Sensenural hearing loss cause

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Sensorineural Hearing Loss (SNHL) — Causes

Sensorineural hearing loss results from damage to the mechanotransduction apparatus of the cochlea or disruption of the electrical conduction pathway from the inner ear to the brain. This accounts for ~90% of all hearing loss. Injury can involve hair cells, supporting cells, auditory neurons, or the central auditory pathway.

Structural Basis

Site of Damage	Consequence
Cochlear hair cells (organ of Corti)	Most common — loss of mechano-electrical transduction
Cochlear division of CN VIII	Neural SNHL
Central auditory pathways / cortex	Central SNHL

Causes — By Category

1. Noise-Induced (Acoustic Trauma)

Prolonged or intense noise exposure damages hair cells
Produces a characteristic "noise notch" at 4000–5000 Hz on audiogram
Can also cause cochlear synaptopathy ("hidden hearing loss") — normal pure-tone audiometry but difficulty hearing in background noise, due to loss of auditory synapses rather than hair cells

2. Age-Related (Presbycusis)

Progressive loss of sensory hair cells and nerve fibers starting at the basal turn of the cochlea (high-frequency region), progressing apically
Most common cause of SNHL in older adults
Presents as high-frequency hearing loss

3. Ototoxic Drugs

Drug Class	Examples
Aminoglycoside antibiotics	Gentamicin, tobramycin, streptomycin
Loop diuretics	Furosemide, ethacrynic acid
Salicylates	High-dose aspirin
Antimalarials	Quinine and synthetic analogues
Chemotherapeutic agents	Cisplatin

4. Infectious

Viral labyrinthitis (mumps, CMV, herpes zoster — Ramsay Hunt syndrome)
Meningitis (bacterial — direct spread to cochlea via CSF pathways)
Chronic otitis media (may damage cochlea directly or via toxins)
Infections of the membranous labyrinth

5. Vascular / Sudden SNHL

Sudden SNHL — often idiopathic; presumed vascular or viral etiology; unilateral, acute onset; medical emergency
Stroke — affecting central auditory pathways

6. Tumors

Vestibular schwannoma (acoustic neuroma) — most important cause of unilateral SNHL; asymmetric SNHL is the hallmark
Meningioma — cerebellopontine angle
Endolymphatic sac tumor
Other CNS tumors

7. Endolymphatic Hydrops — Ménière's Disease

Fluctuating low-frequency SNHL, tinnitus, episodic vertigo, and aural fullness
Due to excess endolymph causing distension of the membranous labyrinth

8. Congenital / Genetic

Congenital inner ear malformations (Mondini deformity, enlarged vestibular aqueduct)
Genetic mutations: > 100 genes implicated; connexin 26 (GJB2) mutations are the most common cause of congenital SNHL
Can be syndromic (Usher, Waardenburg, Pendred, Alport) or non-syndromic
Genetic predisposition may interact with environmental exposures

9. Autoimmune

Immune-mediated SNHL — one of the few treatable causes of SNHL; bilateral, progressive, often responds to corticosteroids

10. Trauma

Temporal bone fracture — transverse fractures typically cause SNHL; longitudinal may cause conductive
Head trauma with perilymphatic fistula

11. Neurological / Demyelinating

Multiple sclerosis — lesions in central auditory pathways
Any CNS infection (encephalitis, meningitis)

12. Radiation Therapy

Cochlear radiation damage following head/neck radiotherapy

Key Clinical Distinction: Asymmetric SNHL

Asymmetric SNHL has very few causes other than vestibular schwannoma. Ménière's disease is in the differential but can usually be distinguished clinically. Any asymmetric SNHL warrants MRI of the internal auditory canals. — Rosen's Emergency Medicine

Sources: Harrison's Principles of Internal Medicine 22E, p. 289 | Histology: A Text and Atlas (Gartner), p. 2476 | Cummings Otolaryngology Head and Neck Surgery

How to undestand audiogram

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Here is a comprehensive guide to reading and interpreting an audiogram.

