Introduction

Sound constantly surrounds us and informs us about many objects in our environment. Its localization is one of the most important biological traits in man William ₁ . Sound localization enables an animal to locate food, avoid predators, find a mate and to communicate ₁ . Sounds from various sources are combined into one complex sound field and travels to the outer ear. The pinnae helped in funneling them toward the external auditory canal from where it reaches the tympanic membrane, it is then transmitted through the middle ear either by bone conduction, air conduction or across the middle ear cavity by means of the ossicular chain on to the fluids and the structures of the cochlear in the inner ear. The intricate motions and interactions of the various parts of the cochlear lead to the generation of electric potentials. This is then transmitted centrally toward the auditory cortex. Diseases of the external ear, the middle as well as any diseases affecting the cochlear, the auditory nerve and centrally the auditory cortex can result in deafness. In the United State of America, Census data showed that 3% of the population in the work force reports some hearing loss ₂. Hearing loss was found to occur in both sexes, and it was found to be Sensorineural (SNHL) in 10 out of every 1000 children in the US. A Worldwide, SNHL occurs in 9-27 per 1000 children ₂,₃,₄ . Conductive hearing loss results from any lesion that prevents sound transmission from the outside world to the cochlea, while SNHL result from the disruptions in transmission beyond the cochlea or damage to the eighth cranial (auditory) nerve. Mixed hearing loss has components of both conductive and sensorineural loss ₄ . The objective of this study is to investigate the pattern of hearing loss, the severity (or degree), and the etiology among the first 1000 patients attending the audiology clinic for the first time at the Obafemi Awolowo University Teaching Hospital Complex Ile-Ife, Nigeria

Materials and Methods

The first 1000 patients that were seen at the audiology clinic of the OAUTHC Ile –Ife for the first time between December 2001-May 2003 were recruited for this study. Only patients who had hearing difficulty as one of the major complaints were admitted into this survey. A symptom questionnaire was administered to each patient. The questionnaire sought information about the patient's demographic data, the work experience, the history of the hearing loss and the possible aetiology, especially the history of exposure to noise, ear infections, use of ototoxic drugs. Other information that was sought in the questionnaire are the history suggestive of birth asphyxia, Neonatal jaundice or infections and history of admission into intensive neonatal care unit in children. Other areas of interest were the past medical and surgical history. The patients were then clinically examined including otological examination and had pure tone audiometry and acoustic impedance measurements.

Audiometric and acoustic impedance measurements: The diagnostic audiometer Interacoustics model AD28 was utilized. Pure tone audiometry using Hughson-Wetslake procedure was carried out on all patients at frequencies of 125, 250,500,750,1000,2000,4000,and 8000Hz. Children below the age of 4 years were screened using the Free field Screening methods. Impedance measurement was carried out using the Interacoustic Tympanometer model AT 22t. The impedance as well as the stapedial reflex was measured for all the patients at frequencies 500, 1000, 2000, 4000,Hz. These assessments were carried out in a soundproof audiology booth.

Data Analysis: The questionnaire of each patient was reviewed for the completeness of the information. The data was analyzed using SPSS version 10. The results were presented using the frequency and percentages.

Results

One thousand patients were studied.

Sex Distribution: There were 595 males and 405 females.

Age Distribution Table I: showed the age distribution of the study population.. Deafness was found to be more prevalent in the third decade of life; this was closely followed by the sixth decade. The ninth and the tenth decades of life are seen less commonly presenting with deafness.

Etiology of Hearing Loss Table II: Showed the common etiologies of deafness as seen in the study population. Sensorineural hearing loss of unknown etiology was seen in 41.2%, Infective disorders was responsible for deafness in 14.6% of the population, Deaf mutism in 9.6%, Presbyacusis in 9.0% while the least prevalence was found in patients with Sickle cell anaemia with 0.2%

Impedance Measurements in the study population Table III: Type A tympanogram was found in 58.2% of the patients, Type B was found in 36.6% Type As was found in 3.2% while Ossicular discontinuity as represented by type Ad was the least in 0.1% of the patients.

Age specific prevalence of Sensorineural hearing Loss Fig. 1: This showed the age specific prevalence of SNHL. The peak prevalence was in the third decade while the least was found in the 9 ^th and the 10 ^th decade of life.

