Interaction between noise and chemical effects on hearing

Occupational hearing loss is the most common disease in Europe. But noise is still considered as the only factor responsible for occupational deafness even though many workers are exposed to noise and chemicals. For decades; research has shown that some chemicals can be ototoxic or can potentiate the traumatic effects induced by noise. Such findings deserve to be taken into consideration since workers can be exposed to noise and/or chemicals, the latter coming from working environment or from drugs; such co-exposures can be simultaneous or sequential. If the ototoxicity of aminosides, or that of antitumor substances are well known, the cochleotoxic effects of aromatic solvents, heavy metals, or asphyxiating gases are not so popular. Because of that we need to be concerned about the pertinence of the prevention when workers are exposed o both noise and ototoxics.
Today, the European legislation (Directive 2003/10/EC) and the hearing conservative programs in the USA and Europe are focused on noise-exposed subjects whose the cochleae are not contaminated by ototoxics. But a cochlea contaminated by an ototoxic agent is more susceptible to noise disturbances than an ear exposed only to noise. The synergistic effects on hearing of a co-exposure to noise and ototoxics have been demonstrated in animals as in humans. Therefore it is fair to be concerned about the pertinence of the threshold limit values recommended for noise (LEx,d), or for ototoxics (TLV) when people are exposed to several factors. There is really a preventive problem about that particular point. Indeed, an occupation deafness can be recognized in case of a cochlear dysfunction, but the prevention and the detection of hearing disturbances lie mainly on a pure-tone audiometry (PTA), technique which allows integrated auditory messages to be detected. The main problem is that PTA cannot detect a peripheral traumatism because of the plasticity of the central nervous system. We are not talking about neuronal reorganizations within the brainstem or the cortex, but rather the physiological mechanisms used by the two major nuclei of the afferent pathways: the cochlear dorsal nucleus and the inferior colliculus. Their plasticity can modify the perception of the peripheral acoustic signals. As a result it is fair to question the pertinence of the PTA which measures only auditory sensations to detect cochlear dysfunctions provoked by complex exposures. The use of tools capable of evaluating only the inner ear function would be a real progress in prevention. The evoked oto-acoustic emissions could be the clue of the problem. An acoustic stimulation with two pure tones: f1 & f2 generates acoustic oto-emissions at 2f1-f2. With a contralateral stimulation, it is possible to measure the impact of the inner-ear and middle-ear reflexes on the amplitude of the oto-acoustic emissions. EchoScan is a new equipment which allows to detect the threshold of these reflexes trigger. The measurements are objective since the subject does not have to be active in the procedure and the CNS does not play a key role in such a measure. Moreover, the equipment allows to assess increases in thresholds after noise exposure and decreases after a solvent exposure. Advantages and drawbacks are considered in this presentation.