Diabetes mellitus and hearing
Diabetes Mellitus (DM) has been noted for its numerous effects on the body. Its effects on the heart, kidneys, eyes and other organs are well documented. However, its relationship with hearing loss has been less trumpeted.
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The discovery of indirect influence of diabetes mellitus on hearing was first reported in 1857 by Jordao who identified hearing impairment in a patient with diabetic coma.
The incidence of hearing loss among diabetics according to Axellson and Fagerberg (1968) is in the range of 10-55 per cent. Kakarlapudi et al (2003) found 13.1 per cent, Jorgen and Buch (1961) found 61 per cent of the participants having hearing loss, Diniz and Guida (2009) found significant worse hearing thresholds among patients with type II diabetes mellitus compared with the control group.
In 2014, the first study on prevalence of hearing loss among type II diabetics was conducted in Ghana by Ofori-Atta at the Korle-Bu Teaching Hospital. Participants were diabetic patients taking only oral hypoglycemics; the study found 52.9 per cent of them having hearing loss.
Type of hearing loss among type II DM patients
Kakarlapudi et al (2003) found more diabetics to have sensori-neural hearing loss (SNHL) and similar findings were reported in other studies; including Ofori-Atta (2014). Rozanka et al (2002) established that diabetics have high hearing thresholds which are bilateral, progressive and sensori-neural in nature in the high frequencies.
SNHL comprises the cochlea and/or auditory nerve (Gelfand, 2009). The cochlea is the organ of hearing and forms part of the inner ear. It measures 5 mm long and 9 mm across its base and shaped like a snail.
It has sensory hair cells (outer and inner), the outer hair cells amplify all sound signals that get into the inner ear, transfer it to inner hair cell before moving along the auditory nerve. It is reported that poorly controlled diabetes mellitus affect the micro-vessels in the inner ear, reducing blood supply to the stria-vascularis which is responsible for transporting potassium ions into the fluid surrounding the outer hair cells (endolymph).
Potassium is noted for its role in maintaining endolymphatic potential which helps in the transduction of acoustic signals. The impaired physiological activity of the stria vascularis and subsequent reduction in potassium ions reduce the amplification quality of the outer hair cells. Hearing loss as a result of disease of the cochlea is termed sensory hearing loss.
Moreover, neural hearing loss which is a component of SNHL is a disease process that affects peripheral and central auditory pathway (Gelfand, 2010). Jerger and Jerger in 1981 identified complications of DM to be:
1. Atrophy of spiral ganglion.
2. Degeneration of myelin sheath of 8th nerve.
3. Decrease in number of nerve fibres in the spiral lamina.
4. Thickening or narrowing of the capillary wall of stria-vascularis and the smaller arteries within the internal auditory canal.
Spiral ganglion is a group of nerve cells that serve the sense of hearing by sending a representation of sound from the cochlea to the brain. Atrophy (shrinking) of the spiral ganglion reduces its ability to effectively send acoustic signals, thereby predisposing diabetics to neural hearing loss.
Also, vestibulocochlear nerve (8th cranial nerve), which is highly myelinated, extends from the cochlea to the brainstem. Myelin is a mixture of proteins and phospholipids forming a whitish insulating sheath around many nerves fibres. It allows impulses to be sent faster along the auditory nerve. Demyelination of the eighth nerve (removing or reduction of myelin content) impairs or reduces the speed of sound transmission to the brain for interpretation.
It is, therefore, necessary for all diabetics both type I and type II to have proper control of their blood glucose level because Sumathi et al (2012) found diabetics with high HbA1c (glycated haemoglobin) greater than seven per cent (>seven per cent ) to have more hearing loss between 250 Hz and 4000 Hz than those with less HbA1c (<seven per cent ).
In addition, Moadab (2011) reported bilateral symmetrical SNHL in insulin -dependent diabetics. Due to significant progressive effect of diabetes on hearing, Ofori-Atta (2014) recommended that:
1. All diabetics must have hearing test annually.
2. All regional and district hospitals must have an audiologist or at least one visiting audiologists to take care of hearing health of diabetics.
3. Diabetics with sense of hearing loss must seek immediate intervention.