Sensorineural Hearing Loss: Features, Aetiology And Management

SNHL results from lesions of the cochlea or CN VIII or central auditory pathway. It may be present at birth(congenital) or start later in life (acquired).

The characteristics of sensorineural hearing loss are:

1. Rinne +ve ie AC>BC
2. Weber lateralised to better ear
3. Bone conduction reduced on Schwabach and ABC test
4. More often involving higher frequencies
5. No gap between air and bone conduction curve on audiometry
6. Loss may exceed 60 dB
7. Speech discrimination is poor
8. Difficulty in hearing in presence of noise.

Aetiology:

Congenital Causes:

• Lack of development (aplasia) of the cochlea
• Chromosomal syndromes (rare)
• Congenital cholesteatoma - squamous epithelium is normally present on either side of the tympanic membrane. Externally, within the external auditory meatus or ear canal and internally within the middle ear. Within the middle ear the simple epithelium gradually transitions into ciliated pseudostratified epithelium lining the Eustachian tube now known as the pharyngotympanic tube becoming continuous with the respiratory epithelium in the pharynx. The squamous epithelium hyperplasia within the middle ear behaves like an invasive tumour and destroys middle ear structures if not removed
• Delayed familial progressive

Acquired Causes:

• Inflammatory
  • Suppurative labyrinthitis
  • Meningitis
  • Mumps
  • Measles
  • Viral
  • Syphilis
• Ototoxic drugs
  • Aminoglycosides (most common cause; e.g., tobramycin)
  • Loop diuretics (e.g., Furosemide)
  • Antimetabolites (e.g., Methotrexate)
  • Salicylates (e.g., Aspirin)
• Physical trauma - either due to a fracture of the temporal bone affecting the cochlea and middle ear, or a shearing injury affecting cranial nerve VIII.

• Noise-induced - prolonged exposure to loud noises (>90 dB) causes hearing loss which begins at 4000Hz (high frequency). The normal hearing range is from 20 Hz to 20,000 Hz.
• Presbycusis - age-related hearing loss that occurs in the high frequency range (4000Hz to 8000Hz).

• Sudden hearing loss
  • Idiopathic (ISSHL: idiopathic sudden sensoneurinal hearing loss), H91.2
  • Vascular ischemia of the inner ear or CN 8
  • Perilymph fistula, usually due to a rupture of the round or oval windows and the leakage of perilymph. The patient will most likely also experience vertigo or imbalance. A history of an event that increased intracranial pressure or caused trauma is usually present).
  • Autoimmune - can be due to an IgE or IgG allergy (e.g. food).
• Autoimmune - a prompt injection of steroids into ear is necessary.

• Cerebellopontine angle tumour (junction of the pons and cerebellum) (the cerebellopontine angle is the exit site of both the facial nerve(CN7) and the vestibulocochlear nerve(CN8). Patients with these tumors often have signs and symptoms corresponding to compression of both nerves)
  • Acoustic neuroma (Vestibular schwannoma) - this is a schwannoma (benign neoplasm of Schwann cells)
  • Meningioma - benign tumour of the pia and arachnoid maters
• Ménière's disease - causes sensorineural hearing loss in the low frequency range (125 Hz to 1000 Hz). Ménière's disease is characterized by sudden attacks of vertigo lasting minutes to hours preceded by tinnitus, aural fullness, and fluctuating hearing loss.

Diagnosis:

1. History: whether disease is congenital or acquired, stationary or progressive, associated with other syndromes or not, whether familial or not.
2. Severity: whether mild, moderate, moderately severe, severe, profound or total. can be determined by audiometry.
3. Type of Audiogram: whether loss is of high frequency or low frequency or mid-frequency or flat type.
4. Site of Lesion: whether cochlear or retrocochlear or central
5. Laboratory Tests: X-ray or CT scan of temporal bone for evidence of bone destruction (congenital cholesteatoma,  glomus tumour, middle ear malignancy or acoustic neuroma), blood counts(leukemia), blood sugar(diabetes), VDRL(syphilis), thyroid function tests(hypothyroidism), kidney function tests, etc

Management:

Early detection of SNHL is important as measures can be taken to stop its progress or reverse it or to start an early rehabilitation.

Syphilis of ear is treatable with high doses of penicillin and steroids.

Hearing loss of hypothyroidism is treatable with replacement therapy.

Serous labyrinthitis can be reversed by treating middle ear infection.

Early management of Meniere’s disease can prevent further episodes of hearing loss and vertigo.

SNHL due to perilymph fistula can be corrected surgically by sealing the fistula in the oval or round window by fat.

Ototoxic drugs should be discontinued, if found to be causing hearing loss.

Noise induced hearing loss can be prevented from further deterioration by removal of the person from the noisy surroundings.

Rehabilitaion of hearing-impaired with hearing aids.