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Introduction The concept of occupational voice disorders Philippe H. Dejonckere "People using their voice professionally are at risk for occupational voice diseases, and require specific prevention and treatment" was the topic focused on by the third Pan European Voice Conference, organized in August 1999 at Utrecht University. The present book includes the main tutorial lectures, with reviews of the most relevant research data and opinions regarding this specific area of concern. Occupational voice users include not only singers and actors, but also teachers, politicians, lawyers, clergymen, telephone operators, etc.(1). The pathogenesis of voice disorders in such patients can be primarily related to their occupation, and thus, after adequate differential diagnosis, these need to be recognized as true occupational diseases, in the same way as, for example, occupational hearing loss (2). A surfeit of information is available on the potential damage from exposure to excessive noise levels(3,4). Noise-induced hearing loss is generally recognized as a typical occupational disease. The relationship between dose and effect is clear, as is documented in publications by the International Organization of Standardization (ISO) (5). The dose combines intensity and duration, and therefore, the concept of dosimetry is of major importance. Also of importance is the definition of the safe limits for exposure to noise. However, factors regarding individual susceptibility to noise and the reversibility of early effects also have to be considered, as well as possible preventive indices of noise-induced hearing loss (6). In some - but not all - respects, noise-induced hearing loss may be considered as a useful model for occupational voice disorders. Epidemiology Titze (7) compared the percentage of the US working population and of the voice-clinic load for different occupation categories: for example, telephone marketers constitute only 0.78% of the total workforce, but 2.3% of the clinic load; teachers represent 4.2% of the US workforce and 20% of the voice-clinic load. Studies based on questionnaires have suggested that teachers and aerobic instructors are at high risk for disabilities from voice disorders, and that these health problems may have significant work-related and economic effects (8,9). For example, Russel et al.(10) investigated the prevalence of self-reported voice problems in teachers: 16% of teachers reported voice problems on the day of the survey, 20% during the current teaching year, and 19% at some time during their career. Roughly speaking, we can conclude from the several studies published during the 1990s that about 20% of teachers experience voice disorders (11). Voice dosimetry Objective measurement of vocal use and vocal load is necessary for the identification of activities and working conditions that are at risk. Voice dosimeters can provide information on the total vocalization time and sound pressure level over a whole working day, in a real life situation (12-14). Just as noise dosimeters define acceptable levels of noise exposure, voice dosimeters help to define the average acceptable limits for vocal load. Hyperphonation Repeated mechanical vibrations transmitted to the body tissues by engines or machines are known to be able of eliciting - in certain conditions - specific kinds of pathology, which are also considered to be occupational diseases (15,16). The vibration may involve the whole body (e.g., in a vehicle) or mainly the hand, wrist, elbow, or shoulder (hand-held power tools). There are standards in the field of occupational health that stipulate the acceptable limits for tissue acceleration values, depending on the frequency (17). Titze's calculations suggest that the risk of damage from tissue vibration is exceeded by occupational vocalists, such as telephone marketers and teachers (18). In the last few years, much new and important information has materialized on the dangers of 'hyperphonation', i.e., loud and prolonged phonation beyond the physiological range. Laboratory experiments on canine larynges, hyper-phonated in vivo under anesthesia, demonstrated obvious damage to vocal fold epithelia (19). The basement membrane shows early lesions and seems to be particularly sensitive (20). A clinical study by Mann et al.