Severity of Hearing Loss Fig. 2: This Bar chart showed that bilateral disease was common. About 50% of the patients had socially significant hearing loss requiring rehabilitation

Types of Hearing Loss Fig 3: This pie chart showed that SNHL was the most common type of hearing loss in Nigerians deaf population (51.5%), Mixed deafness was the second most common (24.6%), while conductive deafness was the least prevalent. It was found in 17.2%.

Figure 1

Table 1: Shows the age distribution of the patients presenting with hearing impairment at a Nigerian Audiology clinic.

Mean age 35.2years.
Std Dev. 22.68

Figure 2

Figure 1: This shows the age specific prevalence of SNHL in the study population.

The peak prevalence was in the first decade of life, and there is a lesser second peak prevalence in the sixth decade of life.

Mean dB Loss 63.3 56.1 St Dev. ± 28.11db

Figure 3

Figure 2: This figure shows the severity of the hearing loss in the study population. Socially significant hearing loss was occurred in about 50% of the population. Normal hearing constitute 12% of the population, majority of them had wax impaction.

Figure 4

Table 2: Etiology of Hearing loss

Figure 5

Table 3

The summary of the acoustic impedance findings according to Jergers classification.

Figure 6

Figure 3: This showed that SNHL was the most common type of hearing loss in Nigerians deaf population.

Discussion

Hearing loss is more prevalent than many congenital abnormalities and all pediatrics cancers and other numerous medical conditions ₄ combined yet clinicians pay very little attention to it.

The prevalence of hearing loss in the Nigerian community is not known and there is an incomplete knowledge of the causes and natural history of ear diseases and deafness. The prevailing level of poverty in the community and the non-availability of diagnostic facilities and hearing Aids had made management of these patients unrewarding. The age distribution of the study population showed that the third decade of life had the highest prevalence rate while the ninth and the tenth decade was found to have the lowest prevalence rate (Table I). Age specific prevalence rate of sensorineural hearing loss in this study population showed a high occurrence in the first decade of life.. High prevalence of childhood infections due to bacteria, viral, fungal, perinatal diseases and congenital malformation of the cochlea might contribute to this high prevalence. The second peak prevalence was found in adulthood. The prevalence of SNHL drops steadily by the second decade of life but rises steadily until it peaks again in the sixth decade of life Fig I. This could be due to the steady rise in the incidence of Cardiovascular, metabolic, and degenerative disorders that are common with aging ₅ . More than 80% of the children in the 1 ^st decade of life presented with deaf-mutism usually at the age of 5 ± 1.65 years. The observed causes of this were found to be birth asphyxia, neonatal sepsis, meningitis, measles and prematurity. These causes were similar to the findings by several investigators in Africa over 2 decades ago ₆,₇,₈,₉,₁₀ . These conditions are largely preventable with better Obstetric services, improved immunization and improvement in the socio-economic status of the people. Immunization with Haemophilus influenza type b (Hib) vaccine in the Gambia showed that it was effective in preventing meningitis and blood stream infections and 100% in preventing pneumonia ₁₁,₁₂ . Improvement in the socio-economic status and the living condition of the people in the then Soviet Union led to a significant fall in the prevalence of deaf-mutism ₁₃ It is hoped that an improvement in the social-economic status and living conditions of Nigerians will also witness a decline in the prevalence deaf-mutism in Nigerian children. In the adult population, some of the common identified causes of SNHL were: Presbyacusis, chronic diseases like Diabetes mellitus, vascular lesions, tumors, trauma and chronic renal failure (Table II). This pattern agrees with the existing literature ₅ .

Mixed Hearing Loss

Conductive Hearing loss

Severity of hearing loss

Tympanometry

Conclusion

It is not known whether the pattern and the causes of hearing loss observed in this study is typical of other communities in Nigeria. Therefore there is a need for further investigations. The further research should focus on detailed study of the genetic of deafness, characteristics of these ear diseases that are found to be associated with hearing loss. The natural history, associated risk factors and management. There is also the need to develop audiological medicine and to provide early screening facilities for children as part of the national deafness prevention programme in Nigeria.

Correspondence to

AO Adeniji ORL unit, Dept of surgery, Obafemi Awolowo University Ile-Ife, Osun state Nigeria. e-mail: gbolaad@yahoo.com