Many diagnostic procedures are available to measure the auditory and vestibular systems indirectly. These tests are usually performed by an audiologist who is recognized by the American Speech-Language-Hearing Association with a certificate of clinical com-petence in audiology.
In detecting hearing loss, audiometry is the single most impor-tant diagnostic instrument. Audiometric testing is of two kinds: pure-tone audiometry, in which the sound stimulus consists of a pure or musical tone (the louder the tone before the patient per-ceives it, the greater the hearing loss), and speech audiometry, in which the spoken word is used to determine the ability to hear and discriminate sounds and words.
When evaluating hearing, three characteristics are important: frequency, pitch, and intensity. Frequency refers to the number of sound waves emanating from a source per second, measured as cycles per second, or Hertz (Hz). The normal human ear perceives sounds ranging in frequency from 20 to 20,000 Hz. The fre-quencies from 500 to 2,000 Hz are important in understanding everyday speech and are referred to as the speech range or speech frequencies. Pitch is the term used to describe frequency; a tone with 100 Hz is considered of low pitch, and a tone of 10,000 Hz is considered of high pitch.
The unit for measuring loudness (ie, intensity of sound) is the decibel (dB), the pressure exerted by sound. Hearing loss is mea-sured in decibels, a logarithmic function of intensity that is not eas-ily converted into a percentage. The critical level of loudness is approximately 30 dB. The shuffling of papers in quiet surroundings is about 15 dB; a low conversation, 40 dB; and a jet plane 100 feet away, about 150 dB. Sound louder than 80 dB is perceived by the human ear to be harsh and can be damaging to the inner ear.
Table 59-2 classifies hearing loss based on decibel level. In surgi-cal treatment of patients with hearing loss, the aim is to improve the hearing level to 30 dB or better within the speech frequencies.
With audiometry, the patient wears earphones and signals to the audiologist when a tone is heard. When the tone is applied di-rectly over the external auditory canal, air conduction is measured. When the stimulus is applied to the mastoid bone, bypassing the conductive mechanism (ie, ossicles), nerve conduction is tested. For accuracy, audiometric evaluations are performed in a sound-proof room. Responses are plotted on a graph known as an audio-gram, which differentiates conductive from sensorineural hearing loss. Speech discrimination is also measured (Fig. 59-7).
A tympanogram, or impedance audiometry, measures middle ear muscle reflex to sound stimulation and compliance of the tym-panic membrane by changing the air pressure in a sealed ear canal. Compliance is impaired with middle ear disease.
The auditory brain stem response is a detectable electrical poten-tial from cranial nerve VIII and the ascending auditory pathways of the brain stem in response to sound stimulation. Electrodes are placed on the patient’s forehead. Acoustic stimuli, usually in the form of clicks, are made in the ear. The resulting electrophysio-logic measurements can determine at which decibel level a patient hears and whether there are any impairments along the nerve pathways (eg, tumor on cranial nerve VIII).
Electronystagmography is the measurement and graphic recording of the changes in electrical potentials created by eye movements during spontaneous, positional, or calorically evoked nystagmus.
It is also used to assess the oculomotor and vestibular systems and their corresponding interaction. It helps in diagnosing conditions such as Ménière’s disease and tumors of the internal auditory canal or posterior fossa. Any vestibular suppressants, such as seda-tives, tranquilizers, antihistamines, and alcohol are withheld for 24 hours before testing. Prior to the test the procedure is explained to the patient.
Platform posturography is used to investigate postural control capabilities. The integration of visual, vestibular, and propriocep-tive cues (ie, sensory integration) with motor response output and coordination of the lower limbs is tested. The patient stands on a platform, surrounded by a screen, and different conditions such as a moving platform with a moving screen or a stationary plat-form with a moving screen are presented. The responses from the patient on six different conditions are measured and indicate which of the anatomic systems may be impaired. Preparation for the testing is the same as for electronystagmography.
Sinusoidal harmonic acceleration, or a rotary chair, is used to assess the vestibulo-ocular system by analyzing compensatory eye movements in response to the clockwise and counterclockwise ro-tation of the chair. Although such testing cannot identify the side of the lesion in unilateral disease, it helps identify disease and evaluate the course of recovery. The same patient preparation is required as for electronystagmography.
With endoscopes with very small diameters and acute angles, the ear can be examined endoscopically by an endoscopist spe-cializing in otolaryngology. Middle ear endoscopy is performed safely and effectively as an office procedure to evaluate sus-pected perilymphatic fistula and new-onset conductive hearing loss, the anatomy of the round window before transtympanic treatment of Ménière’s disease, and the tympanic cavity before ear surgery to treat chronic middle ear and mastoid infections. The tympanic membrane is anesthetized topically for about
10 minutes. Then, the external auditory canal is irrigated with sterile normal saline solution. With the aid of a microscope, a tympanotomy is created with a laser beam or a myringotomy knife, so that the endoscope can be inserted into the middle ear cavity. Video and photo documentation can be accomplished through the scope.
Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.