Hearing loss

Author: Dr.G.K.Hebbar

Hearing loss (HL) is one of the most prevalent but least recognized physical ailment.  Probably because HL is painless and invisible to others it does not draw as much attention as other disabilities.  However for those who are profoundly deaf and condemned to live in a world of silence the emotional pain is devastating.

There are two main types of hearing loss: Conductive hearing loss and Senorineural hearing loss.

Conductive hearing loss

Conductive hearing loss occurs when sound waves are prevented from reaching the inner ear because of disease in the external or middle ear.  This type of hearing loss is partial and never total, because some sound does manage to reach the inner ear and those suffering from this type of hearing disorder are never totally deaf.  Abnormalities or damage to the external or middle ear impede transmission of sound impulses to the inner ear.  The most common types of conductive hearing loss in children are due to collection of fluid in the middle ear (Middle ear effusion) as a result of inflammation.  Such hearing losses in children usually go unnoticed unless it is gross. Chronic otitis media resulting in damage to the ear drum and/or middle ear ossicles result in significant conductive hearing loss. Fixation  of the Stapes bone in the middle ear caused by a disease known as Otosclerosis results in delayed  conductive hearing loss in both ears generally.  Genetic disorders, exposure to viral infections or teratogenic drugs during the first trimester of pregnancy may cause deformities of the ear canal or the conducting structures of the middle ear leading to a conductive hearing impairment in the new born.

Sensorineural hearing loss

Sensorineural hearing loss (SNHL) occurs due to disease of the inner ear or the hearing nerve.  Such hearing loss may be a result of genetic disorders but in most cases the damage to the inner ear or the nerve is secondary to aging, noise, drugs, or disease.  Such a hearing loss may be partial or total.  This type of hearing loss is generally not correctable by either medications or surgery.  Persons with this type of hearing loss are more incapacitated than those with conductive hearing loss.  These individuals can sometimes hear sounds but have difficulty in understanding speech because of poor discrimination.  Sometimes such losses can  occur suddenly.  They are usually unilateral and caused by a viral infection of the inner ear or a clot in the internal auditory artery.  Other causes of sudden deafness is due to exposure to loud sounds (firecrackers or explosives), fracture of the temporal bone.

Both Sensorineural and conductive hearing losses may be congenital (present at birth) or delayed. Both types may result from either genetic or acquired factors such as disease or injury.  Hearing loss with both conductive and sensorineural elements is called as mixed hearing loss.

SNHL in infants may be due to genetic causes or non genetic causes.  Genetic causes should be identified because the affected families need genetic counselling.  Non genetic causes of hearing loss in infants include maternal use of ototoxic drugs, viral infections that affect the fetus (maternal rubella), metabolic disorders (cretinism), Rh incompatibility, irradiation in the first trimester, prematurity, fetal distress, low birth weight, hyperbilirubinaemia, bacterial meningitis and anatomic malformations of the head and neck.  The most prominent signs of deafness in infants are the lack of startle reflex and delay in acquiring speech.  Totally deaf children do not learn to speak because they cannot hear speech.   The need to hear is very important for development of speech.  Early detection and treatment  of hearing impairment in infants are essential for acquiring the ability to speak.

SNHL in adults may also occur due to genetic aberrations or from acquired causes such as aging trauma or disease.   Familial progressive sensorineural loss (genetic) and presbyacusis (acquired) are common types of delayed sensorineural loss. Presbyacusis is a progressive loss of hearing acuity that occurs with advancing age.  Acquired SNHL may also result from infections caused by bacteria or viruses (measles & mumps), ototoxic drugs, neoplastic diseases, temporal bone fractures damaging the cochlea, Meniere’s disease, hypothyroidism, vascular insufficiency and central nervous system  disorders like multiple sclerosis.

Deafness resulting from SNHL is permanent and there is no medical or surgical cure in most of the cases.  Using a hearing aid or a cochlear implant in selective cases may benefit those suffering from this type of HL.

Deafness resulting from Middle ear effusion 

A collection of fluid in the middle ear or middle ear effusion (MEE) is a common cause of conductive hearing impairment.  It usually follows upper respiratory tract infections or Otitis Media or Barotrauma (result of pressure changes) which causes malfunction of the Eustachian tube.  The fluid in the middle ear may be thin, mucoid or mucopurulent.

This condition is more commonly seen in children than in adults. Enlarged adenoids is the commonest cause of tubal obstruction in children.  The disorder is more chronic in children with thick mucoid fluid being retained in the middle ear causing deafness.  More often than not this condition goes unnoticed in children and is one of the common causes of poor school performance.  Hearing loss, pressure, fulness and some discomfort in the ear are the usual symptoms of MEE.  In adults the fluid is usually a thin serous type that frequently resolves with medical treatment.  The diagnosis is confirmed by an audiometry and tympanometry.

