Pathophysiology of Carhart Notch
The Carhart notch is an artifactual depression in bone conduction thresholds (typically 5-15 dB) caused by altered middle ear mechanics when the stapes becomes fixed, which disrupts the normal resonance characteristics of the ossicular chain and middle ear system.
Mechanism of the Phenomenon
The Carhart notch represents a mechanical artifact rather than true cochlear bone conduction loss. When the stapes footplate becomes fixed (most commonly from otosclerosis), the normal resonance pattern of the ossicular chain is disrupted. The intact ossicular chain normally contributes to bone conduction hearing through inertial effects, and when the stapes is immobilized, this contribution is lost, creating an apparent bone conduction threshold elevation.
The key evidence for this mechanism comes from the reversibility of the notch after surgical correction—studies demonstrate that bone conduction thresholds improve by a mean of 7.5-8.8 dB after stapedotomy, with the greatest improvement occurring at the notch frequency 1. This improvement in bone conduction after restoring stapes mobility confirms the notch is not true sensorineural hearing loss but rather a mechanical artifact of the fixed ossicular system.
Clinical Characteristics and Variability
Frequency Distribution
Contrary to the classic teaching that the Carhart notch occurs exclusively at 2 kHz, the notch shows significant variability in frequency location:
Importantly, a 2 kHz notch suggests stapes footplate fixation (93% in otosclerosis/tympanosclerosis), while a 1 kHz notch suggests a mobile stapes footplate (55% in chronic otitis media) 3. This distinction has practical value for predicting intraoperative findings.
Diagnostic Limitations
The Carhart notch is neither sensitive nor specific for stapes fixation. Critical findings include:
- Only 31-74% of confirmed stapes fixation cases demonstrate a Carhart notch preoperatively 4, 1
- The notch occurs with similar frequency (26-30%) in incudostapedial joint detachment and malleus/incus fixation 4
- The presence of a Carhart notch increases the probability of otosclerosis by only 2-33% depending on the clinical context 5
The most recent systematic review concluded there is insufficient high-quality evidence to use the Carhart notch to confirm a diagnosis of otosclerosis, though it may serve as a useful clinical hint 5.
Alternative Diagnostic Approach
Rather than relying on the Carhart notch, the air-bone gap (ABG) at 2 kHz provides better diagnostic discrimination. An ABG ≤15 dB at 2 kHz distinguishes stapes fixation from chain disconnection with 60.4% sensitivity and 89.2% specificity 6. This represents a more reliable audiometric parameter than the bone conduction dip itself.
Clinical Implications
Prognostic Value
The presence of a preoperative Carhart notch has mixed prognostic implications:
- Positive: Disappearance of the notch after stapedotomy correlates with improved speech audiometry outcomes 7
- Negative: Long-term follow-up shows patients with preoperative Carhart notch have less favorable air-bone gap closure and smaller improvements in average bone conduction 7
- Preoperative Carhart notch combined with progressive sensorineural hearing loss correlates with higher rates of tinnitus and vertigo 8
Common Pitfall
Do not exclude otosclerosis or stapes fixation based on the absence of a Carhart notch—approximately 25% of surgical otosclerosis cases lack the typical notch configuration 1. The diagnosis must integrate clinical history, otoscopy findings, tympanometry (typically type A with reduced compliance), and the overall audiometric pattern rather than relying on this single finding.