Speech Understanding Ototoxicity Characteristics the Disease Stems

Speech

Understanding Ototoxicity

Characteristics

The disease stems from toxic reactions to structures of the ear, including the cochlea, vestibule, semicircular canals and otoliths. Drug-induced damage to these structures of the auditory and balance system can result in hearing loss, tinnitus, disequilibrium or dizziness (Ryback 2007).

Etiology

The propensity of specific classes of drugs to cause ototoxicity has been well established and over 100 classes of drugs have been associated with ototoxicity. Hearing loss can be temporary but is usually irreversible with most agents. Generally, antibiotic-induced ototoxicity is bilaterally symmetrical, but it can be asymmetrical. The usual time of onset is often unpredictable, and marked hearing loss can occur even after a single dose (Ishiyama 2006). Additionally, hearing loss may not manifest until several weeks or months after completion of antibiotic or anti-neoplastic therapy (Dille 2003). Vestibular injury is also a notable adverse effect of aminoglycoside antibiotics and may appear early on with positional nystagmus. If severe, vestibular toxicity can lead to disequilibrium and oscillopsia (Fausti et al. 2009). Permanent hearing loss or balance disorders caused by ototoxic drugs may have serious communication, educational, and social consequences. Therefore, the benefits of ototoxic drugs must be weighed against the potential risks and alternative medications should be considered when appropriate. Management emphasis is on prevention, as most hearing loss is irreversible. No therapy is currently available to reverse ototoxic damage (Fausti et al. 2009).

Assessment of Ototoxicity

The definition and criteria for ototoxicity has been well established by the American Speech-Language-Hearing Association (ASHA). The ASHA defines ototoxicity as: (a) 20db or greater decrease in pure-tone threshold at one frequency, (b) 10db or greater decreased at two adjacent frequencies or (c) loss of response at three consecutive test frequencies in which responses were previously obtained. It is critical that proper initial assessment be achieved to monitor disease progression. For this reason a variety of tests exist:

Tympanogram -- This is an objective test of middle ear function. However, it is not a hearing test but rather records the degree energy transmitted across the middle ear. Therefore, it should not be used to assess the sensitivity of hearing loss but rather distinguishes conductive hearing loss and sensorineural hearing loss, such as caused by Ototoxicity (Campbell 1993).

SRS Scoring -- This is a test used to detect changes in conductive and sensorineural hearing loss. A perfect score on the SRS means that one was able to hear and repeat every word correctly. A normal score is 90%. Levels below 50% means that a hearing loss will not be very helpful in corrected any ototoxic damage and that a cochlear implant might be useful. Typically, sensorineural hearing loss is more severe in ototoxicity than conductive loss (Grant 1998).

SRT Scoring - This test determines the level at which a patient begins to recognize speech. This score will change and can serve as an early detection marker for ototoxcic effects. (Grant 1998).

Acoustic Reflex Testing - Acoustic reflexes measure the stapedius and tensor tympani reflex of the inner ear which is generates eardrum movement in response to intense sound. They can be helpful in checking for particular types of hearing loss in situations where patient reliability is questionable. This test helps elucidate the location of the ototoxic lesion and what the best intervention is in regard to hearing aids or cochlear implants (Campbell 1993; Grant 1998)

Acoustic Reflex Decay -- This is a test is given to rule out acoustic neuromas which might mimic the effect of ototoxicity. The sound stimulus is presented in the ear contralateral to the probe tip. A continuous pure tone stimulus is presented 10 dB above acoustic reflex threshold for 10 seconds. The test is positive if the magnitude of the reflex decreases by more than 50% in ten seconds. In a normal ear, the reflex should stay contracted for the full 10 seconds. Note, the ear being tested is the one receiving the stimulus not the probe (Dille 2010).

Otoacoustic Emission -- The purpose of this test is to determine cochlear status, specifically hair cell function. This information can be used to: 1) screen hearing, particularly in neonates or individuals with developmental disabilities, 2) estimate hearing sensitivity, 3) differentiate between the sensory and neural components of sensorineural hearing loss, and 4) test for functional (feigned) hearing loss. The information can be obtained from patients who are comatose because no behavioral response is required (Campbell 1993; Rybak 2007)

Auditory Brainstem Response - This test is similar to ARD as it also is a screening method to rule out acoustic neuromas or vestibular schwanomas of the eighth cranial nerve. The test is performed by generated an evoked potential by a brief click or tone transmitted from a transducer in the form of an insert earphone. The elicited response is measured by surface electrodes typically placed on the scalp and ear lobes. An abnormal ABR finding is suggestive of retrocochlear pathology and indicates the need for MRI to look for abnormalities (Roland 2004).