An Epidemiologic Study of the Effects of Carbon Disulfide on the Peripheral Nerves

Effect of environmental toxicants on neuronal functions

In the last few years, neurodegenerative diseases like Alzheimer's disease (AD) and Parkinson's disease (PD) have attracted attention due to their high prevalence worldwide. Environmental factors may be one of the biggest reasons for these diseases related to neuronal dysfunctions. Most of neuronal disorders are strongly associated with pre- and postnatal exposure to environmental toxins released from industries. Some of the neurotoxic metals such as lead, aluminum, mercury, manganese, cadmium, and arsenic as well as some pesticides and metal-based nanoparticles have been involved in AD and PD due to their ability to produce senile/amyloid plaques and NFTs which are the main feature of these neuronal dysfunctions. Exposure to solvents is also majorly responsible for neurodegenerative disorders. The present review highlights the impact of omnipresent heavy metals with some other neurotoxins on human health and how they give rise to neuronal dysfunctions which in turn causes socio-economic consequences due to increasing pollution worldwide. Graphical abstract.

Effects of co-exposure to CS2 and noise on hearing and balance in rats: continuous versus intermittent CS2 exposures

Background

Carbon disulfide (CS₂) exacerbates the effect of noise on hearing, and disrupts the vestibular system. The goal of this study was to determine whether these effects are also observed with intermittent CS₂ exposure.

Methods

Rats were exposed for 4 weeks (5 days/week, 6 h/day) to a band noise at 106 dB SPL either alone or combined with continuous (63 ppm or 250 ppm) or intermittent (15 min/h or 2 × 15 min/h at 250 ppm) CS₂. Hearing function was assessed by measuring distortion product otoacoustic emissions (DPOAEs); balance was monitored based on the vestibulo-ocular reflex (VOR). Functional measurements were performed before, at the end of exposure and 4 weeks later. Histological analyses of the inner ear were also performed following exposure and after the 4-week recovery period.

Results

The results obtained here confirmed that CS₂ exposure exerts two differential temporary effects on hearing: (1) it attenuates the noise-induced DPOAE decrease below 6 kHz probably through action on the middle ear reflex when exposure lasts 15 min per hour, and (2) continuous exposure to 250 ppm for 6 h extends the frequency range affected by noise up to 9.6 kHz (instead of 6 kHz with noise alone). With regard to balance, the VOR was reversibly disrupted at the two highest doses of CS₂ (2 × 15 min/h and continuous 250 ppm). No morphological alterations to the inner ear were observed.

Conclusion

These results reveal that short periods of CS₂ exposure can alter the sensitivity of the cochlea to noise at a dose equivalent to only 10 times the short-term occupational limit value, and intermittent exposure to CS₂ (2 × 15 min/h) can alter the function of the vestibular system.

Continuous exposure to low-frequency noise and carbon disulfide: Combined effects on hearing

Carbon disulfide (CS₂) is used in industry; it has been shown to have neurotoxic effects, causing central and distal axonopathies.However, it is not considered cochleotoxic as it does not affect hair cells in the organ of Corti, and the only auditory effects reported in the literature were confined to the low-frequency region. No reports on the effects of combined exposure to low-frequency noise and CS₂ have been published to date. This article focuses on the effects on rat hearing of combined exposure to noise with increasing concentrations of CS₂ (0, 63,250, and 500ppm, 6h per day, 5 days per week, for 4 weeks). The noise used was a low-frequency noise ranging from 0.5 to 2kHz at an intensity of 106dB SPL. Auditory function was tested using distortion product oto-acoustic emissions, which mainly reflects the cochlear performances. Exposure to noise alone caused an auditory deficit in a frequency area ranging from 3.6 to 6 kHz. The damaged area was approximately one octave (6kHz) above the highest frequency of the exposure noise (2.8kHz); it was a little wider than expected based on the noise spectrum.Consequently, since maximum hearing sensitivity is located around 8kHz in rats, low-frequency noise exposure can affect the cochlear regions detecting mid-range frequencies. Co-exposure to CS₂ (250-ppm and over) and noise increased the extent of the damaged frequency window since a significant auditory deficit was measured at 9.6kHz in these conditions.Moreover, the significance at 9.6kHz increased with the solvent concentrations. Histological data showed that neither hair cells nor ganglion cells were damaged by CS₂. This discrepancy between functional and histological data is discussed. Like most aromatic solvents, carbon disulfide should be considered as a key parameter in hearing conservation régulations.

Changes of median nerve conduction velocity in rayon manufacturing workers: A 6-year cohort study

Objectives:

We conducted a 6-year cohort study to evaluate the relationship between carbon disulfide (CS₂) exposure and reductions in the motor and sensory nerve conduction velocity (MCV and SCV) of the median nerve.

