Brain, Vol. 123, No. 1, 82-92,
January 2000
© 2000 Oxford University Press
The spectrum of hearing loss due to mitochondrial DNA defects
1 Departments of Neurology, 2 Physiological Sciences and 3 Radiology, The University of Newcastle upon Tyne and 4 Department of Medical Physics, Freeman Hospital, Newcastle-upon-Tyne, UK
Correspondence to:
Dr P. F. Chinnery, Department of Neurology, The Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH, UK E-mail: P.F.Chinnery{at}ncl.ac.uk
Heteroplasmic mitochondrial DNA (mtDNA) defects are an important cause of neurological disease. Although hearing impairment is common in patients with mtDNA defects, the spectrum and pathophysiology of the hearing loss is not well characterized. We therefore studied the relationship between cochlear and brainstem auditory function in 23 patients harbouring a range of different mtDNA mutations. Based upon the pure tone audiogram, patients fell into three distinct groups: (i) normal hearing, (ii) mild to moderate predominantly high frequency hearing loss, and (iii) severe or profound hearing loss at all frequencies. Within this study group only certain genetic defects were associated with hearing loss, and for individuals harbouring the A3243G point mutation, the severity of the hearing loss correlated with the percentage level of mutated mtDNA (mutation load) in skeletal muscle. The 10 patients who had a moderate hearing loss or less had normal brainstem auditory evoked responses and MRI, but it was not possible to interpret the brainstem auditory evoked responses in 13 patients with severe hearing loss. Otoacoustic emissions were absent in patients with a moderate or more severe hearing loss. These findings are consistent with a predominantly cochlear origin for the hearing deficit, which is determined by the precise genetic defect and the percentage mutation load.
mitochondrial encephalomyopathies; mtDNA mutation; mitochondrial hearing loss; genetic hearing loss
FSE = fast spin echo; MELAS = mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes; mtDNA = mitochondrial DNA; NARP = neurogenic weakness with ataxia and retinitis pigmentosa; PCR = polymerase chain reaction
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