AUDIOLOGICAL EVALUATION

Hearing tests are the primary means of determining the type of hearing loss.  However accurate diagnosis and cause of deafness require a thorough history and physical examination.  Assesment of deafness in an infant requires a complete review of the gestational and family history.  Based on the findings of the history and physical examination more specific tests and imaging studies may be indicated.  Screening with tuning forks might provide a clue to a hearing abnormality.  However more sophisticated techniques of hearing and vestibular evaluation might be required to localize the sensorineural or conductive lesion and determine the degree of hearing loss and the cause.

A pure tone audiometer and impedance audiometer are used to determine the hearing functions of the ear.

Pure tone audiometry (PTA):  This technique evaluates both bone conduction and air conduction and determines the degree of hearing impairment.  The audiometer presents a series of tones (measured in decibels) at frequencies (measured in Hz) from 250 Hz to 10,000 Hz.  The patient wears headphones through which these tones are presented.  Both air and bone conduction thresholds are measured.  The results are plotted in a graphic form called as an audiogram.

Speech audiometry:  This type of audiometry evaluates the patient’s ability to hear and understand the spoken word.  The patient is presented with ten familiar two-syllable words recognized primarily by their vowel sounds.  The sound intensity required for the patient to correctly repeat 50% of these words is called the speech reception threshold (SRT).

Speech discrimination test (SDT) determines comprehension of speech rather than loudness and helps detect abnormalities of the inner ear and the hearing nerve. Even a small lesion of the hearing nerve may impair the ability to understand speech.  The SD scores above 90% are normal, and the score should be close to 100% with conductive hearing loss.  SD in small tumors of the hearing nerve is consistently low.  Slight to moderate hearing loss with severely depressed SD scores should raise suspicion of hearing nerve tumors.

Impedance audiometry  This test has three components Tympanometry, Acoustic Reflex Threshold, Reflex decay.

Tympanometry measures the compliance of the ear drum as air pressure in the ear canal is increased or decreased by 200 to 300 mm.H2O.  The results plotted on a graph called as a tympanogram are categorized as Type A,B, or C. Type
A is normal type and has a peak of maximum compliance at 0 mm H2O.  Type B shows little or no compliance over the entire range of air pressures and suggests a fluid in the middle ear or obliteration of  middle ear air space.  Type C has a peak which occurs at a negative pressure seen in patients with blocked Eustachian tubes. Tympanometry  is a quick and easy test to administer and hence used to screen children with suspected serous otitis media. 

Acoustic Reflex test (ART) measures the normal bilateral contraction of the stapedius and tensor tympani muscles (muscles in the middle ear) in response to loud acoustic stimuli.  These movements stiffen the middle ear system, thus affecting impedance of the middle ear and compliance of the ear drum.  The change in impedance is measured with an impedance audiometer.  The AR is normally elicited by sound intensities from 70 to 100 dB above the hearing threshold.  Absence of reflex suggests lesions of the hearing nerve, sensorineural hearing loss, or Otosclerosis.