The human middle ear transmits sound from the external to the inner ear and includes structures such as the tympanic cavity, eustachian tube, aditus, attic, antrum, and mastoid air cells ¹. These components work together to ensure effective sound conduction and pressure regulation ^{2 1}. The eustachian tube also helps ventilate the middle ear and clear secretions ³. Understanding the volume of the middle ear is important, especially in cases of tympanic membrane (TM) perforation, as it can influence hearing loss and clinical outcomes ^{4, 5}..

Mastoid pneumatization, the formation of air cells in the mastoid bone, is important for sound transmission. The mastoid antrum, a large and consistent mastoid air cell, communicates with the attic of the middle ear through the aditus³. Mastoid pneumatization commences from 33rd week before birth, and develops until the age of 15years for male and 10years for female ⁵. In a typical adult, the mastoid has a volume of about 2 milliliters ³. The mastoid antrum connects to the middle ear via the aditus. Well-pneumatized mastoids, with many thin septa, enhance sound conduction, while sclerotic mastoids, lacking air cells, impair sound conduction ^{1, 2}.. The volume and status of mastoid air cells significantly influence hearing efficiency and are key factors in assessing and managing hearing loss ².

Research shows that middle ear volume is a key factor in hearing loss, particularly in cases of tympanic membrane (TM) perforation. Studies across different populations (Asia and America) have found that smaller middle ear spaces correlate with larger air-bone gaps and greater conductive hearing loss ^{5, 6}. Mehta et al., Park et al., and Ahn et al. all support this link, emphasizing that reduced mastoid air cell volume or sclerotic mastoids worsen hearing ^{4 5, 7}.

Middle ear (ME) volume has been identified as a key factor in determining the degree of hearing loss in ears with tympanic membrane (TM) perforations. Smaller ME volumes are associated with larger air-bone gaps, and identical perforations can result in hearing losses that differ by up to 35 dB if ME volumes vary significantly ^{4, 7, 8}.

The degree of mastoid pneumatization varies widely among individuals. Normal mastoid volumes range from 2 to 20 ml ⁵. A Korean study reported an average mastoid pneumatization volume of 7,095.2 mm³ with a large standard deviation (3,087.1 mm³), indicating significant individual variation ⁵. Similarly, a Norwegian study found the average middle ear volume to be 6.5 ± 3.7 ml ⁶. These differences may result from postnatal pathological changes, such as otitis media and eustachian tube dysfunction ⁵.

Mehta et al. (2006) using tympanometry found that ears with ME volumes ≤4.3 ml had significantly greater conductive hearing loss than those with volumes >4.3 ml⁴. Park et al. (2015) found an inverse correlation between hearing loss and the combined volume of the middle ear and mastoid ⁵. Using temporal bone CT scans, they measured these volumes and grouped patients into four categories based on volume size, (group A: 1–2,400 mm3; group B: 2,401–4,800 mm3; group C: 4,801– 7,200 mm3; group D: >7,200 mm3)⁵. Importantly, their analysis showed that middle ear and mastoid volume was a more significant predictor of hearing loss than the size of tympanic membrane perforation ⁵.

Ahn et al. compared middle ear (ME) volume measurements using tympanometry and CT scans in patients with unilateral chronic otitis media. ME volumes measured by CT in normal ears were significantly larger than in lesioned ears. In ears with tympanic membrane (TM) perforation, tympanometry showed a strong linear correlation with CT measurements ⁷. Manjuran & Bahuleyan assessed mastoid air space volume via digital X-ray, grouping patients into sclerotic and cellular mastoids. They found that mastoid air space volume is inversely proportional to conductive hearing loss ².

