Portuguese Version

Year:  2002  Vol. 68   Ed. 5 - (17º)

Artigo Original

Pages: 705 to 713

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Auditory and extra-auditory effects of occupational exposure to noise and vibration

Author(s): Márcia Fernandes,
Thaís Catalani Morata 1

Keywords: vibration, noise, hearing loss, prevention

Abstract:
Aim: The present study aimed to investigate the health complaints and the audiological findings of 2 groups of workers. Study design: Clinical prospective randomized. Material and method: Group 1 was exposed to high sound pressure levels and vibration transmitted by hands and arms through the use of power brush cutter/string trimmers. Group 2 was exposed to high sound pressure levels and whole-body vibration transmitted by heavy machinery such as vibrating compactor rollers, skid-steer loaders, backhoes and compact hydraulic excavators. The 73 participants underwent an interview, otoscopy, and pure-tone audiometry. Regarding general health, group 2 workers, exposed to whole-body vibration presented the highest number of complaints. Results: All the participants from group 1 use hearing protectors and 11% of them complained about tinnitus. Not all workers from group 2 use hearing protectors and 17% of them 2 reported tinnitus. However, group 1 workers, exposed to hand-arm vibration was the group with the highest percentage of abnormal audiograms. Conclusion: This study revealed a series of weaknesses in the health surveillance of these populations and indicated the need for the implementation of preventive programs focusing on their exposures to noise and vibration.

INTRODUCTION

Physical agents such as noise, heat, vibration, pressure, radiation and chemicals such as fumes, dusts, gases and vapors are some of the environmental stressing agents found in different workplaces. Organizational stressers are factors related to work organization, such as for example shifts, rhythm and ergonomics, or the relation between the workers and their tasks. They affect the functioning of the whole body and sleep, increase sensitivity to environmental stressing agents and, consequently, increase the risk of industrial accidents. When combined, such stressing agents can cause a series of effects on health and wellbeing of workers.

Among the occupational risk agents we can include noise. It is known that workers exposed to this risk complain of hearing loss and tinnitus and various other symptoms, such as headache, nervousness, stomach problems, and others. Vibration is also an occupational risk factor and owing to its complexity and difficulty to identify and its effects on men, it still deserves further studies.
The purpose of the present study was to analyze the complaints resultant from exposure to vibration and high levels of sound pressure, as well as findings of audiological assessment, and by correlating all findings we analyzed the relevant aspects for prevention of hearing losses.

Noise induced hearing loss - NIHL
Noise-induced hearing loss (NIHL) is understood as abnormal sensorineural hearing loss thresholds resultant from systematic occupational exposure to high sound pressure levels. The main characteristic is irreversibility and gradual progression as a result of exposure to risk. The natural history shows initially the impairment of the one or more frequencies at the range of 3.000 to 6.000Hz. The other frequencies may take longer to be affected. When the exposure ceases, there is no progression of the hearing loss (Brasil, 1998).

Hétu and Phaneuf (1990) stated that among hearing losses, NIHL is the most common preventable disease. However, noise is not the only cause of hearing loss in the workplace. Other factors may influence its occurrence. Among them, we can mention vibration, exposure to ototoxic agents and extreme temperature (Morata and Lemasters, 1995). Concomitant exposure to noise, chemicals (Morata, 1986; Souza, 1994) and vibration (Carnicelli, 1994), may aggravate the hearing loss induced by high levels of sound pressure. Hétu and Phaneuf (1990) reported that when the recommendations for permitted levels of noise exposure were made, the hearing losses detected were based only on noise only and other physical and chemical agents were ignored. The authors suggested that further studies should be conducted with noise and other harmful occupational exposure and leave some unanswered questions, such as: "Should allowed levels of noise exposure be reduced in the presence of other agents that cause hearing loss?".

Vibration
The human body is exposed to vibrations in different environments and we can classify it based on how it is transmitted to the body: vibration in the whole body or vibration transmitted through the hands. The whole body vibration is present when the body is being supported by a vibrating surface. It is produced in three ways: when we sit on a vibrating seat, when we stand up and the floor is vibrating or when we lie down on a vibrating surface. This type of vibration is felt in all means of transportation.

