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Auditory neuropathy in CAPOS syndrome
18 April 2018 (online)
CAPOS (Cerebellar ataxia, Areflexia, Pes cavus, Optic atrophy and Sensorineural hearing impairment) syndrome (OMIM 601338) is a rare disorder of autosomal dominant inheritance. The remarkable clinical picture is characterized by sudden acute episodes of cerebellar ataxia triggered by febrile illness during childhood. It is caused by the missense mutation c.2452G>A, p.Glu818Lys, in the gene ATP1A3 which encodes for the neuron specific alpha subunit of Na+/K+-ATPase α3. This ion pump is highly expressed in spiral ganglion neurons and essential for maintaining a low intracellular Na+ concentration.
We show in a large multicentric study with 18 patients from 11 families that the hearing impairment in CAPOS syndrome is an auditory neuropathy. Pure tone audiometric thresholds are variable between patients and in time but tend to affect low frequencies most. While auditory brainstem responses are typically absent, otoacoustic emissions can be present at high amplitudes. In some cases, electrocochleography was performed to show the presence of cochlear microphonic potentials. The compound action potential had a very low amplitude and appeared broadened. These findings indicate that active cochlear amplification is initially normal and hearing dysfunction is primarily due to a defect of action potential generation and propagation in the auditory nerve.