What does an air‑bone gap greater than 15 decibels at 500, 1000, and 2000 Hz indicate?

Air-Bone Gap Greater Than 15 dB at 500,1000, and 2000 Hz

An air-bone gap (ABG) greater than 15 dB at 500 Hz, 1000 Hz, and 2000 Hz indicates conductive or mixed hearing loss and requires immediate evaluation to differentiate between middle ear pathology and inner ear disorders that can mimic conductive loss.

What This Finding Means

An ABG occurs when bone conduction thresholds are better (lower) than air conduction thresholds by more than 15 dB. This pattern traditionally suggests:

• Conductive hearing loss: Sound transmission is impaired through the outer or middle ear
• Mixed hearing loss: Both conductive and sensorineural components are present
• Asymmetric hearing loss: When the ABG differs between ears by >15 dB at these frequencies 1

Clinical Significance by Frequency

The specific frequencies where you see the ABG matter:

• Low frequencies (500-1000 Hz): ABGs at these frequencies are most commonly associated with middle ear pathology like otitis media with effusion (OME), tympanic membrane perforation, or ossicular chain problems 2, 3
• Average ABG at 500-2000 Hz: In children, a pure tone average >15 dB at these three frequencies indicates hearing loss requiring referral to audiology 2

Critical Differential Diagnosis

Common Middle Ear Causes (Most Likely)

• Otitis media with effusion (average hearing loss 28 dB, with 20% exceeding 35 dB) 4
• Tympanic membrane perforation
• Ossicular chain disruption or fixation
• Chronic suppurative otitis media

Inner Ear Disorders That Mimic Conductive Loss (Must Not Miss)

Several inner ear conditions can present with ABGs and must be ruled out to avoid unnecessary middle ear surgery 5:

• Superior semicircular canal dehiscence (SSCD): Produces ABGs when dehiscence >3 mm, with larger dehiscences correlating with larger ABGs (R² = 0.828) 6. The ABG results from a "third window" effect that shunts air-conducted sound away from the cochlea while improving bone conduction 7
• Large vestibular aqueduct syndrome (LVAS): Shows mean ABGs of 51 dB at 250 Hz, 31 dB at 500 Hz, and 12 dB at 1000 Hz 8
• Endolymphatic hydrops/Ménière's disease: Typically presents with low-to-mid frequency sensorineural hearing loss but can show conductive components 1

Immediate Diagnostic Workup Required

1. Comprehensive Audiologic Evaluation

You need more than just pure tone thresholds 9:

• Air and bone conduction thresholds at 250-8000 Hz (bone conduction at 250-4000 Hz)
• Speech recognition threshold (SRT) and word recognition scores (WRS)
• Tympanometry to assess middle ear function
• Acoustic reflex testing at 500-4000 Hz

2. Key Clinical Red Flags

Look specifically for symptoms suggesting inner ear pathology rather than middle ear disease 5:

• Vertigo, oscillopsia, or dizziness
• Pulsatile tinnitus
• Autophony (hearing one's own voice loudly)
• Tullio's phenomenon (vertigo with loud sounds)
• Hennebert's sign (vertigo with pressure changes)

3. Imaging Indications

Order high-resolution temporal bone CT when 1:

• ABG present with normal tympanometry and acoustic reflexes
• Vestibular symptoms accompany the ABG
• Asymmetric sensorineural hearing loss (>15 dB difference between ears)
• WRS worse than expected for the pure tone average

Management Algorithm

Step 1: Perform otoscopy and tympanometry

• If abnormal → Treat middle ear pathology
• If normal → Proceed to Step 2

Step 2: Check acoustic reflexes and obtain comprehensive audiometry

• Reflexes present + normal tympanometry with ABG → Suspect inner ear "third window" disorder
• Reflexes absent + flat tympanogram → Middle ear effusion likely

Step 3: For persistent ABG with normal middle ear findings

• Order temporal bone CT to evaluate for SSCD, LVAS, or other bony abnormalities
• Consider vestibular testing (VEMP, caloric testing)

Step 4: Refer to otolaryngology when:

• ABG persists >3 months despite treatment 4
• Inner ear pathology suspected
• Hearing loss affects speech/language development (children)
• Surgical intervention being considered

Common Pitfalls to Avoid

1. Assuming all ABGs are middle ear disease: Inner ear disorders like SSCD and LVAS can produce significant ABGs that will not improve with middle ear surgery 8, 5

2. Operating without proper workup: Exploratory tympanotomy in a patient with LVAS or SSCD will be negative and won't help the patient 8

3. Ignoring unilateral findings: Asymmetric ABGs (>15 dB difference between ears) warrant imaging to rule out retrocochlear pathology 1

4. Delaying treatment in children: ABGs in children with OME averaging >15 dB can impair language acquisition and academic performance, particularly when sustained over time 2, 4

5. False ABGs at high frequencies: Above 2000 Hz, ABGs can occur in normal listeners due to incorrect reference threshold levels, not true pathology 10