degenerative disease known as retinitis pigmentosa (RP) is a common feature of this disease.Individuals with Refsum disease are unable to metabolize phytanic acid. As a result, high levels of phytanic acid accumulate in blood and tissue. It is thought that disorders associated with this disease result from harmful levels of phytanic acid. Phytanic acid is found in meat, fish, and dairy products. The body also converts phytol, a substance found in green leafy vegetables, to phytanic acid.
Causes
Refsum's disease is caused by faulty enzymes during the alpha-oxidation of phytanic acid resulting in buildup of phytanic acid and its unsaturated fatty acid derivatives in the plasma and tissues. This in turn can be due to deficiencies of phytanoyl-CoA hydroxylase (chromosome 10) or peroxin-7 (chromosome 6).
Diagnosis
Nerve conduction studies are abnormal, with slowing of conduction velocities. CSF protein levels are usually elevated. The electroretinogram may be grossly abnormal. Nerve biopsies from affected patients have shown ‘onion bulb’ formation, and targetoid inclusions have been described in Schwann cells which have a similar appearance on electron microscopy to those seen in cultured fibroblasts. Plasma levels of phytanic acid measured by gas chromatography-mass spectroscopy are consistently elevated (normal range 800 mmol/l are not uncommon at presentation.
Prognosis
ARD is the most treatable of the leukodystrophies because phytanic acid is not produced by the body, but is only found in foods. With treatment, muscle weakness, numbness, and dry and scaly skin generally disappear. However, vision and hearing problems may persist and the sense of smell may not return. Untreated, ARD can lead to death caused by heartbeat abnormalities.
Treatment
The primary treatment for ARD is to restrict or avoid foods that contain phytanic acid, including dairy products; beef and lamb; and fatty fish such as tuna, cod, and haddock. Some individuals may also require plasma exchange (plasmapheresis) in which blood is drawn, filtered, and reinfused back into the body, to control the buildup of phytanic acid.