Vibration can also be classified as being transmitted through the hands. It is produced by different industrial, agricultural, mining and construction processes, when the vibrating machine is manipulated with the hands or fingers (Griffin, 1982 apud Carnicelli, 1994). In general, the typical vibration of equipment is greater than 0.316 m/s2 (Matoba, 1994), whereas the level of sound pressure is usually below 95dB HL. The repetitive action of human body stressing agents can overload and harm the peripheral nervous system as well as the central nervous system.

Regardless of how vibration is transmitted, it is important to know its effects so that we can not only prevent but also make the differential diagnosis between diseases with similar symptoms, resulting from exposure to vibration or work-related osteoarticular disorders. There is a great inter-individual variability concerning their capacity to perceive vibration and consider it uncomfortable or unacceptable. The subject has to be exposed to vibration for many years to detect health abnormalities.

According to ISO 2631 standard (1997), exposure to whole body vibration causes a complex distribution of oscillation movements and strengths in the body. There may be a wide variation of complaints and severity concerning the biological effects caused by it. The vibration can cause sensation of discomfort and bad mood, influence performance or pose health and safety at risk. To Pekkarinen (1995), whole body vibration is a diffuse stimulus that excites various receptors simultaneously and causes general stress. The effects of whole body vibration are closely related to the effects of low frequency noise. The author reported that whole body vibration is held responsible for blood abnormalities in the inner ear and a temporary reduction of the auditory threshold between the frequencies of 2 and 4kHz has also been related to whole body vibration.

Matoba (1994) reported headache, insomnia, forgetfulness, irritability, depression, tinnitus and impotence in subjects exposed to hand-arm induced vibration as signs and symptoms progress. However, the most common abnormalities are impairment of peripheral circulation, nervous and muscle systems, articulation and central and autonomic nervous system associated to hearing loss, nystagmus and vertigo. These abnormalities are observed in 60 to 70% of the patients. Physical evaluation revealed signs of blood supply, nervous and muscular abnormalities in the fingers and arms.

Noise and Vibration
The action of combined exposure to noise and vibration may cause a synergic effect to workers health. Mannimen (1984) apud Castaño and Fernandez (1989) described a systematic increase in stress and other harmful effects in workers exposed to the combined action of noise and vibration in general towards workers exposed to one or the other isolated risk.

Many studies reported that workers who suffered of the white finger syndrome developed greater hearing loss that the control group (Pyykko et al., 1981, 1994; Axelsson et al., 1989; Iki, 1994; Miyakita et al., 1997). They referred that the most likely mechanism of high sound pressure level noise induced hearing loss is the vasoconstriction of the inner ear, caused by exposure to noise and the increase demand of oxygen to strengthen the cells and prolong the excitation of cell receptors that increase the ischemic risk of hair cells.

Murata, Araki and Aono (1990) examined the effects of hand-induced vibration on the central and peripheral nervous systems using auditory evoked potential tests. The authors concluded that combined exposure to stressants such as vibration, noise, weather differences and hard work affect not only the peripheral nervous system, but also the central nervous system.

MATERIAL AND METHOD

According to the Occupation Health Department of the studied company, the sound pressure level to which both groups were exposed was the same, approximately 92 dBHL. Owing to unavailability of the appropriate equipment, the vibration to which the workers were exposed was not formally measured but is described below.

All workers in the company were submitted to periodical hearing tests every year or at the occupational physician's own discretion. Since the present study was a transversal epidemiological trial, previous tests were not considered. All workers had previously undergone audiometric test at least once, so they were familiar with the test. The workers were assessed at the Clinic of Speech and Language Pathology and Audiology of University Tuiuti of Parana.

During the interview, we collected data concerning the work routine of subjects. The routine of Group 1 workers was to prepare the equipment, including fuel supply to put it into operation. The fuel lasted for about 45 minutes and the workers had to interrupt the activity to replenish the tank and to sharpen the cutting blades. The process took about 15 minutes on average. During the second break both in the morning and afternoon, the workers had a snack, extending the duration of the break.
Group 2 workers followed a different routine. There were no breaks during the work day. They had to turn on the equipment and keep it running until lunch time and then up to the end of the shift, in the afternoon.