(21) in drill sergeants, demonstrated significant increases in vocal fold edema, erythema and edge irregularity, and decreases in vocal fold mucosal wave and amplitude of excursion, following a five-day training period Voice fatigue, relief and recovery According to Titze (18) , two different aspects must be considered: Muscle fatigue: the muscle chemistry needs to be reset for the following contractions. Epithelial cells may die and be shed, due to repeated traumata. New cells have to develop underneath. Collagen and elastin fibers may have separated from the structural matrix of the lamina propria, and have to be removed and replaced by the fibroblasts. Detached protein debris will be removed and re-used by the fibroblasts to make new protein fibers that will support the connective tissue structure. Therefore, minor destruction and repair is continuous. Can the regenerative processes keep up with the destructive process, and what are the physiological time constants in these processes? When there is damage to the joints, ligaments, tendons, or other connective tissue, the recovery time will be proportional to the amount of localized tissue injury that has occurred. If muscle fatigue is the only complaint, the recovery period required will probably be shorter. Hypothetical curves for tissue injury and the recovery period for human phonation have been suggested by Titze (18). Nevertheless, vocal fatigue is still difficult to identify in practical and clinical situations, and Buekers has questioned the clinical relevance of voice endurance tests (13,14). Environmental factors The relative humidity of the air affects vocal function: the most common subjective complaints of teachers with regard to their working environment are the dryness and dustiness of the air. Professional singers note that singing is more difficult in a dry environment: dry air puts an increased strain on the phonatory apparatus and raises the demands on its efficacious and appropriate use (22). The human voice is very sensitive to decreases in the relative humidity of inhaled air because, in experimental conditions, even after short provocation, a significant increase in perturbation measures has been found (23). Noise is also a very common and relatively well-known risk factor in the working environment of professional voice users. It has been observed that the sound level of the speaking voice significantly increases in ambient noise levels starting from 40 dB (A) (about 3 dB for each 10 dB increase in ambient noise), due to the Lombard effect (24,25). In kindergartens, for example, noise levels have been found to vary between 75 and 80 dB (A) (26,27). Effects of stress Mendoza and Carballo investigated the effects of experimentally induced stress on voice characteristics (28). In conditions of stress, induced by means of a stressful environment and cognitive workload tasks, they observed: 1. an increase in F o with respect to baseline; 2. a decrease in pitch perturbation quotient and in amplitude perturbation quotient; 3. a lower presence of turbulent noise in the spectral zone in which the existence of harmonic components is not expected (2800-5800 Hz), with respect to harmonic energy in the 70-4500 Hz range; 4. an increase in harmonic energy in the 1600-4500 Hz range with respect to harmonic energy in the 70-1600 Hz range. The increase in F o seems to be considered a universal indicator of stress and of cognitive workload, as is the lowering of F o perturbation. The response to a stressful stimulus demands a high level of activation, which in turn produces elevated ergotropic arousal that would cause an increase in the tension of the vocal muscles, producing a higher and more tense voice. Mattiske et al. (29) report that teachers seem to experience a significant degree of stress during their work (30) , and there is some research evidence that anxiety and stress are associated with the development of voice problems (31). Marks (32) compares teachers' voices with those of nurses, and finds that psychological stress is reported more frequently by teachers. There are indications that stress, psychological tension, personality, and other psychological factors, may play an important role in voicing problems among teachers (30,33,34). Vocal fold lesions Phonotrauma may result in typical vocal fold lesions, to be interpreted as a direct consequence of mechanical stress and/or as tissue reaction to that stress. Vocal fold nodules and polyps are classical examples (35,36) , but also contact ulcerations/ granulomas of the vocal processes (35,36,37) , if not induced by acid re-flux. Vocal fold hemorrhage is generally consecutive to acute phonotrauma (35). Depending on reversibility and context, microsurgery may become indicated as an important element of the treatment (38,39). Care and cure Patients with occupational voice disorders should benefit from specific medical and paramedical treatments, as well as from technical aids, with respect to their particular pathogenesis. There are major economical aspects at stake, and occupational rehabilitation plays an important role. In a few cases, compensation and professional re-orientation is necessary. Outcomes of the various possible treatment strategies have still to be investigated. However, prevention is still essential. There are indications that vocal hygiene education programs could improve the voice by reducing vocal abuse in daily life and by practising specific strategies to maintain classroom order and to reduce the use of the voice during teaching (40). Further research is needed to demonstrate the usefulness of prevention strategies on the incidence of actual voice disorders.