Diabetes mellitus is considered by many as the “disease of the twentieth century,” affecting more than 450 million people worldwide, with far-reaching consequences that span age, sex, creed, socioeconomic class, and geographical location [1],[2]. It is a global pandemic that requires definitive action to control and prevent long-standing complications. Type 2 diabetes accounts for more than 90% of the disease burden [3].

More commonly than not, long-standing poorly controlled diabetes mellitus results in complications that affect major organs, including the brain, eyes, heart, kidneys, and nerves. One complication of diabetes mellitus that is seldom mentioned is sensorineural hearing loss. Diabetes and its associated complications have a debilitating effect on the health and quality of life of affected individuals [4],[5],[6],[7]; however, adding the loss of sense of hearing makes the situation much worse. The relationship between diabetes and sensorineural hearing loss has been an area of interest since it was reported in 1857 by Jordao [8]. Sensorineural hearing loss is caused by damage to sensory cells or nerve fibers (or both) of the inner ear. Sensorineural hearing loss in diabetics is also characterized by gradual, progressive, and bilateral onset, affecting the highest frequencies [9],[10],[11]. Some authors have reported hearing loss of a more sudden onset and can be unilateral [12]. The higher prevalence of sensorineural hearing loss in diabetics is well documented over the past decades [13],[14],[15],[16] and in Nigeria few studies have reported on this disease [17],[18], although a direct causal relationship has not yet been established and some researchers have even found no significant difference in hearing loss between diabetics and non-diabetic controls [19],[20],[21]. It is also known that the duration of the disease and the level of control can affect the development of sensorineural hearing loss.

Long-term diabetics tend to have a more profound degree of hearing impairment than newly diagnosed individuals [22],[23].

Several theories for the mechanisms of sensorineural hearing loss in diabetics have been proposed: diabetic microangiopathy, diabetic neuropathy, and a mixture of both. Chronic hyperglycemia tends to damage the cochlea, which is highly microvascular and sensitive to injury.

There is endothelial proliferation, accumulation of glycoproteins in the intima, and thickness of the basal membrane of the stria vascularis, the main arterial supply to the cochlea [24],[25]. Microangiopathy markers such as elevated serum creatinine levels, microalbuminuria, and higher levels of stromal cell-derived factor 1a (SDF-1a) have been found in diabetics with higher hearing threshold levels [26],[27],[28]. On the other hand, some researchers have found atrophy of spiral ganglion neurons and demyelination of the vestibulocochlear nerve [25], rather than a vasculopathy as the vascular changes are argued to be nonspecific [29].

Auditory brainstem response is an objective measurement of auditory pathway function from the auditory nerve to the mesencephalon and tests synchronous neural function and can estimate hearing sensitivity thresholds. Pure-tone audiometry is a behavioral test used to measure hearing sensitivity. Puretone thresholds (PTTs) indicate the softest sound audible to an individual at least 50% of the time.

Pure tone audiometry is most commonly used in the assessment of hearing acuity [10],[11],[30]. However, other more specific tests such as otoacoustic emission (OAE) and auditory brainstem response audiometry have been used [10],[31].

This is a hospital-based study that used specific tests to evaluate hearing loss among type 2 diabetics in Nigeria. The results will be useful for making comparisons and spurring action toward recognition of hearing loss as a complication of diabetes and to promote early referral to the otolaryngologist.

The objective of this study is to assess hearing loss among type 2 diabetics diagnosed in the endocrinology unit of a private tertiary health facility in southwest Nigeria.

The study was carried out in the Otolaryngology and Endocrinology units of Babcock University Teaching Hospital, a 140-bed tertiary health institution located in the western part of Nigeria.

Inclusion criteria: All type 2 diabetic patients attending the endocrinology clinic and normal healthy individuals who gave their informed consent.

Exclusion criteria: History of previous ear infection, meningitis, exposure to ototoxic drugs, prolonged exposure to noise, previous ear surgery, and family history of deafness.

Adult diabetics who met the inclusion criteria had their age and sex matched with healthy controls.

The minimum sample size was determined to be 120.

One hundred and twenty participants were recruited and assigned to two groups, each consisting of 60 members. The control group (Group A) consisted of normal and healthy individuals matched for age and sex, and the diabetic group (Group B) consisted of people diagnosed with type 2 diabetes mellitus.

An interviewer-administered questionnaire was designed to obtain sociodemographic information from each participant, along with their hearing history, duration and treatment of diabetes, presence of complications, and history of smoking. A detailed ear examination was performed on all participants. Samples were obtained for the measurement of baseline fasting blood glucose, glycated hemoglobin (HBA1c).

Otoacoustic emission using a handheld interacoustics OAE device and auditory steady-state response audiometry (ASSR) using the interacoustic ABR/ASSR device were carried out in all participants.

Otoacoustic emission tests the integrity of the outer hair cells of the cochlear while ASSR tests the retrocochlear hearing loss and also gives the hearing threshold of the patient. These tests are objective (not dependent on patients response).

World Health Organization hearing loss grading was used to determine the degree of hearing loss in the study participants.

The approval for the study was obtained from the Babcock University Health Research Ethics Committee. Written informed consent was obtained from each participant and their personal details were left anonymous. Data obtained from this study were entered into the Statistical Package for Social Sciences (SPSS) version 21 program and analyzed.