Subjects

The studied workers were employed in a company that provided cleaning and preservation services for streets and avenues in the city of Curitiba and they worked in fixed shifts from 7 to 5h30pm. The minimum daily noise exposure was four hours for at least one year. When the audiological assessment was conducted they had had at lest 14 hours of auditory rest and we did not detect tympanic membrane perforation, acoustic canal obstruction or extra-occupational exposure to noise and vibration.
The seventy three studied workers were divided into two groups considering the way vibration was conducted through the body: Group 1 - hand and arm vibration in 38 workers who were operators of power grazers and group 2 - whole body vibration - in 35 workers who were operators of power grader, power shovel, paving roller, backhoes and compact hydraulic excavators.. Table 1 shows the mean age, duration of exposure in previous jobs and total noise exposure, including the whole working life of the subjects.

We observed that both groups are very similar in terms of mean age and occupational exposure was very homogenous concerning previous, current and total exposure. However, as to daily exposure, there was a slight difference concerning the number of hours that the two groups were exposed to. The workers in Group 1 had a mean daily exposure of 6 hours and group 2, of 7 hours. Both exceeded the maximum daily exposure level for such an intensity (92 dBHL), advocated by the Brazilian Regulation Standards n. 15 (NR 15) by the Ministry of Labor (Brasil, 1998), which is a maximum of 4 daily hours, and according to the American Conference of Governmental Industrial Hygienists (ACGIH), the maximum is 2 hours.

Interview
The topics of the interview included: occupational history of exposure to high levels of occupational sound pressure, non-occupational exposure to the same agents, history of ear pathologies, diseases that can have audiological impact, use of drugs and their action over hearing, use of hearing protective equipment and frequency of use (see Annex 1). The interview was conducted in order to list auditory and extra-auditory complaints of the studied subjects.


Annex 1. Interview form

Name: (initials)
Age: Education:
Date:

1- What is your profession?
2- Do you work with noisy equipment?
( ) no ( ) yes. Which one?
3- How many hours a day?
4- For how long?
5- Do you feel bad after working with such piece of equipment?
( ) no ( ) yes. For how long?
6- Before your current job, did you use to work in a noisy environment?
( ) no ( ) yes. How long did you work there?
7- Do you live or go to noisy places where you have to shout to be heard?
( ) yes ( ) no
8- Have you practiced or do you practice shooting?
( ) yes ( ) no
9- Did you serve in the army?
( ) yes ( ) no
10- Do you go to discos or places where amplified music is played?
( ) no ( ) yes. ( ) once a week
( ) once a month ( ) once a year
11- Do you practice any noisy activity after work? ( ) no ( ) yes. Which one?
How often?
12- Do you wear hearing protective equipment?( ) no
( ) yes( ) phone
( ) inserted plugs
( ) yes, sometimes
( ) almost always when I am exposed to loud noise
( ) always when I am exposed to loud noise
13- Can you hear well?( ) yes
( ) no ( ) R ear ( ) L ear ( ) both
14- Do you understand what people tell you? ( ) yes ( ) no
15- Do you have earache?( ) no ( ) yes ( ) R ( ) L
When
16- Do you have tinnitus?( ) no ( ) yes ( ) right ( ) left ( ) both
How often?
Since when?
In which situations?
Does the tinnitus sound: ( ) low ( ) high ( ) whistle ( ) rain
17- Does anyone in your family have hearing problems? ( ) yes ( ) no
Family relationship?
18- Do you have headache? ( ) yes ( ) no
19- Dizziness? ( ) yes ( ) no
20- Stomach problems? ( ) yes ( ) no
21- Kidney problems? ( ) yes ( ) no
22- Cervical spine problems? ( ) yes ( ) no
23- Do you have orthopedic problems?
( ) no ( ) yes. Which one?
24- Do you have difficulty to sleep? ( ) yes ( ) no
25- Do you have insomnia? ( ) yes ( ) no
26- Are you easily distracted ( ) yes ( ) no
27- Are you nervous? ( ) yes ( ) no
28- Are you anxious?( ) yes ( ) no
29- Are you depressed? ( ) yes ( ) no
30- Can you see well? ( ) yes ( ) no
31- Do you have blood pressure problems?
High pressure( ) yes ( ) no Low pressure ( ) yes ( ) no
32- Do you have tachycardia?( ) yes When?_____________ ( ) no
33- Do you have any cutaneous sensitivity problem? ( ) yes ( ) no
34- Have you observed finger whitening? ( ) yes ( ) no
( ) during work ( ) after work
35- Has the doctor prescribed medications to any of these problems? ( ) no ( ) yes. Which medication?
To which problem?
36- On average, how many glasses of alcoholic drink do you take?
( ) more than six a day
( ) 3 to 5 a day
( ) 1 to 3 a day
( ) 3 to 4 a week
( ) never
37- Do you smoke? ( )yes ( ) no
How much?
For how long?
38- Are you frequently sick? ( )yes ( ) no
39- When was the last time you visited a doctor?(month/year)
40- What was the reason?
41- Did he/she prescribe any drug?
( ) no ( ) yes Which one?
42- Did you take the medication as prescribed?
( )yes ( ) no. Why not?
43- Are you still taking the medication?
( ) yes.How frequently?
( ) no. For how long did you take it?
44- Do you have any other health problems not listed here? ( ) no ( ) yes. Which one?
45- Are you currently taking any medication?
( ) no ( ) yes. Which one?
What for?
46- Did you notice any hearing decrease after a serious disease?
( ) no ( ) yes. hich disease?
47- Did you notice any hearing problems after taking the medication? ( ) no ( ) yes ( ) improved ( ) worsened

