Refsum disease

Phytanic acid comes from the diet, particularly beef and dairy products. Normally it is broken down by alpha-oxidation inside peroxisomes. In Refsum disease an enzyme from the PHYH gene is missing, or in a small minority a protein from PEX7 that helps import enzymes into peroxisomes is affected. Phytanic acid then accumulates in tissues. It is toxic to cells, although it is not clear exactly how an excess affects vision and smell and causes the other features.

The prevalence is unknown but the condition is thought to be uncommon, so the diagnosis can be reached late, and several specialties, including eye, neurology and metabolic teams, may be involved.

Loss of night vision from retinitis pigmentosa is often the first sign and may be noticed in childhood. Over years the disease narrows peripheral vision and may eventually lead to blindness. Loss of smell is present in almost everyone. Other features vary and may appear later: about one third are born with bone differences of the hands and feet; later there can be progressive muscle weakness and wasting, poor balance and coordination, hearing loss, dry scaly skin (ichthyosis) and a degenerative nerve disease. Spinal fluid protein is often raised.

An abnormal heart rhythm and related heart problems can develop and may be life-threatening, so cardiac review is part of care.

• Eyes: night blindness, then tunnel vision.

• Nerves: numbness, weakness and unsteadiness.

• Heart: rhythm problems that need monitoring.

A doctor suspects Refsum disease when retinitis pigmentosa, loss of smell and a neuropathy occur together. A raised level of phytanic acid in blood or urine is diagnostic when it fits the symptoms, and genetic testing for PHYH and PEX7 confirms the type. Ophthalmology tests such as visual fields and electroretinography follow the retina over time, and nerve conduction studies and hearing tests assess other involvement.

Metabolic Support UK describes treatment as a strict diet low in phytanic acid and high in calories. The foods that supply phytanic acid include dairy products, beef, lamb and some seafoods, so a specialist metabolic dietitian designs an eating plan that avoids them while still meeting energy and nutrient needs.

Removal and reinfusion of blood plasma, called plasmapheresis, may also be required for those whose levels are very high or who become acutely unwell. Other care is symptomatic and supportive: low-vision services, hearing aids, physiotherapy and foot care, skin care for ichthyosis, and heart rhythm monitoring.

Because this is an adult-onset or adolescent-onset condition, care often moves between paediatric and adult metabolic services and several hospital departments, so a named coordinator is helpful.

Refsum disease is autosomal recessive. If both parents are carriers, each child has a one in four chance of the disorder and a one in two chance of being a carrier. Rarely, carriers have mild symptoms. Metabolic Support UK notes that if a family has already had an affected child, a later baby's newborn screening test should be carried out within 24 to 48 hours of birth. A genetic counsellor can discuss testing for relatives.

The aim of treatment is to limit further build-up of phytanic acid and manage the effects already present. Lifelong follow-up with ophthalmology, neurology, cardiology and dietetic teams is recommended.

Call 999 for chest pain, fainting or a racing or very irregular heartbeat. Seek urgent medical advice for rapidly worsening weakness, difficulty walking or a sudden loss of vision, and for any illness that stops you eating, so your team can advise.