Why there is dip at 2k Hz in osteosclerosis

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The Carhart's Notch: Why There Is a Dip at 2 kHz in Otosclerosis

The dip in bone conduction at 2000 Hz in otosclerosis is called Carhart's notch. It is a mechanical artifact, not a true sensorineural hearing loss, and the mechanism is explained below.

How Bone Conduction Normally Works

When the skull is vibrated by a bone conduction transducer, sound reaches the cochlea via three routes (as described in Scott-Brown's):

Route	Pathway
A	Direct vibration through skull bone to cochlea
B	Vibration of the ossicular chain (suspended within the skull), which adds to cochlear stimulation
C	Sound radiating out into the external canal and reaching the ear via air conduction

In a normal ear, all three routes contribute to bone conduction sensitivity.

What Happens in Otosclerosis

In otosclerosis, the stapes footplate becomes fixed in the oval window due to abnormal bone growth. This fixation disrupts Route B - the ossicular chain can no longer vibrate freely and transmit energy to the cochlea during bone conduction stimulation.

The natural resonant frequency of the ossicular chain (stapes-incus-malleus system) in humans is approximately 2000 Hz. At this frequency, the ossicular chain normally contributes the maximum amount of energy to bone conduction. When the stapes is fixed:

The contribution from the ossicular chain (Route B) is blocked at the frequency where it normally matters most - around 2 kHz
The result is a selective reduction (dip) in apparent bone conduction sensitivity specifically at 2000 Hz
This manifests as a notch on the audiogram

As Cummings Otolaryngology describes it, the Carhart notch occurs due to "an impedance mismatch of the cochlea from stapes fixation" - the stiffness introduced by the fixed stapes creates maximum disruption at the natural resonant frequency of the chain.

Key Characteristics of the Carhart Notch

Feature	Detail
Frequency	2000 Hz (the ossicular chain resonance)
Type	Bone conduction dip (apparent sensorineural loss)
Magnitude	~15 dB at 2 kHz (Carhart, 1950); averages ~12 dB improvement after surgery
Nature	Mechanical artifact - NOT true cochlear/sensorineural damage
Reversibility	Disappears (bone conduction improves) after successful stapedectomy
Specificity	Not exclusive to otosclerosis - occurs in any conductive hearing loss, but is most prominent and classic in otosclerosis

The Proof: Post-Operative Recovery

The most compelling evidence that this is a mechanical phenomenon is that after successful stapes surgery, bone conduction thresholds at 2 kHz improve and the notch is eliminated. This is called the Carhart effect - it was actually first described because stapes surgery caused "overclosure" of the air-bone gap when pre-operative bone conduction thresholds were used. When post-operative bone conduction was measured instead, it had genuinely improved, particularly at 2 kHz. - Scott-Brown's Otorhinolaryngology, Vol. 2, pp. 1113-1114

Why NOT True Sensorineural Loss?

The notch is erroneously labeled as sensorineural on the audiogram because it appears as a bone conduction dip. However:

The cochlea is structurally normal
The dip is proportional to the degree of stapes fixation
It reverses with surgery
It occurs in all conductive losses (ossicular chain disruption, middle ear effusion), not just otosclerosis

Classic Audiogram in Otosclerosis

Audiogram showing mixed hearing impairment in otosclerosis with Carhart notch

Audiogram in otosclerosis showing mixed hearing impairment. The Carhart notch at 2 kHz can be obscured when a significant sensorineural component co-exists. - Scott-Brown's Otorhinolaryngology, Vol. 2

Summary

The 2 kHz dip exists because:

The ossicular chain (stapes) has its peak resonant contribution to bone conduction at ~2 kHz
Stapes fixation in otosclerosis blocks this ossicular contribution
The loss is greatest where the ossicular chain normally contributes the most - at 2000 Hz
This is a purely mechanical, reversible artifact, not cochlear damage

Why there is rinnes positive in severe SNHL

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Why Rinne's Test is Positive in Sensorineural Hearing Loss (SNHL)

The Core Principle: What Rinne's Test Actually Measures

Rinne's test does not measure the absolute sensitivity of hearing. It only compares two pathways to the same cochlea:

Air Conduction (AC): Sound → pinna → ear canal → tympanic membrane → ossicular chain → oval window → cochlea
Bone Conduction (BC): Sound → mastoid → skull bone → cochlea directly

In a normal ear, AC > BC because the outer and middle ear act as a sound amplifier/impedance matcher - this gives AC roughly a 2:1 time advantage over BC.

In SNHL: Why It Stays Positive

In sensorineural hearing loss, the problem lies in the cochlea (or auditory nerve) - not in the outer or middle ear. The middle ear mechanics are completely intact.

	Normal Hearing	SNHL	Conductive HL
Middle ear	Intact	Intact	Damaged/blocked
AC threshold	Normal	Raised	Raised
BC threshold	Normal	Raised equally	Normal
AC vs BC ratio	AC > BC	AC still > BC	BC > AC
Rinne result	Positive	Positive	Negative

Since both AC and BC must pass through the same damaged cochlea in SNHL, both are reduced equally and proportionally. The 2:1 ratio of AC over BC is preserved. Air conduction still delivers more sound energy to the cochlea than bone conduction does - the cochlea just processes all of it poorly.

As Swanson's Family Medicine Review states: "In the presence of sensorineural hearing loss, the durations of both AC and BC are reduced; however, the 2:1 ratio remains the same (i.e., a positive Rinne test result)." - Swanson's Family Medicine Review, p. 295

The Analogy

Think of a broken loudspeaker. If you play music through a large amplifier (AC) vs a small one (BC), both will sound distorted and quiet through the broken speaker - but the large amplifier still sounds relatively louder. The defect is downstream and affects both equally.

Important Clinical Pitfall: The "False Negative Rinne" in Severe/Profound Unilateral SNHL

This is where severe SNHL creates a dangerous trap:

In a patient with a dead ear (profound unilateral SNHL), when you place the tuning fork on the mastoid of the deaf ear, the vibration travels transcranially (through the skull) and is heard by the opposite normal cochlea. The patient says "yes, I hear it via bone conduction" - but they are actually hearing it in the other ear.

When you then move the fork to the front of the deaf ear canal, no sound reaches either cochlea well - so the patient says BC > AC.

Result: The deaf ear gives a falsely negative Rinne - mimicking conductive hearing loss when none exists.

Situation	What patient actually hears	Rinne result recorded	True pathology
Moderate SNHL	Ipsilateral cochlea (poorly)	Positive (correctly)	SNHL
Profound/dead unilateral SNHL	BC heard by contralateral cochlea	False Negative	SNHL misread as CHL

This is why Weber's test and masking are essential alongside Rinne's in any severe/unilateral hearing loss, and why the Stenger test or formal audiometry must be used in suspected dead ear scenarios.

Summary

Rinne's is positive in SNHL because:

The middle ear is structurally normal - no impedance barrier exists
Both AC and BC are degraded equally at the level of the cochlea
AC still delivers more acoustic energy than BC (2:1 ratio preserved)
Therefore AC > BC remains true, giving a positive Rinne

The test is positive but at a lower absolute level - both thresholds are shifted up together. Rinne only tests the relationship between the two pathways, not their absolute values. - Shambaugh Surgery of the Ear, p. 205; Cummings Otolaryngology

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