Audiological Assessment
The audiometry was conducted in a sound proof booth, audiometer brand Maico MA 41 and phones TDH 39, submitted to annual calibration complaint to ISO R 389 (1998) for air conduction and ISO 7566 (1987) for bone conduction. We conducted daily biological calibration in order to identify the occurrence of any calibration problems in the device.

Graph 1. Number of complaints presented by the studied subjects according to exposure group.

RESULTS

The workers in Group 1 had complaints of nervousness, stomach and vertebral spine problems and the workers in Group 2 reported nervousness, anxiety, headache and visual problems, vertebral spine problems, tinnitus and stomach problems, in addition to numbness and white fingers. The occurrence of complaints was similar between the two groups, but they were more frequent in Group 2 (headache, vertebral spine problems, difficulties to sleep, insomnia, nervousness, tinnitus, visual problems, lack of attention and finger numbness). The exceptions were reduction of cutaneous sensitivity and stomach and orthopedic problems.

Our audiometry results were analyzed according to the classification criteria defined by Fiorini (1994). This criterion classifies findings into the following categories: normal (all thresholds equal or below 25dBHL), normal with notch (decrease in frequencies of 3, 4 or 6kHz with a difference of at least 10dB compared to the previous or next frequency) and audiometric tracing suggestive of noise induced hearing loss (NIHL), that is, NIHL configuration but with auditory thresholds above 25dB HL in the frequency range of 3 to 6kHz.

In Table 2, we can find the audiometric classification according to duration of exposure. Note that in Table 3 audiometric curves with a notch were prevalent in the group with 11-26 years of noise exposed in group 1.

In group 1, the greatest percentage of normal audiometry (50%) was detected in workers who had been exposed from 0 to 10 years. This percentage is reduced as the exposure increased in both groups, but it was more marked in group 1. The greater number of NIHL cases after 10 years of exposure was evident.

All workers of Group 1, with no exceptions, reported use of hearing protective devices and 13 of them used phones and 11, plugs. One worker reported occasional use, 2 reported they used it always, and 21 workers reported they used it when exposed to loud noise, that is, when the machine was on. Even with the use of hearing protective devices, we detected 6 cases of NIHL.

In Group 2, 9 workers with abnormal hearing admitted they did not use hearing protective equipment. Among the 12 who reported its use, 7 worn phones and 5, plugs, 4 of them used it occasionally, one used almost always, and 7 stated that they used it when exposed to loud noise, that is, when the machine was on. The number of workers that used hearing protective equipment and had normal thresholds was higher in group 1.

The workers of group 1, with no exception, used hearing protective equipment and only 4 (11%) reported tinnitus. In Group 2, there were more workers with tinnitus that did not use the hearing protective equipment that those who used it. Out of 35 workers, 14 did not use hearing protection and 6 (43%) presented tinnitus; 21 used hearing protective equipment and 3 (14%) had tinnitus. Out of the total of investigated workers, 59 used hearing protective equipment and only 7 (10%) reported tinnitus, 14 did not use the protective device and among them 6 (8%) presented tinnitus. Even though they did not use the protective equipment, eight people (11%) did not report tinnitus.
We did not observe an association between finger numbness complaint and hearing impairment.