Sociodemographic characteristics

The sociodemographic result revealed that the mean age and standard deviation of the case and control groups was 6.05+1.13 and 5.98±1.33, respectively. The result also revealed that a total of 30 (50.0%) males and 30 (50.0%) females participated in the case group, while 28 (46.7%) males and 32 (53.3%) female participated in the control group.

Plasma glucose level

Plasma glucose level among the case group revealed that the majority of 44 (73.3%) of the participants had a high level of plasma glucose while on 16 (26.6%) had a normal level of plasma glucose. In terms of the control group, the data revealed that 55% (33) of the participants had a normal plasma glucose level, while 45% (27) of the participants had a high plasma glucose level.

HB1c assessment revealed that for the case group, 22 (36.7%) participants have normal while 38 (63.3%) participants have high HB1c. But for the control group, the data revealed that all participants, 60 (100%), have normal HB1c (Table 1 and Table 2).

Otoacoustic emission revealed that the case group has a pass in the right ear and pass in the left ear of 61.7% respectively, refer in the right ear and refer in the left ear of 21.7% respectively, pass in the right ear and refer in the left ear of 8.3% respectively, refer in the right ear and pass in the left ear of 8.3% respectively. The following results were revealed for the control group; pass in right ear and pass in left ear of 76.7%, refer in right ear refer in left ear of 21.7%, and pass in right ear refer in left ear of 1.7%.

ASSR; ASSR-audiometry; and ASSR-audiometry Rt an Lt for both groups

Auditory steady-state response audiometry for the two ear (right and left) for both the Case and Control groups revealed that those in the case group with mild mild audiometry are higher at 63.3%, those with moderate moderate were 16.7%, those with mild moderate were 10%, while those with moderately severe moderately severe were 5%. Furthermore, data from the control group revealed that those with mild audiometry for both ears were also higher at 51.7%, those with mild moderate were 5%, those with normal normal were 20%, those with normal mild were 13.3%, while those with mild moderately severe were 5% (Table 3 and Table 4).

Diabetes mellitus is known to affect the nervous system; sensory, motor, and autonomic divisions, including the auditory pathway [31]. Jardao was the first to report hearing loss in a diabetic patient [32].

The pattern of hearing loss in diabetics is commonly bilateral, progressive sensorineural hearing loss to all frequencies of sound, but particularly high frequency sounds [31],[33].

Diabetes mellitus has been isolated as an independent etiology of hearing loss [34].

Hearing loss in diabetics as mentioned above could be bilateral and progressive sensorineural could be erroneously diagnosed as presbycusis in elderly patients with diabetes. However, hearing loss is more severe than is usually observed for that age [8]. Some studies also listed diabetes as a known cause of sudden unilateral hearing loss [12].

The incidence of diabetes continues to increase exponentially across the globe [31],[35],[37]and a plethora of individuals are progressing toward undiagnosed diabetes and unawareness of complications [31].

Many theories have been postulated as the etiology of hearing loss in diabetes, the pathophysiological pathways include microangiopathy, deposition if advanced glycosylated end products and oxidative stress all culminating in metabolic dysfunction ultimately affecting auditory function and hearing loss [11],[14],[36],[37].

Due to the burden of diabetes and its strong association with hearing loss and its negative synergistic impact on quality of life, it is imperative to increase screening programs to curb the growing tide of undiagnosed and untreated diabetes and hearing loss as it’s complication [37].

In the screening for hearing loss, 3 assessment tools are usually used. Pure tone audiometry used to determine a hearing threshold, i.e., the softest sound audible to the person being studied. Otoacoustic emissions (OAE) used to evaluate the status of the cochlea, particularly the outer hair cell, and lastly, auditory brain stem response audiometry is a neurological test used to determine the retro-cochlear part of the auditory pathway, up to the brain stem [12]. However, in our study auditory steady-state response audiometry (ASSR) was used in place of pure tone audiometry as many studies have classified ASSR to be on par or superior to PTA [38],[39],[40].

The majority of participants in the test group and control group are in the age range of 40–49 years (Table 5), accounting for 33.5% and 25%, respectively. Similar findings were observed in a study with a demographic close to ours [10],[41].

Our study has an equal number of sexes in the case group and more females in the control group (Figure 1). There is conflicting information on the correlation between sex and hearing loss in diabetics. A report does not show a relationship between hearing loss and sex [33], some reported hearing loss to be more common in diabetic women compared to their male counterparts [41], while some reported hearing loss to be more common in male diabetics [8]. However, our study didn’t find any correlation between both variables.

Threshold frequencies are known to be significantly higher in individuals who have had DM for more than five years [34], another study showed no correlation between them [10].

The fasting plasma glucose level was deranged in 73.3% and 45% of the case and control groups, respectively. Sixty-three percent (63%) of the case group had abnormal glycated hemoglobin (Table 1). Studies show that there is no correlation between fasting plasma glucose and hearing loss [10]; however, persistently high or deranged plasma glucose (glycated hemoglobin) is associated with sensorineural hearing loss in diabetic patients [42]. Our study showed no correlation between hearing loss and glycated hemoglobin levels.

The otoacoustic emissions performed showed that at least 38.3% and 23.4% of the participants had a referral in at least one ear for the case and control groups, respectively, revealing that hearing loss is more common in people with diabetes. However, the analysis did not show a significant correlation between hearing loss and diabetes.

In conclusion, hearing loss is an established complication of type 2 diabetes, proactive measures should be taken to make early diagnosis, start treatment, and regular screening to prevent hearing loss in patients with type 2 diabetes.