Children are usually treated with antibiotics and decongestants.  If there is no improvement and a conductive hearing loss remains, a myringotomy (incision in the ear drum) with insertion of pressure equalizing tubes (tympanostomy tubes) is performed.  Adenoidectomy may also be required if the adenoids are found to be hypertrophied and obstructing the eustachian tube. 

Deafness due to Meniere’s disease

Fluids in the inner ear chambers are constantly being produced and absorbed by the circulatory system.  Any disturbance in this delicate relationship results in over production or under absorption of the fluids.  The resulting increased pressure in the inner ear causes the symptom triad of episodic dizziness, fluctuating hearing impairment and head noise or tinnitus.  This condition is called as Endolymphatic hydrops or Meniere’s disease (so called after Prosper Meniere who first recognized this condition).  In 85% of the patients suffering from Menieres only one ear is affected, in the remaining 15% involvement of the other ear occurs within 2 years. In the early stages the hearing loss fluctuates always returning to normal levels after the episode has passed.  As the disease progresses hearing loss becomes more severe and permanent.  In the early stages the tinnitus is often intermittent usually occuring during an attack.  Within a short time, it becomes constant, increasing in intensity during attacks.  The types of noise experienced by patients are usually described as low-pitched roaring or hissing type of sounds.  The frightening aspect of the disease is the vertigo.  During an attack, the patient feels as if the room is spinning or sometimes even oneself spinning.  The feeling of vertigo makes sitting and walking difficult and the patient must lie down quietly avoiding all head movements.  Vertigo may be associated with sweating, nausea and vomiting.  The frequency, duration and severity of attacks vary from patient to patient.  Vertigo may come on suddenly and without warning, or may be preceded by an aura.  Vertigo may last for only a few minutes or may last for hours.  Most commonly the acute manifestations may continue for one to three hours. 

A thorough evaluation is necessary in most cases of Meniere’s disease to determine the cause of the increased fluid pressure.  Circulatory, metabolic, toxic, allergic or emotional factors may play a part.  The diagnosis is confirmed by evaluating the the hearing and balance functions.

Most of the patients respond to medical management.  The goal of the medical treatment is to relieve the acute vertigo attack and to reduce the inner ear fluid volume.  If the vertigo is continuous and lasts for days, it is not due to Meniere’s.  The acute vertigo is treated with bed rest, sedation and antiemetic drugs.  Reassurance that the attack will subside and that it is not a sign of a life threatening disease will go a long way in reducing the patient’s anxiety.  After an acute attack, maintainence therapy is instituted to stimulate the circulation.  Caffeine and nicotine have an adverse effect on this disease and has to be avoided.  A low salt diet helps in reducing the volume of the inner ear fluids.  Drugs are now delivered to the inner ear by injecting drugs into the middle ear. Those cases not responding to medical management may have to undergo surgery to relieve the pressure or more radical surgeries like destruction of the labyrinth or section of the nerve conducting the giddiness sensation to the brain.

Deafess resulting from Otosclerosis

This hereditary disease of bone is unique to the human otic capsule and is one of the most important causes of conductive hearing loss in adults.  Otosclerosis occurs most frequently after puberty and affects women more than men. 

This condition occurs when minute areas of bone of the otic capsule are resorbed and replaced with sclerotic bone in an irregular mosaic pattern.  In Stapedial Otosclerosis this process spreads to the stapes (the tiniest bone in the body) fixing it, thus causing conductive hearing impairment.  This type of hearing impairment can be corrected by a surgical technique known as Stapedotomy.  In Cochlear Otosclerosis the process spreads to the inner ear causing sensori-neural hearing impairment.  This type of hearing impairment is permanent and may even be progressive in nature.  On occasion the otosclerosis may spread to the balance organs and cause dizziness.

The initial symptom of Otosclerosis is the inablity to hear a whisper or to understand someone speaking from a distance.  They also experience a permanent ringing sound or tinnitus in their ears.  Those suffering from bilateral conductive hearing loss, their own voice seems loud and thus they pitch their voice lower and lower until it is barely audible to others.  The relative increase in bone conduction intensifies the sound of chewing.  Hence the patient’s hearing appears to be poorer during meal times.  Patients with otosclerosis hear fairly well on the telephone.  They also hear much better in noisy places where people must talk above surrounding noises.  They are thus comfortable at party settings.  Diagnosis is usually confirmed by audiological evaluation.