Table 1. Description of subjects according to age and occupational noise exposure in their current and previous jobs



Table 2. Number and percentage of audiometric classifications of participants, according to group and duration of exposure



Table 3. Results of audiometric findings and use of hearing protective equipment, according to exposure group



Table 4. Relation between tinnitus and use of protective equipment, according to exposure groups



Table 5. Audiometric classification concerning complaints of finger numbness and whitening according to exposure groups

DISCUSSION

Extra-auditory complaints
The most frequent complaints were nervousness, anxiety, headache, tinnitus and stomach problems. These findings confirm the observations published by Bevilacqua et al. (1987) and Andrade et al. (1998). The complaints of arterial hypertension are confirmed by the reports of Griffin (1982) and Axelsson (1989), and the complaints of finger numbness and whitening are in accordance with Carnicelli (1994) and Okada (1990), whereas visual problems and reduction of cutaneous sensitivity confirm the observations made by Gerges (1997). Cervical spine and orthopedic problems and lack of attention have also been referred by Griffin (1982) apud Carnicelli (1994) and Matoba (1994).

The difference in percentages of health complaints between the two groups suggested that the exposure to whole body vibration and work routine without breaks can affect the health of workers in a more frequent and marked fashion, which was also noticed by Pekkarinen (1995) and ISO 2631, which stated that the whole body vibration effects are mainly applied to seated people, since the effects on people standing, reclined or leaning against something are little known. This observation indicates the need for measures to reduce such exposure.

Auditory complaints and audiometric findings
In Group 2, the number of workers who complained of tinnitus was twice greater than in group 1. This fact can possibly be explained by the daily use of hearing protective device by all workers in group 1 during the period in which the machine was working. In group 2, only 21 or 60% of the workers used hearing protective device and even so, only occasionally. Among those that used hearing protective device, 3 (14%) reported tinnitus. The most frequent justifications by workers in group 2 were that the devices made the ears hot and that they needed their hearing to assess and monitor the operation of the equipment.

It was observed that among the 11% of the workers in group 1 who presented complaints of tinnitus, 5% presented normal audiometry. In Group 2, 23% complained of tinnitus; 11% presented normal audiometry. The same data were not observed by Bevilacqua et al. (1987), Matoba (1994) and Andrade et al. (1998) which found tinnitus more commonly associated with NIHL.

Hempstock and O'Connor (1978) demonstrated that when there is an interruption of exposure to vibrations during the day, the result could be beneficial to the workers. The authors observed that before the introduction of anti-vibration devices in electrical saws, workers presented the first signs of white finger syndrome after a latency period of 3 years. After the introduction of the anti-vibration device, there was a clear improvement in symptoms. Similarly, intervals during noise exposure have also reduced the total exposure and the risk it poses.

An observation that is in accordance with the findings by Ferraz (1995) was that many workers do not associate complaints with the occupational risk. Some workers were even resistant to report their complaints and health problems, because they feared retaliations at work by the company, such as for example transfer to a new function that did not provide the same compensation over unhealthy workplace or discrimination from work colleagues.

According to Zhu et al. (1997), the combined effects of noise and vibration transmitted through the hands and arms can increase the occurrence of hearing loss, but also to Pekkarinen (1995) this increase may be associated with individual susceptibility and, supposedly, to the autonomic nervous system that regulates the reaction to peripheral vascularization.

As to finger numbness and whitening, of the 6 (17%) workers in group 1 who presented this complaint, 3 presented normal audiometry, one had unilateral notch, one had bilateral notch and one had bilateral NIHL. In Group 2, 8 (23%) presented the complaint, 3 presented normal audiometry, one had unilateral notch, 1 had unilateral NIHL and 3 had bilateral NIHL. These data, however, do not coincide with those reported by Starck et al. (1988): in his study with workers that presented symptoms of white finger syndrome, hearing thresholds were 10dB HL worse in them than in the subjects without symptoms. It is important to point out that the current study is considering complaints and not symptoms as a likely diagnosis of the disease itself. In addition, the sample is too small to enable observation of the effects reported by the literature.

