A discussion about the effect of HNPP on hearing arose in a group forum, and the possibility of damage to the auditory nerve. Some members have said they are suffering from auditory-related issues including tinnitus and hearing loss, which could be seen as a potential manifestation of this inherited condition, but does the research back the claims?
A survey conducted by one of the groups in December 2016 showed that 41 people out of 73 individuals with HNPP suffered from tinnitus, a term for hearing sounds that come from inside your body, rather than from an outside source. This can include ringing in the ears, or even music and singing. It could be purely coincidental, or related to another underlying issue.
So what do the academics say?
How HNPP could be connected to hearing issues
In 2005, Wim Verhagen along with five other researchers from the Department of Neurology, Canisius-Wilhelmina Hospital, Nijmegen, in the Netherlands, held a study measuring sensorineural hearing impairment in participants with HNPP and hereditary motor and sensory neuropathy (HMSN-1a). A sensorineural hearing loss is damage to the hair cells in the cochlea (sensory hearing organ) or damage to the hearing nerve.
“The progressive SNHI in HNPP might be explained by the liability for exogenous factors associated with this disorder.”
Sensorineural Hearing Impairment in Patients with PMP22 Duplication, Deletion, and Frameshift Mutations – Verhagen WI et al, 2005
Sixteen patients with HNPP and another eleven with a frame shift mutation of the PMP22 gene within the same condition were given hearing tests via an audiogram. The authors found that those with HNPP had hearing regression beyond presbycusis – hearing loss that is commonly associated with natural ageing.
They state: “Patients with HNPP showed postnatal onset at age 11 years with progression of SNHI [sensorineural hearing impairment] in excess of presbyacusis by 0.4 dB per year.” The researchers explain that after a certain age, the degree of hearing was recorded to have decreased more than the average person without this type of neuropathy.
They add: “The differences in SNHI may be explained by the differences in PMP22 expression. The progressive SNHI in HNPP might be explained by the liability for exogenous factors associated with this disorder.”
A 2015 case report produced by Martin Gencik and Josef Finsterer from the Human Genetic Laboratory in Vienna, Austria, presented a 37-year-old man with HNPP who had quite serious hearing loss after a fracture in the skull due to a gunshot behind the right ear, as well as a whole host of other issues. Without a question, it is obvious that a gunshot wound would have a significant impact on a person’s functionality, with or without HNPP.
“Since hypoacusis is a frequent phenotypic feature of HNPP, the phenotype suggested hereditary neuropathy rather than any of the other possible differential diagnoses.”
Hereditary Neuropathy with Liability to Pressure Palsies Masked by Previous Gunshots and Tuberculosis – Martin Gencik and Josef Finsterer, 2015
The authors however, attribute the man’s left-side hypoacusis – a medical term which simply means hearing impairment, a partial or total inability to hear – to HNPP stating: “Although it was initially attributed to the trauma from the gunshot, there was no explanation for hypoacusis on the left side.”
Using the 2005 study as the basis of the diagnosis, the authors go on to say: “Since hypoacusis is a frequent phenotypic feature of HNPP, the phenotype suggested hereditary neuropathy rather than any of the other possible differential diagnoses.”
The authors importantly add that a link to HNPP and cerebral cavernoma i.e. collections of small blood vessels (capillaries) in the brain that are enlarged and irregular in structure, has not been established thus the report is purely speculative.
In a German translated study, researchers at the Paediatric Centre, Olga Hospital, Stuttgart, reported that a six-year-old boy with HNPP was suffering from progressive deafness. The translated report states: “Searching for the reason of the deafness we found mild peripheral neuropathy with reduced motor nerve conduction velocity. The MR of the brain showed demyelinated lesions and CSF [cerebrospinal fluid] protein was elevated. Biopsy of the suralis nerve demonstrated thickenings of the nerve, called tomaculae, which are typical for HNPP.”
As a result, they made the assumption: “Because there were no signs of any other disease, we assume that PMP22 has also influence of central myelination or the described chromosomal deletion is responsible for the expression of a other unknown protein with a central function.”
As well-intentioned the authors may be, the report is still highly questionable and much of the diagnoses is based on conjecture.
How HNPP could not be linked to hearing issues
In a more recent report for the Acta Neurologica Scandinavica Journal, the opposite seem to have been revealed.
Researchers from the University Hospital of Leicester, UK, tested eight patients with HNPP, among 23 others with other “demyelinating neuropathies”. A demyelinating neuropathy is any condition that results in damage to the protective covering (myelin sheath) that surrounds nerve fibres. When the myelin sheath is damaged, nerve impulses slow or even stop, causing neurological or neuropathic problems.
The participants were examined on their brain stem auditory and visual responses, focusing more on the central nervous system rather than the peripheral nervous system, which is where most HNPP symptoms derive from.
“Peripheral auditory nerves may be spared in HNPP possibly due to absence of local compression.”
Optic and Auditory Pathway Dysfunction in Demyelinating Neuropathies – Knopp M et al, 2014
According to the research, there were more HNPP participants suffering from optical issues rather than peripheral auditory nerve-related problems. And brain stem involvement was “exceptional” in all groups.
The authors state that lack of hearing issues within HNPP could be down to the fact that compression is required for nerves to become damaged, adding: “Peripheral auditory nerves may be spared in HNPP possibly due to absence of local compression. Evidence for central brainstem pathology appeared infrequent in all four studied neuropathies.
“This study suggests that acquired and genetic demyelinating polyneuropathies may be associated with optic and auditory nerve involvement, which may contribute to neurological disability, and require greater awareness.”
As a fellow HNPP sufferer says the study “poses more questions than it answers” given that they provide opposing evidence to the 2005 report.
There have been frequent reports of bilateral sensorineural hearing loss in patients with various types of peripheral neuropathies including hereditary motor and sensory neuropathy [1], and hereditary sensory and autonomic neuropathy [2]. However, research is mostly based on Charcot Marie-Tooth disorder, and more widely diagnosed inherited neuropathies as well as those suffering from multimorbidities.
And while there may be a fair few HNPP sufferers with complaints of hearing loss, it’s still very unclear there is a direct correlation.
- 1. Musiek et al., 1982; Raglan et al., 1987; Perez et al., 1988
- 2. Denny-Brown, 1951; Hallpike et al., 1980; Wright and Dyck, 1995
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