See also: Congenital hearing loss
Timothy C. Hain, MD of Chicago Dizziness and Hearing.
Last updated: December 28, 2014
This topic was reviewed in 2002 (Edmonds et al, 2002). Mitochondrial disorders usually first manifest in tissues with high metabolic demands such as nerve and muscle. Similarly the complete auditory pathway is at risk from mitochondrial disorders. Hearing loss is common in mitochondrial disorders including MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke like episodes), Kearns-Sayre syndrome (KSS) and MERRF (myoclonic epilepsy with ragged red fibers). Others include complex I deficiency, cytochrome-c oxidase (complex IV) deficiency or COX, pyruvate dehydrogenase deficiency (PDH). These disorders are caused by mutations in mitochondrial DNA, and are characterized by muscular weakness, an abnormal muscle biopsy with "ragged red" fibers, and a lactic acidosis. Edmonds et al found that 80% of persons with severe mitochondrial disease had hearing deficits. They suggested that these patients are more vulnerable than others to bacterial infection and should be managed more aggressively than the general population. This conclusion must be looked at with caution as some antibiotics may have their site of action on mitochondria, which resemble bacteria in many ways.
In MELAS, Sue et al recently reported that the hearing loss is caused by cochlear damage. It resembles presbyacusis in that it is generally symmetrical, gradual, and affects the higher frequencies first (Sue et al, 1998). Edmonds et al (2002) suggested that lesions might also occur elsewhere in the auditory pathway.
Others have also reported hearing loss associated with mitochondrial mutations (Yamasoba et al, 1999; Tsutsumi et al, 2001). Mitochondrial DNA mutations accumulate naturally during life and are presently implicated as an important cause of normal aging.
Patients with Kearns-Sayre syndrome were found to have a significantly prolonged I-V ABR latency in one study (Nakamura et al, 1995).
Mitochondrial defects including a deletion in A1555G have been reported to both cause unusual sensitivity to aminoglycosides as well as nonsyndromic sensorineural deafness (El-Schahawi et al, 1997 -- this paper reviews "mitochondrial deafness). These patients have a mild high-frequency sensorineural hearing loss without aminoglycoside exposure. Some indicate that as much as 15% of persons with aminoglycoside ototoxicity have this mutation. However, we are dubious that this is the case.
Mohr-Tranebjaerg syndrome (DFN-1) is also thought to cause deafness via a mitochondrial disturbance.
An update on current locii can be found on the hereditary hearing loss homepage, which is hosted by the University of Iowa.