Preventive measures
In order to prevent hearing losses at work, many authors agree that control of exposure is the first alternative to be considered (NIOSH, 1996). However, owing to technical and financial difficulties, Gerges (1999) argued that the use of hearing protective device is the world most accepted and popular measure because it is cheap and easy to have. The National Institute for Occupational Safety and Health (NIOSH,1998) stated that the attenuation of the hearing protective device depends on its characteristics and how workers use it. The selected hearing protective device should be capable of keeping the hearing exposure to levels below 85dB HL. Workers and supervisors should periodically check whether the hearing protective device is being correctly used, is well adjusted and appropriate to the noise level to which the worker is subject. The main focus is the attenuation provided by the hearing protective device and the other necessary qualities to guarantee effectiveness, which have been neglected. The people who select the hearing protective device should consider the type of noise it will be used against and who will use it, the need to have equipment compatible with other protective devices, as well as comfort and workplace conditions, such as temperature, humidity and atmospheric pressure.

The intermittent use of hearing protective device reduces drastically its effectiveness. A device that attenuates 30dBHL in 8 hours of exposure will attenuate only 15dBHL if the worker does not use it for a cumulative period of 30 minutes during a 8-hour day of work (NIOSH, 1998). The measures adopted by the studied company were limited to provision of hearing protective device without control of use. The periodicity of audiometric tests was decided by the physician and ranged from once a year to every three years, according to the workers. They reported that some workers had been transferred from their workplace because they presented hearing loss. There was no hearing prevention program in place for the studied population, despite the evident need. Detailed recommendations for hearing loss prevention programs can be found at NIOSH (1996, 1998). Such documents can be accessed in full on the Internet at http://www.cdc.gov/niosh/96-110.html and http://www.cdc.gov/niosh/98-126.html. Some recommendations in Portuguese for this kind of program can be found in Ferraz (1995).

In the manual of the power saw used, we found guidelines on how to minimize the effects of vibration, exposure limits (TVL) defined by ACGIH (1999) apud Associação Brasileira de Higienistas Ocupacionais (1999) and NIOSH (1989) in the document Criteria for a Recommended Standard Occupational Exposure to Hand-Arm Vibration.

In the studied population, we observed only the use of some individual protective equipment (goggles, hearing plug) as preventive measures for vibration effects. However, the need to apply such measures and to prioritize workers' training emphasizing procedures as those suggested by the Criteria for a Recommended Standard Occupational Exposure to Hand-Arm Vibration (NIOSH, 1989) were evident:

 to recognize sings and symptoms resultant from vibration;
 to record all signs and symptoms;
 to create norms for medical supervision to prevent and control diseases resultant from vibration;
 to list the possible effects of health contributing to operate vibration devices;
 to learn about reversibility of signs and symptoms when early detected;
 to create norms to produce and maintain vibration devices;
 to observe the ergonomic aspect of the equipment used;
 to know about heating needs and the procedures to keep the hands and the body both dry and warmed;
 to wear protective clothes and equipment;
 to measure risk agents;
 to define working and rest hours to control duration of exposure;
 to inform the supervisor about any abnormal functioning of the machine;
 to be aware of the possibility of worsening of hand-arm vibration-induced syndrome when the subject smokes and uses illegal drugs.

CONCLUSIONS

The present study assessed the health complaints and audiological findings of workers exposed to excessive noise and hand-arm versus whole-body vibration. As to health related problems in workers exposed to whole body vibration they presented the highest number of complaints: headache, dizziness, spinal problems, sleep disorders, hypertension, anxiety, nervousness, lack of attention, numbness and finger whitening, tinnitus and visual problems. However, the percentage of abnormal audiogram was greater in the group exposed to hand-arm induced vibration.

We concluded that we have to implement measurement and control measures of exposure to vibration and noise. The high number of health complaints indicated the need to apply other tests that could characterize and diagnose the symptoms induced by vibration exposure. This study has also shown the need to guide both the company and the workers concerning the implementation of prevention programs of occupational diseases that include hearing loss prevention.

The study suggested the conduction of other trials to assess the appropriateness of the current exposure limits when there is continuous noise and vibration.

ACKNOWLEDGMENT

We would like to thank Dr Doris Ruth Lewis and Professor Dr Luiz Gonzaga Calefe for their support in the initial phase of this study and to IMAP - Instituto Municipal de Administração Pública (Curitiba) for the granted scholarship.

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1 Ph.D., Professor, University Tuiuti, Paraná.
Final paper submitted for the Master Course in Communication Disorders.

Article submitted on June 13, 2002. Article accepted on July 18, 2002

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