Datos de desempeño

Hearing loss resulting from aminoglycosides is usually gradual, bilateral and symmetrical (Harris & Heinze, 2015; Schmidt, Knief, Lagosch, Deuster & am Zehnhoff-Dinnesen, 2008). However, asymmetry has been noted in some instances (Huizing & de Groot, 1987), with possible explanations presented below. The degrees of the hearing losses tend to differ, and details regarding the degree of hearing loss are not always reported.

Moreover, with increased exposure to the drugs, this high-frequency hearing loss often progresses to involve the lower frequencies. As aminoglycosides can stay within ones system for six months to a year after completion of aminoglycoside drug therapy, hearing loss can continue to occur after the cessation of treatment (Harris & Heinze, 2015). These hearing losses are likely to cause a hearing handicap, defined as the disadvantage imposed by a hearing impairment on a person's communicative performance in the activities of daily living (American Speech-Language-Hearing Association, 1981).

4.5.1. Degree

A typical audiogram from an MDR-TB patient may appear as per Figure 4.1. This figure displays the initial high-frequency hearing loss progressing to include the lower frequencies.

Figure 4.1. Baseline and follow-up audiograms of a patient on treatment for MDR-TB (Adapted from Harris & Heinze, 2015, p. 3)

High-frequency nature aminoglycosides induced hearing loss described in various studies (Sharma et al., 2016; Javadi et al., 2011; Fausti et al., 1992). Although they mostly report the bilateral hearing loss (Sharma et al., 2016; Javadi et al., 2011) in some instances, unilateral hearing loss is described, which depicts asymmetry.

This high-frequency loss was observed in an MDR-TB study using kanamycin in India (Sharma et al., 2016). Of the 100 patients examined, ototoxicity was found in 18% of participants, with majority displaying high-frequency hearing loss. The hearing loss was also

mainly bilateral, yet the hearing loss was unilateral in 5% of these patients (Sharma et al., 2016). High-frequency hearing loss was described further in a paediatric TB study (Ghafari, Rogers, Peterson & Singh, 2015). Furthermore, high-frequency hearing loss was seen in the majority of the patients that experienced ototoxicity with 62% being bilateral and 48% being unilateral (Javadi et al., 2011).All these studies only tested up to 8 kHz. However, when testing frequencies higher than 8 kHz (9 kHz to 20 kHz), as with Fausti et al. (1992), it was possible to identify an early change in hearing in 82% of the ears.

4.5.2. Laterality and symmetry

Unilateral and asymmetrical hearing loss is not understood in relation to ototoxicity, yet it has been reported in the above studies. Various ideas have however been proposed.

McFadden (1993) discusses two main differences known in hearing sensitivity, namely gender and ear when measured with OAEs. McFadden (1993) describes this as females being more sensitive than males, and right ears being more sensitive than left ears. This right ear advantage is described mainly in individuals’ exposure to intense sounds, where the right ear displayed a lesser hearing loss than the left ears. However, the difference of this was

minimal; the magnitude of the right-ear advantage in sensitivity was rarely more than 2-4 dB at a given frequency, and it differed as a function of several variables. This article noted the plausibility of a developmental linkage between efferent-based asymmetries insensitivity and asymmetries in adult cortical lateralisation (McFadden, 1993). Furthermore, in a study by Chung, Masoon, Gannon and Willson (1983), with a participant group of more than 50 000 people, differences of about 1.5-2.5 dB were noted for male adults in the range 2-6 kHz between the ears. The right ear always appeared to have the superior hearing levels.

A variety of studies by Khalfa, Morlet, Micheyl, Morgon and Collect (1997) also show ear differences. Khalfa et al. (1997) showed higher amplitudes of TEOAEs in right ears. Khalfa et al. (1997) showed right ears to have more significant tone decay and for left

ears to show a higher temporary threshold shift. The right medial olivocochlear system also shows significantly higher activity (Khalfa & Collet, 1996).

This right ear advantage thus may explain differences in the symmetry of ototoxic hearing loss, where the left ear displays more hearing loss than the right ear. Yet the differences observed in the aminoglycoside studies were more than 2-4 dB as proposed by McFadden (1993).

Further, possible asymmetry with ototoxicity from cisplatin has been observed in the study by Schmidt et al. (2008). They investigated the possibility of the right ear advantage in children undergoing chemotherapy (cisplatin). The study included 55 children and found that the left ear was slightly more affected in the range of 2-8 kHz to 8 kHz. The difference was significant at 4, 6 and 8 kHz. McFadden (1993) concludes that it is possible that the

asymmetry in the efferent system may be related to the well-known cortical asymmetries that are believed to underlie speech perception, speech production, and other human abilities. This study shows that contrary to other literature, the cisplatin-induced hearing loss is not

necessarily symmetrical. Additionally, asymmetrical or unilateral hearing loss in the case of cancer is often a result of tumour location or surgical or radiotherapeutic intervention (Schmidt et al., 2008). There is only limited evidence of asymmetric hearing loss, such as a case with initially bilateral, symmetric hearing loss after aminoglycoside therapy, in which the right ear recovered although the left ear remained hearing impaired (Moffat & Ramsden, 1977).

Another study by Huizing and de Groot (1987) mentioned that aminoglycoside induced hearing loss is not necessarily symmetrical. This study, however, occurred in 1987 and did not explicitly describe reasoning for this. Most studies appear to describe hearing loss resulting from ototoxicity as symmetrical; however, vestibular disturbances are described more often as asymmetrical (Waterston & Halmagyi, 1998; Elidan, Lin & Honrubia, 1987).

It appears that many studies are reporting high frequency and mostly bilateral hearing loss. Also, the degree of the hearing loss varies. In many instances, unilateral components have been reported, indicating asymmetry. Although some possible ideas have been

presented to explain this, there is no conclusive evidence as to why these unilateral hearing losses are occurring.

4.6. Impact of Ototoxic Induced Hearing Loss on Quality of