Absolute Bone Conduction (ABC) Test and Schwabach Test

Absolute Bone Conduction (ABC) Test

• Bone conduction bypasses the external and middle ear and directly stimulates the cochlea.
• It therefore reflects the sensorineural reserve (cochlear + nerve function).
• A reduced ABC indicates cochlear or retrocochlear (sensorineural) pathology.
• A normal or enhanced ABC in the presence of hearing loss points to a conductive lesion (outer/middle ear problem).

• A tuning fork (typically 512 Hz) is struck and its base placed firmly on the mastoid process.
• The patient is asked to report when they can no longer hear the tone.
• The duration or loudness perceived is noted.

Schwabach Test

• The tuning fork is set into motion and its stem placed alternately on the mastoid process of the patient and the examiner.
• When the patient no longer hears the sound, the examiner listens to the fork to check if the tone is still audible to them.
• This alternation continues until a comparison is established.

Comparing Both Tests in Context of Hearing Loss

Why Conductive Loss Prolongs Bone Conduction?

Clinical Significance

• The Schwabach test is particularly useful for differentiating conductive from sensorineural hearing loss using only a tuning fork.
• It is not commonly used in modern practice because formal audiometry provides more precise, quantifiable results. However, it remains a valid bedside/clinical assessment tool.
• A classic teaching scenario: a patient with otosclerosis will show prolonged Schwabach + negative Rinne + Weber lateralizing to the affected ear.

Clinical Case Write-Up

Case Presentation

1. Right ear discharge - 8 months duration
2. Gradually progressive hearing loss in the right ear - 8 months duration

History of Present Illness

• Ear pain (otalgia)
• Ear bleeding (haematotympanum)
• Tinnitus or vertigo
• Facial palsy or deviation
• Headache or neck stiffness
• Previous ear surgeries or trauma
• Aural polyp noted by the patient

Past Medical History

• No significant past medical history
• No history of chronic nasal obstruction, allergic rhinitis, or recurrent URTIs (though discharge worsens with URTIs, suggesting underlying Eustachian tube dysfunction)
• No history of diabetes mellitus, hypertension, or immunosuppression

General Examination

• Well-built, well-nourished, afebrile
• No pallor, icterus, cyanosis, clubbing, lymphadenopathy, or edema
• Vital signs within normal limits

ENT Examination

External Ear

• Pinna: normal shape, size, and position; no tenderness
• Pre- and post-auricular areas: no swelling, redness, or sinus
• External auditory canal (EAC): no signs of active discharge or stenosis

Otoscopic / Endoscopic Findings (Right Ear)

• Tympanic membrane (TM): Thickened, dull, and pearly white in appearance with loss of normal translucency
• Perforation: A central perforation located in the antero-inferior and postero-inferior quadrants (inferior pars tensa), approximately 30-40% of the total TM area - medium to large in size
• Perforation margins: Smooth, rounded, and fully epithelialized - indicating a chronic, inactive (dry), stable perforation
• Tympanosclerosis: Prominent chalky-white calcified plaque visible in the posterior quadrants of the TM (myringosclerosis) - consistent with longstanding chronic inflammation
• Middle ear: Promontory visible through the perforation; middle ear mucosa appears dry, pale, and healthy - no active granulation tissue or mucosal edema
• Malleus: Handle visible; umbo terminating near the superior-posterior edge of the perforation; mildly retracted
• No cholesteatoma: No squamous debris, keratin pearls, or retraction pockets seen
• Left ear: Normal (not shown)

Diagnosis

Basis of Diagnosis

Differential Diagnosis

1. CSOM - Atticoantral (Unsafe) Type - Excluded by: central (not marginal/attic) perforation, mucoid (not foul-smelling, scanty, non-cheesy) discharge, no evidence of cholesteatoma on endoscopy
2. Otitis Media with Effusion (Glue Ear) - Excluded by: presence of frank discharge through perforation
3. Acute on Chronic Suppurative Otitis Media - Possible during exacerbations (URTI/water exposure), but currently in dry phase
4. Tuberculous Otitis Media - Excluded by: no multiple perforations, no pale granulations, no painless discharge in atypical pattern

Investigations Planned

1. Audiometry (Pure Tone Audiogram): Expected to show conductive hearing loss in the right ear (low-frequency > high-frequency loss); assess air-bone gap
2. Tympanometry: Type B (flat) or type C curve expected due to perforation
3. Throat swab / Ear swab for C&S: During active discharge phase - to identify organisms (commonly Pseudomonas aeruginosa, Staphylococcus aureus, Proteus, E. coli)
4. High-Resolution CT Temporal Bone (HRCT): To assess:
   • Extent of middle ear disease
   • Ossicular chain integrity
   • Mastoid pneumatization
   • Any occult cholesteatoma
   • Tympanosclerotic involvement of ossicles
5. Nasal examination / PNS X-ray or CT: To evaluate for adenoid hypertrophy, DNS, or chronic rhinosinusitis contributing to Eustachian tube dysfunction

Management

Conservative (Medical) Management - Inactive Phase

• Ear toilet: aural cleaning and drying
• Strict water precautions: use of ear plugs during bathing; avoid swimming
• Treat underlying nasal/nasopharyngeal pathology (contributing to Eustachian tube dysfunction)
• Topical antibiotic ear drops (ciprofloxacin/ofloxacin) during active discharge episodes
• Systemic antibiotics during acute exacerbations

Surgical Management (Definitive)

• Myringoplasty (Type I Tympanoplasty): Repair of the central perforation using a temporalis fascia graft - indicated once the ear has been dry for at least 6-8 weeks
  • Goals: close the perforation, prevent recurrent infections, and improve hearing
  • Approach: underlay or overlay technique
• Tympanoplasty (Type II or higher): If ossicular chain involvement (erosion or fixation by tympanosclerosis) is found on intraoperative assessment
• Tympanosclerotic plaques causing ossicular fixation may require ossiculoplasty

Expected Audiological Findings

Prognosis