There is a lack of research in Port Harcourt, Nigeria, on how middle ear volume affects hearing loss in patients with tympanic membrane (TM) perforation. While previous studies have explored the size and site of TM perforations, the impact of middle ear volume, including mastoid pneumatization, remains under-investigated in this population. This study aims to address that gap by evaluating how different patterns of middle ear pneumatization (sclerotic vs. well-pneumatized) influence hearing outcomes in Nigerian patients. Understanding the inverse relationship between mastoid volume and conductive hearing loss could improve diagnostic accuracy. The study will assess the correlation between middle ear volume and hearing loss in patients with TM perforation. This research will contribute valuable knowledge to the clinical understanding of middle ear anatomy’s role in hearing loss and help shape future practices and research in the region.

Materials And MethoDS

Study Area: This study was conducted at the University of Port Harcourt Teaching Hospital (UPTH), Rivers State, Nigeria, a reference hospital with a bed capacity of 800. The ENT clinic operates four days a week, serving approximately 4,200 patients annually.

Study Design: A hospital-based cross-sectional study was employed, focusing on patients with unilateral tympanic membrane (TM) perforation attending the ENT clinic.

Study Population: The study included adult patients ≥18 years with unilateral TM perforation, while the contralateral non-perforated ear served as the control.

Inclusion Criteria

• Adult patients ≥18 years.
• Patients with unilateral or multiple TM perforations who gave consent.

Exclusion Criteria

• Bilateral TM perforation.
• Previous ear surgery.
• History of ototoxicity, diabetes mellitus, sickle cell anemia, or HIV infections.
• Severe external auditory canal stenosis.
• All patient > 60 years

Study Duration: The study spanned from August 1, 2018, to July 31, 2019.

Scope of Study: The study assessed the degree, pattern, and type of hearing loss associated with TM perforations, alongside middle ear volume. Assessment methods included otoscopy, video otoscopy, pure tone audiometry, and tympanometry.

Sample Size Determination: Using Cochran’s formula, the minimum sample size was calculated to be 61, with a 10% allowance for non-response, resulting in a final sample size of 70 patients (140 ears).

Materials

• Otoscope: Heine mini 3000 otoscope, Germany.
• Audiometer: Madsen Itera model, Denmark.
• Tympanometer: Madsen Zodiac 901, Denmark.
• Pure Tone Audiometry Equipment: Soundproof booth, Kahntact BB 100 model, USA.
• Video Otoscopy: MD Scope, Apple Biomedical, USA.
• Data Analysis Software: Image J version 1.51p (Wayne Rasband, NIH, USA).

Sampling Method: A cluster sampling technique was employed, selecting two clinic days (Monday to Thursday) randomly. All patients with unilateral TM perforation on those days were included in the study.

Data Collection

A structured, interviewer-administered questionnaire was used to gather demographic and clinical data from each participant. The questionnaire included questions on the patient's age, gender, and medical history, with particular focus on ear-related symptoms, such as pain, discharge, and hearing loss. It also addressed potential causes of tympanic membrane perforation, such as trauma, ear infections, or ototoxicity due to medication. Further details were collected regarding the patients’ medical history, including conditions like diabetes, sickle cell anemia, and prior ear surgeries. Information on any history of ototoxicity or retroviral infections was also documented. Following the interview, patients underwent pure tone audiometry (PTA), which was performed using a calibrated Madsen Itera audiometer inside a soundproof booth. The PTA evaluated both air and bone conduction thresholds at frequencies of 250, 500, 1000, 2000, 4000, and 8000 Hz to assess the type and degree of hearing loss ^{9, 10}. Air conduction testing was done using supra-aural earphones, while bone conduction testing was performed using a bone oscillator placed over the mastoid bone ¹⁰. Air conduction masking was performed when the air conduction threshold in the perforated ear was 40 dB greater than the bone conduction threshold in the normal contralateral ear, using the plateau seeking method. Bone conduction masking was done for all patients at all frequencies. Pure-tone averages were calculated for air and bone conduction at 500, 1000, 2000, and 4000 Hz ^{10, 11}. An air-bone gap greater than 10 dB in these frequencies was considered significant for diagnosing conductive hearing loss ¹⁰. Sensorineural hearing loss was defined as air and bone conduction averages within 10 dB, with thresholds above 25 dB HL ¹⁰. Mixed hearing loss was defined as a gap greater than 10 dB between air and bone conduction averages, with thresholds above 25 dB HL at any frequency ¹⁰. Results was used to grade hearing threshold into mild, moderate, severe and profound hearing according to WHO ¹².

