Types of hearing loss

Conductive hearing loss (CHL) results in a decrease in the transmission of sound from the outside world to the cochlea. The causes of CHL include abnormal formation of the external ears called the auricle or helix, impaction of cerumen (ear wax) in the ear canal, fluid collections in the middle ear, or dysfunction of middle ear bones that are inflexible usually due to scarring or fibrosis. Otosclerosis is an overgrowth of bone in the inner ear and is one such cause of CHL.

A significant cause of CHL is a cholesteatoma, or a destructive benign growth or cyst filled with air, fluid, or tissue behind the eardrum. Other growths can affect the middle ear such as glomus tympanicum or glomus jugulare, schwannomas of the facial nerve, and hemangiomas. Separation of the roof of the middle ear (tegmen mastoideum) can result in CHL and is commonly due to an encephalocele (cystic protrusion of the brain and surrounding membranes through the skull). Sounds perceived by the brain are reduced but sound normal, not garbled.

Sensorineural hearing loss (SNHL) may result from disconnections in the transmission of sound after entering the cochlea. These disruptions may be due to hair cell destruction in the cochlea or damage to the eighth cranial nerve. Sounds received by the brain are perceived as both diminished and distorted. The degree of distortion does not depend on the amount of hearing loss. Poor speech discrimination can occur with minimal hearing loss. 

Auditory dyssynchrony is found when there is no auditory brainstem response (ABR), lack of middle-ear muscle response, lack of normal sounds generated in the ear known as otoacoustic emissions, or no normal conversions of mechanical to electrical impulses in the cochlea known as cochlear microphonics.

Mixed hearing loss combines both CHL and SNHL.

Categories of hearing loss

The American National Standards Institute defines hearing loss by the degree of decibels (dB) lost: 

· Slight hearing loss - 16-25 dB

· Mild hearing loss - 26-40 dB

· Moderate hearing loss - 41-55 dB

· Severe hearing loss - 71-90 dB

· Profound - More than 90 dB

Genetic causes

Research demonstrates that genetic causes account for 50% of cases of hearing loss. They are divided into syndromic and non-syndromic types. Genetic causes of hearing loss may be due to dominant or recessive genes, linked to the X chromosome, defects in the mitochondria in the cell, or sporadic.

Non-syndromic deafness accounts for about two-thirds of all causes of genetic deafness. People with non-syndromic deafness are deaf or hard of hearing but are normal otherwise with no other physical or mental defects. Some children have siblings or other family members with deafness. 

Mutations in hearing genes result in a functional or structural defect. One such gene defect that affects the normal development of the structural organization of the pillar cells of the inner ear. 

Syndromic deafness accounts for the other cases of genetic deafness and there are more than 300 syndromes recognized. Some syndromes are Waardenburg syndrome, Jervell Lange-Nielson syndrome, Gernet syndrome, and Winter syndrome. The X linked syndromes are Alport syndrome and Rosenberg syndrome. The random syndromes are cat-eye syndrome, Turner syndrome, and Klinefelter’s syndrome.

Prenatal causes of deafness

Prenatal causes result in up to 10% of the causes of hearing loss. Congenital infections such as cytomegalovirus (CMV) infections, varicella, herpes, rubella, syphilis, and toxoplasmosis can lead to SNHL. 

Fetal exposure to known teratogens such as methyl mercury, retinoic acid, trimethadione, and thalidomide can result in SNHL. 

Perinatal (Before or Shortly After Birth) Causes

Perinatal causes such as prematurity, low birth weight, loss of oxygen, elevated bilirubin, and blood infections are responsible for 5-15% of SNHL. 

Postnatal causes

Overall 10-20% of hearing losses are due to causes that occur in infancy and early childhood. Childhood infections, such as mumps or meningitis may proceed SNHL. Treatment with medications such as aminoglycosides or furosemide are toxic to the ear tissues and nerves can lead to SNHL. Otitis media or major head injury may lead to SNHL or CHL. Iron-deficiency anemia can be associated with an increased chance of developing SNHL in children and adolescents. 

Unknown causes

In approximately 20-30% of deaf children, no cause can be determined. Most are likely due to an undiagnosed genetic defect or mutation. Psychogenic hearing loss is another rare cause of hearing loss.

References

1. R Morton NE. Genetic epidemiology of hearing impairment. Ann N Y Acad Sci. 1991. 630:16-31. [Medline].

2. Erenberg A, Lemons J, Sia C, Trunkel D, Ziring P. Newborn and infant hearing loss: detection and intervention. American Academy of Pediatrics. Task Force on Newborn and Infant Hearing, 1998- 1999. Pediatrics. 1999 Feb. 103 (2):527-30. [Medline].

3. Katbamna B, Crumpton T, Patel DR. Hearing impairment in children. Pediatr Clin North Am. 2008 Oct. 55 (5):1175-88, ix. [Medline].

4. Roizen NJ. Etiology of hearing loss in children. Nongenetic causes. Pediatr Clin North Am. 1999 Feb. 46 (1):49-64, x. [Medline].

5. McGee J, Walsh EJ. Cochlear transduction and the molecular basis of auditory pathology. Cummings Otolaryngology: Head & Neck Surgery. 5th ed. St. Louis: Mosby; 2010. Chap 146.

6. Hildebrand MS, Husein M, Smith RJH. Cochlear genetic sensorineural hearing loss. Cummings Otolaryngology: Head & Neck Surgery. 5th ed. St. Louis: Mosby; 2010. Chap 147.

7. Richardson SO. The child with "delayed speech". Contemp Pediatr. 1992. 9(9):55.