Heredopathia atactica polyneuritiformis (Refsum's diseases). An audiological examination of two patients

Auditory Profile of Children With Some Rare Neurodevelopmental Disorders

Neurodevelopmental disorder is an umbrella term comprising many muscular, skeletal, metabolic, endocrinal, systemic, and immune-related diseases, which are caused due to the improper/inaccurate development of the central nervous system. Most of these disorders are highly prevalent, but some express rarely in human beings. Such disorders with least prevalence rates are known as rare neurodevelopmental disorders. The sensory system is affected in all individuals with these rare neurodevelopmental disorders, although to a varying extent. Sensory processing in terms of hearing loss is reported by many researchers in many rare neurodevelopmental disorders, but the pathophysiology of audiological findings are seldom investigated. In this chapter, the authors highlight the possible relationship between underlying cause and the resultant audiological symptoms in some of the rare neurodevelopmental disorders. Further, the research studies on the audiological profiling in such disorders are discussed.

Audiological findings in Infantile Refsum disease

Audiological manifestations in a four-year-old child with Infantile Refsum disease are reported. He was born to non-consanguineous parents and had normal birth history and mildly delayed milestones prior to presentation. Clinical features were characterized by neuroregression, retinitis pigmentosa, hearing loss, peripheral neuropathy and white matter signal changes on magnetic resonance imaging. Biochemical evaluation showed elevated serum levels of long chain fatty acid and phytanic acid confirming the diagnosis. The audiological profile was characterized by absent auditory brainstem responses with robust otocoustic emissions, which indicated auditory neuropathy as the possible cause of hearing loss.

Hearing loss in adult Refsum's disease

Refsum's disease is characterized by defective peroxisomal alpha oxidation of phytanic acid, with clinical features that include retinitis pigmentosa, polyneuropathy, anosmia and hearing loss. Although hearing loss in Refsum's disease is common, there are few detailed assessments of the site of the abnormality. We examined the audiometric findings in patients with biochemically diagnosed Refsum's disease in order to assess the site of origin of the hearing loss. We found hearing loss, ranging from mild, predominantly high frequency to moderate degree, in seven out of nine patients with biochemically diagnosed adult Refsum's disease. In addition, we found evidence to suggest subtle auditory nerve involvement in six out of the seven patients with hearing loss and in one out of the two patients with a normal pure tone audiogram, on the basis of the ABR test results. We conclude that patients with Refsum's disease who report hearing difficulties should have full audiometric investigations in order to provide appropriate audiological rehabilitation.

The site of the hearing loss in Refsum's disease

Refsum's disease is a disorder of lipid metabolism with pigmentary retinopathy, demyelinating neuropathy, ataxia, and hearing loss. Previous histological studies have located the site of hearing impairment in the inner ear, but it has never been confirmed audiologicaly in the literature. In this reported case of Refsum's disease, despite hearing loss and absence of response in ABR, robust otoacoustic emissions were measured. Together with these and other audiological findings, we conclude that our case might be the first reported case of Refsum's disease with auditory neuropathy. The site of the hearing abnormality in Refsum's disease may be 'post-outer hair cells' in some cases as in the current case. Because of their limited benefits and risk of noise-induced damage to outer hair cells, the use of hearing aids before otoacoustic emission measurements should be considered cautiously in Refsum's disease.