What are the causes of conductive hearing loss with a normal type‑A tympanogram?

Conductive Hearing Loss with Normal Tympanometry: Causes and Clinical Approach

The most important causes of conductive hearing loss with a normal type-A tympanogram are ossicular chain abnormalities (particularly otosclerosis and ossicular fixation), tympanic membrane perforations with patent ventilation tubes, and inner ear malformations such as enlarged vestibular aqueduct syndrome. 1, 2

Understanding the Clinical Paradox

A normal type-A tympanogram indicates normal tympanic membrane mobility and normal middle ear pressure, yet conductive hearing loss is present. This apparent contradiction occurs because:

• Tympanometry only measures tympanic membrane compliance and middle ear pressure, not the integrity of the entire sound transmission pathway 1
• Ossicular chain pathology distal to the tympanic membrane can exist without affecting membrane mobility 3
• Inner ear malformations can paradoxically present as "conductive" hearing loss on audiometry despite an intact middle ear 2

Primary Etiologies by Mechanism

Ossicular Chain Abnormalities (Most Common)

Otosclerosis and stapes fixation are the leading causes in adults with normal tympanic membranes and normal tympanometry 4, 3:

• The stapes becomes fixed at the oval window, preventing normal sound transmission
• The tympanic membrane remains mobile, yielding a normal type-A tympanogram
• Presents with progressive conductive hearing loss, often bilateral

Malleus-incus fixation or ossicular discontinuity also produces this pattern 4:

• Fixation of the malleus head or incus body prevents ossicular chain movement
• A loose incus-stapes joint creates discontinuity without affecting tympanic membrane mobility
• These conditions were detected in 26% of conductive hearing loss cases in one study 4

Tympanic Membrane Pathology with Altered Ear Canal Volume

Tympanic membrane perforation or patent ventilation tube creates a unique tympanometric pattern 5:

• Produces a type-B tympanogram (flat) but with high ear canal volume
• The high volume distinguishes this from middle ear effusion (which shows normal ear canal volume)
• This is a critical pitfall: the tympanogram is abnormal (type-B), but the high volume indicates the middle ear is air-filled

Inner Ear Malformations (Emerging Recognition)

Enlarged vestibular aqueduct syndrome can present exclusively as conductive hearing loss with completely normal tympanic membranes and middle ear structures 2:

• High-resolution temporal bone CT reveals the enlarged vestibular aqueduct
• The air-bone gap results from inner ear malformation, not middle ear pathology
• This prevents unnecessary middle ear exploration

Other inner ear malformations including cochlear dysplasias can produce similar patterns 2, 3

Diagnostic Algorithm

Step 1: Confirm the Tympanometry Result

• Verify that ear canal volume is normal (not high or low) 5
• Low volume suggests cerumen impaction or probe malposition 5
• High volume indicates perforation or patent tube, which is actually a type-B pattern 5

Step 2: Perform Detailed Otoscopy

• Pneumatic otoscopy to assess tympanic membrane mobility directly 6
• Look for subtle perforations, retraction pockets, or tympanosclerosis 1
• Examine for middle ear masses visible through the membrane 4

Step 3: Obtain High-Resolution Temporal Bone CT

This is the definitive diagnostic step for conductive hearing loss with truly normal tympanometry 7, 2:

• Identifies otosclerosis (lucency around oval window)
• Detects ossicular fixation or discontinuity
• Reveals inner ear malformations (enlarged vestibular aqueduct, cochlear dysplasia)
• Distinguishes chronic inflammatory changes from structural abnormalities

Step 4: Consider Specialized Testing

• Video pneumatic otoscopy with deep learning analysis showed 94.1% accuracy in detecting ossicular pathology in research settings 4
• Tuning fork tests (Weber and Rinne) confirm conductive pattern 7

Critical Pitfalls to Avoid

Do not assume normal tympanometry excludes significant middle ear pathology 1, 3:

• Ossicular chain problems distal to the tympanic membrane are invisible to tympanometry
• The test only measures the first step in sound transmission

Do not overlook inner ear causes of apparent conductive hearing loss 2:

• Enlarged vestibular aqueduct and other inner ear malformations can mimic middle ear pathology
• Temporal bone CT is essential before considering exploratory tympanotomy

Distinguish true type-A tympanograms from type-B with high ear canal volume 5:

• Both can present with conductive hearing loss
• High ear canal volume indicates perforation, not normal middle ear function
• This distinction changes management completely

Age-Specific Considerations

In children, congenital ossicular malformations and syndromes affecting first and second pharyngeal arch derivatives are more common 8:

• External auditory canal malformations are invariably associated with middle ear and ossicular abnormalities
• Isolated ossicular malformations are uncommon but possible

In adults, otosclerosis dominates as the primary cause, particularly in women of childbearing age 3