A tympanometric assessment of the middle ear volume (MEV) was conducted for both ears of each patient. A 226Hz frequency tympanometer probe with a clean tip of appropriate size and shape was used to ensure an airtight seal in the ear canal ^{7, 13}. The probe was directed towards the tympanic membrane for accurate measurements. For patients with a perforated tympanic membrane (TM), both the middle ear volume and external auditory canal (EAC) volume were measured, and a type B tympanogram (flat) was expected .^{4, 7}. In contrast, for non-perforated ears, only the EAC volume was recorded.

For patients with unilateral TM perforation, the MEV was calculated by subtracting the EAC volume of the contralateral normal ear from the total estimated MEV of the perforated ear ^{4, 5, 7}. Studies have shown no significant difference between the right and left EAC volumes in individuals ⁷.

The MEV was then categorized into four groups based on Park et al.'s classification: Group A (1–2,400 mm³), Group B (2,401–4,800 mm³), Group C (4,801–7,200 mm³), and Group D (>7,200 mm³) ⁵. The maximum volume measurable with the Madsen Zodiac 901 tympanometer was 9.99 mL.

These assessments provided critical data for analyzing the relationship between tympanic membrane perforations and hearing impairment in the study population..

Data Analysis

Data were analyzed using SPSS version 20. Descriptive statistics (mean, standard deviation) were used for numerical data, while categorical data were analyzed using frequencies and proportions. Paired t-tests, Chi-square tests, Pearson’s correlation, and linear regression analyses were used to assess the correlation between TM perforation size, site, and hearing loss. A p-value of ≤ 0.05 was considered statistically significant.

Ethical Considerations

Ethical approval was obtained from the Research and Ethics Committee of UPTH. Informed consent was obtained from all participants, with full disclosure of the study procedure.

Results

Table 1: Summary measures of external ear volume (control ear) and middle ear volume (perforated ear)

SD – Standard deviation                   

Figure 1. Distribution of middle ear volume categories

Based on the classification by Mehta et al., the figure  2 below Small (≤ 4.3mls) represented 52.9% and large (>4.3mls) representing 47.1% of the patient recruited for this study.

Figure 2. Distribution of middle ear volume in affected ears

As shown in figure 3, there is an inverse correlation between middle ear volume and air conduction PTA values. This is reflected in the p-value of 0.001 being significant and the Pearson’s correlation r = 0.452 showing a near moderate correlation.

Figure 3. Correlation between middle ear volume and air conduction hearing threshold

On the contrary the figure 4 below, shows no correlation between middle ear volume and bone conduction values as noticed. The p-value is not significant 0.600 and the Pearson’s correlation also showing no correlation with values of r = -0.0041

Figure 4. Correlation between middle ear volume and bone conduction hearing threshold

Table 2: Correlations between middle ear volume and patient’s characteristics (age, BMI, duration of symptoms and height)

It was noticed that 8000Hz was the most affected frequency with a mean air conduction value of 46.81dB followed by 250Hz. The least affected frequency was 1000Hz with 34.31dB as seen in  figure 5 below.

Figure 5: Mean values across PTA frequencies (Air Conduction)

Figure 6: Mean values across PTA frequencies (Bone Conduction)

Table 3: Comparison of hearing threshold (dB) between low and high PTA frequencies in the perforated ear

SD – Standard deviation                    *Paired t-test                        **Statistically significant

Table 5: Multiple linear regression analysis of factors associated with hearing loss (PTA air conduction)