How to Understand an Audiogram

An audiogram is a graph that plots a person's hearing sensitivity to pure tones as a function of frequency. It is the foundation of audiometric assessment.

The Grid

Axis	What it represents
Horizontal (X-axis)	Frequency in Hz — from low (125 Hz) to high (8000 Hz); left = low pitch, right = high pitch
Vertical (Y-axis)	Hearing level in dB HL — 0 at top (normal/quiet), increases downward (louder sound needed)

Key point: Down = worse hearing. A point plotted lower on the graph means the patient needed a louder sound to hear it.

Severity of Hearing Loss

dB HL Threshold	Degree
≤ 25 dB	Normal (adults); ≤ 15 dB in children
26–40 dB	Mild
41–55 dB	Moderate
56–70 dB	Moderately Severe
71–90 dB	Severe
> 90 dB	Profound

Symbols Used on an Audiogram

Symbol	Meaning	Ear
O	Unmasked air conduction	Right
X	Unmasked air conduction	Left
Δ	Masked air conduction	Right
□	Masked air conduction	Left
<	Unmasked bone conduction	Right
>	Unmasked bone conduction	Left
[	Masked bone conduction	Right
]	Masked bone conduction	Left

By convention: Red = Right ear, Blue = Left ear.

Air Conduction vs. Bone Conduction

Air conduction (AC): Sound travels through the ear canal → tympanic membrane → ossicles → cochlea → auditory nerve. Tests the entire hearing system.

Bone conduction (BC): A vibrator on the mastoid process sends vibrations directly to the cochlea, bypassing the outer and middle ear. Tests the inner ear and auditory nerve only.

Air-bone gap (ABG): When AC thresholds are worse (lower on the graph) than BC thresholds, there is a gap. This means the outer or middle ear is not conducting sound efficiently but the cochlea is intact.

The Four Audiogram Patterns

1. Normal Hearing

Both AC and BC thresholds ≤ 25 dB HL across all frequencies.

2. Conductive Hearing Loss

AC elevated (shifted down on graph)
BC normal (at top of graph)
Air-bone gap present
Cause: outer or middle ear problem (e.g., otitis media, otosclerosis, cerumen)

3. Sensorineural Hearing Loss (SNHL)

Both AC and BC equally elevated — they overlap, no air-bone gap
Cause: cochlea, auditory nerve, or central auditory pathway

Classic SNHL: high-frequency loss sloping downward — air conduction (X) and bone conduction (<, >) track together

4. Mixed Hearing Loss

Both AC and BC elevated, but AC is worse than BC
Air-bone gap is present, but BC is also not normal
Cause: conductive pathology superimposed on sensorineural pathology

Configuration (Shape) of the Audiogram

The shape gives clues about etiology:

Configuration	Description	Classic Cause
Flat	Equal loss across all frequencies	Otosclerosis, some SNHL
Sloping / High-frequency	Normal low-freq, loss at high freq	Presbycusis, noise-induced
Falling	Progressive drop toward high freq	Age-related, ototoxicity
Notched	Dip at 4000 Hz, recovers at 8000 Hz	Noise-induced hearing loss
Rising	Worse at low frequencies	Ménière's disease
Saucer-shaped	Better at high and low, worse in mid	Various

Pure-Tone Average (PTA)

The PTA is the average threshold at 500, 1000, and 2000 Hz — the speech frequencies. It summarizes the degree of hearing loss in a single number and should correlate with the patient's speech perception ability.

In Summary: A 4-Step Approach

Look at the thresholds — are they in the normal zone (top 25 dB)?
Compare AC and BC — is there an air-bone gap?
Classify the type — conductive / SNHL / mixed
Describe the configuration — flat, sloping, notched, etc., and identify the frequency range affected

Sources: K.J. Lee's Essential Otolaryngology; Cummings Otolaryngology Head and Neck Surgery; Guyton and Hall Textbook of Medical Physiology

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