HHTM staff: This longer-than-expected series on leprosy nears an end with one more to follow after today’s post. Leprosy is a complicated disease, hard to identify, easy to treat, hard to pin down auditory effects prior to diagnosis and treatment. Even after the disease itself is diagnosed and hearing loss is reported, the audiologic diagnostic process is fraught.
Past posts reviewed limited available data and arrived at a few conclusions which boil down to one main one for our purposes: leprosy can cause sensorineural hearing loss, perhaps to a larger extent than is reported at present.
Site of lesion is the last point of discussion. We’ll complete the series in this and the next post by picking up where the last post left off, examining
…the scant literature on putative cochlear, peripheral and central origins of such hearing losses.
Rawlani et al performed audiometry and established a higher incidence of sensorineural hearing loss in those with leprosy. But their study did not distinguish between cochlear and neural (VIIIn) site of lesion by test results. Speech testing, acoustic reflex, ABR, or central auditory tests were not included in their Methods. Despite that, Rawlani et al. concluded that the losses they observed were “cochlear in origin” based on the following negative logic:
In the absence of any local or systemic disease or drugs likely to have side effects on the cochleovestibular function, leprosy affects the cochleovestibular system, and effect on cochlear function is seen more often than effect on the vestibular system. Thus hearing loss which is seen in patients suffering from Hansen’s disease is of cochlear origin.
In other words, since there were few if any vestibular effects in the presence of non-conductive hearing loss, then the proximal cause of the observed hearing loss must be cochlear in origin, rather than due to more central vestibulocochlear neural involvement.
Earlier work by Singh et al (1984) supported this view, stating:
The perceptive deafness and vestibular hypofunction were due to end organ lesion probably due to E.N.L. reaction. The vestibulococlear nerve involvement was considered to be unlikely.
In sum, the conclusion that sensorineural hearing losses are cochlear remains open for debate: Singh et al. did not perform quantitative testing or state a basis for their judgement. Rawlani et al. did not perform retrocochlear evaluations to test their cochlear assumption.
Despite the known effect of the leprosy bacilli on Schwann cells in peripheral nerves, the question of neural hearing loss was not addressed until 1970, when El Arini et al mounted a systematic, controlled investigation of hearing loss in 60 leprosy patients. After eliminating confounded hearing losses due to age (3 subjects) and/or ototoxicity (3 subjects with histories of streptomycin treatment), the authors concluded that 25% of study subjects had gradual onset, progressive hearing loss best attributed to “eighth nerve hearing loss…related… to [their] leprosy condition.”
Their presumption of neural origin of hearing losses was based once again on exclusionary logic. Those with hearing loss demonstrated an
“absence of recruitment or abnormal loudness adaptation…lack of diplacusis, hyperacusis or hearing distortion … hence, the possibility of eighth cranial nerve disease must be considered.“
The neural mechanism suggested by the authors was ischemia and gradual destruction of the eighth nerve via “invasion of Schwann cells wrapping the … nerve, but not the receptors.”
In sum, the conclusion that sensorineural hearing losses are due to peripheral VIIIn destruction remains open to debate.
Please tune in for the next and last post on the fascinating subject of Hansen’s disease. That post will look at ABR studies on peripheral and central auditory pathway effects in patients with leprosy and well as another rare neural disease that masquerades with similar symptoms.
All of which underscores the necessity of thorough diagnostic audiologic testing and history-taking for new patients and those returning with changes in hearing or travel patterns. Audiologists must protect their patients and themselves by assuming a diagnostic role in addition to the more lucrative role of hearing aid fitting.
Editor’s Note: Click for the beginning post in this 10-part series.
El Arini, F et al. Eighth cranial nerve affection in leprosy. Int’l J Leprosy, 1970; 38: 164.
Rawlani S et al. Evaluation of hearing impairment in leprosy patients taking multidrug therapy. Indian J Lepr, 2013; 85(4); 171-6.
Singh TR et al. Evaluation of audiovestibular status in leprosy. Indian J Lepr, 1984; 56(1): 24-9.
Feature Image courtesy of Haiku Deck