Auditory evoked cortical responses to frequency glides in subjects with retrocochlear hearing impairment

Audiological disturbances caused by long-term exposure to industrial solvents. Relation to the diagnosis of toxic encephalopathy

Sixty workers, consecutively admitted due to suspicion of solvent-induced chronic toxic encephalopathy (CTE), were investigated with pure-tone audiometry, determination of speech recognition of monosyllabic words and distorted speech and cortical response audiometry (CRA). Eighteen workers not exposed to occupational solvents and noise were also investigated. The scores in the distorted speech test were significantly lower and the CRA latencies were significantly longer in the solvent group than in the control group. There was no difference between the groups in the pure-tone and monosyllabic speech recognition tests. In the solvent group, 19 subjects had one or several pathological audiological test results (values exceeding the mean result of the control group by 2 SD). Independently of the audiological examination all the workers in the solvent group underwent the traditional clinical assessment of CTE, which is based on symptoms, history of exposure, clinical neurological examination and a neuropsychological investigation. They were classified in three groups--CTE, incipient CTE and non-CTE. There was no correlation between these groups and the audiological test results. A previous report on vestibular pathology in the same group of subjects and the present investigation on hearing deficits suggest that long-term exposure to solvents causes disturbances of the central pathways in the otovestibular system. Hitherto, no attention has been paid to these disturbances in the definition of the CTE syndrome.

Otoneurologic disturbances caused by solvent pollution

Subjects exposed to industrial solvents may experience vertigo and nausea. Solvents are usually volatile hydrocarbon compounds, which are important parts of everyday life in a modern society. They may also cause neurastenia, personality changes, and reduced intellectual capacity. The syndrome that may develop was formerly named psycho-organic syndrome (POS), but in modern terminology it is called chronic toxic encephalopathy (CTE). The syndrome develops slowly, and during the first years no pathological findings will be found using various test batteries. Somewhat later, when the syndrome still might be reversible, psychometric, auditory, and otoneurologic testing may well unveil disturbances within the posterior fossa structures. Animal experiments suggest one site of effect for solvents to be within the cerebellum and brainstem regions with close relationship to the gamma-amino-butyric acid (GABA) transmission. In the otoneurologic test battery, visual suppression and smooth pursuit are of extreme value, as are some auditory tests such as discrimination of interrupted speech and cortical response audiometry using frequency glides as stimuli. Dynamic posturography and magnetic resonance imaging (MRI) have recently proved valuable in the diagnosis. Research is needed concerning the most efficient test battery for early detection of solvent-induced lesions. During further research it is important to unveil other toxic agents, like heavy metals and alcohol, and their damage to the central nervous system and to make comparisons between these substances and the lesions caused by hydrocarbon solvents.

Audiometric profile in presbycusis

A group of 40 elderly subjects were evaluated using a variety of audiological tests. Their mean age was 68 years. They had all come to the clinic because of hearing problems. The subjects were selected for this study because no cause of their hearing loss could be found other than presbycusis. They all had the typical moderately sloping pure tone audiogram configuration. The pure tone average hearing loss (PTA, average of 0.5, 1 and 2 kHz) ranged from 11 to 70 dBHL with a median of 40. The average air-bone gap was 3 dB. In addition to pure tone audiometry the following tests were performed: most comfortable loudness level and uncomfortable loudness level for pure tone; stapedius reflex thresholds; speech recognition threshold and maximum speech recognition score in quiet; distorted speech recognition; detection thresholds for frequency and intensity glides of a pure tone; slow evoked cortical potentials in response to frequency and intensity glides of a pure tone. The test results, when compared with those obtained on young subjects with normal hearing and with cochlear lesions, show a mixture of characteristics typical of cochlear, retrocochlear and central lesions.

Posturography findings in workers exposed to industrial solvents

Postural control was investigated by static posturography in 18 workers exposed to industrial solvents, 9 patients with psycho-organic syndrome due to industrial solvent exposure, and 52 controls. Both groups of exposed subjects showed larger sway areas with eyes open as well as closed, compared with the controls. No differences were found in the Romberg quotient (the relationship between sway areas with eyes closed/open). The correlation between static posturography and the otoneurological test battery was positive for the visual suppression test in the styrene group. In the industrial solvent group no significant correlations were found. The visual suppression test and some auditory tests were pathological in the exposed groups. The otoneurological test battery--especially the visual suppression test and static posturography--seems to contribute in the assessment of solvent related CNS lesions.

Otoneurological findings in psycho-organic syndrome caused by industrial solvent exposure

Nine subjects with long-term (8-30 years) occupational exposure to industrial solvents and a confirmed diagnosis of psycho-organic syndrome (POS) have been studied with audiological and otoneurological test batteries. The results were compared to a matched control group of nine industrial workers not exposed to solvents and to normal data from healthy volunteers. In the clinical examination, the Romberg test identified 5/9 workers as pathologic and concurrently the stabilometry showed significantly larger sway areas in the POS-group. In the audiological test battery, the significantly pathologic tests were discrimination of interrupted speech and evoked cortical responses to frequency glides (CRA-delta-f). The saccade test disclosed abnormal findings in 5/9 workers. In the smooth pursuit test, abnormality was found at some test frequencies using pseudorandomized stimulus. The VOR-suppression test was significantly abnormal at all test frequencies. The test battery used strongly indicates CNS lesions due to industrial solvents.

Auditory stimulation brain map

The topography of auditory evoked potentials (AEPs) was studied in 12 neurologically normal, adult, right-handed subjects of either sex. The AEPs were recorded with seventeen active electrodes in response to 500 Hz tone bursts with a level of 75 dB HL presented either to the left or the right ear. AEP brain maps were obtained with the BRAIN ATLAS III system. A region of maximum negativity was obtained corresponding to N1 at vertex and defined as focus of N1 (FN1). The reproducibility within and between sessions of FN1 was studied and found to be good for most of the subjects. The interindividual variability was moderate and the centre of FN1 on the map was localized 7 mm in front of the vertex electrode and 8.4 and 8.0 mm to the contralateral side of the vertex electrode upon stimulation of the left and the right ear, respectively. The approximate position of the skull surface is obtained by multiplication by a factor of 3. It was concluded that a standardized test session including six single maps based on 50 stimuli each gives an average map with identifiable and reproducible focus. The basis seems to be fulfilled to use the brain map in further analysis on clinical material.

Vestibulo-oculomotor disturbances caused by occupational hazards

Air-borne pollutants inhaled by workers when welding or in environments containing hydrocarbon solvents can result in occult CNS disturbances. The disturbances can appear as a psycho-organic syndrome, but early symptoms such as vertigo or dizziness may indicate the presence of a vestibular involvement.

Audiological and vestibulo-oculomotor findings in workers exposed to solvents and jet fuel

Three groups of subjects with long-term (5-41 years) occupational exposure to industrial solvents have been evaluated with extensive audiological and vestibular test batteries. Group A comprised 16 subjects with a confirmed diagnosis of psycho-organic syndrome (POS), while group B consisted of 7 subjects with suspected POS. Both groups had been exposed to mixtures of aliphatic and aromatic solvents. Eight subjects with long-term exposure to jet fuel constituted group C. In the audiological test battery, discrimination of interrupted speech and evoked cortical potentials in response to frequency glides were the tests that yielded significantly abnormal results. In the vestibular test battery, considerable pathology was seen in electronystagmography, and in addition, visual suppression test and saccade test indicated CNS disturbance. In general, when a test yielded pathological results, the incidence of pathology was highest in group A and lowest in group C.