Methods:

Study subjects at baseline included 432 exposed workers and 402 unexposed workers. Among the exposed workers, 145 workers terminated CS₂ exposure during the follow-up period (ex-exposed workers). MCV and SCV were measured at baseline and followed up. CS₂ personal exposure concentration was measured two times a year during a 6-year follow-up period and mean (range) CS₂ exposure concentrations (ppm) were 5.96 (0.8-16.0) and 3.93 (0.6-9.9) in the exposed and ex-exposed workers, respectively.

Results:

Reductions in MCV during the follow-up period did not differ among the exposed, ex-exposed, and unexposed workers. Reduction in SCV (m/s) of the exposed workers (-4.47±3.94) was significantly larger than that of the unexposed (-3.38±3.97) and ex-exposed workers (-3.26±3.79). For SCV reduction, a partial multiple regression coefficient of (ex-exposed workers) / (unexposed workers) was significantly positive (+0.915, p < 0.01) after adjustment for confounding variables.

Conclusions:

This cohort study showed that 6-year CS₂ exposure around a mean level of 6 ppm did not affect MCV reduction but induced significant SCV reduction beyond the influence of aging. The effect of CS₂ on SCV around a mean exposure level of 4 ppm may be reversible, since it disappeared in the ex-exposed workers after CS₂ exposure cessation for a mean period of 4.1 years.

Neurobehavioral and clinical effects in workers exposed to CS2

OBJECTIVES

Diffuse vascular encephalopathy, parkinsonism and peripheral polyneuropathy (PNP) due to CS(2) exposure are known since long. Data concerning low exposure are sparse. The current study aimed to evaluate whether the decrease of exposure below the threshold limit value (TLV) of 31 mg/m(3) results in an absence of neurological health effects using the same health outcomes as in the first study.

METHODS

The study was conducted in a group of workers of a viscose rayon plant and a control group (CG, n=66). The exposed group was divided into two groups, according to the average CS(2) exposure being below (EG1; n=60) or above 31 mg/m(3) (EG2; n=25). If the current TLV seemed insufficient to prevent effects, the analysis was repeated with three subgroups (10, >10 to 30, >30 mg/m(3)) to estimate the NOEL. Every participant filled in the neurotoxicity symptom checklist (NSC-60) and the Q16. Clinical neurological examination, computer-assisted neurobehavioral tests (NES) and neurophysiological examinations (nerve conduction velocities and electromyography (EMG)) were performed.

RESULTS

Sensorimotor complaints (NSC-60) were significantly worse in both EG1 and EG2 compared to CG (p=0.032) and both exposure groups had a higher proportion of positional tremor compared with CG (p=0.046). Finger tapping in the dominant and non-dominant hand (NES) was worse in EG1 and EG2 compared with CG (both p<0.001). A significant increase in sural nerve SCV, a significant decrease in sural nerve SNAP amplitude and duration and sympathetic skin response (SSR) amplitude, and higher prevalence of EMG abnormalities and PNP was also found in both EG1 and EG2 (p<0.003). All effects remained significant after controlling for possible confounding factors. Further analysis suggested that even average CS(2) exposures of 10 mg/m(3) could still cause effects on positional tremor, finger tapping dominant and non-dominant hand, sural nerve amplitude, SSR amplitude and EMG abnormalities.

CONCLUSIONS

An excess of psychomotor slowing, tremor and PNP was found in workers on average exposed to CS(2) concentrations below 31 mg/m(3). Data suggest that even average exposures as low as 10 mg/m(3) can cause the deleterious effects. This indicates that the current TLV insufficiently protects against neurological effects and, as a consequence, should be lowered.

The meaning of distal sensory loss and absent ankle reflexes in relation to age

CONTEXT

Polyneuropathy is a common disease and is more prevalent (at least 3 %) in elderly people. However, routine neurological examination of healthy elderly people may show distal sensory loss and absent tendon reflexes, which can obscure the distinction from polyneuropathy.

OBJECTIVE

To investigate the relation between age and the prevalence of distal sensory loss, absent tendon reflexes, or muscle weakness, and to ascertain above which age these neurological signs could be considered as normal in ageing.

DATA SOURCES

PubMed, Embase, the Cochrane Library, and Current Contents from 1960 until 2004. Reference lists of relevant studies were searched for additional studies, reviews or textbooks.

STUDY SELECTION

Studies reporting on neurological signs upon routine neurological examination in generally healthy adult persons were considered for inclusion. Two reviewers independently assessed study eligibility and performed study inclusion. Of 629 studies initially identified, 50 (8 %) met the inclusion criteria.

DATA EXTRACTION

Two reviewers independently performed data extraction and assessed study quality based on study design and the rigour by which confounding co-morbidity was excluded.

DATA SYNTHESIS

The 50 included studies comprised a total of 9,996 adult persons. Assuming heterogeneity between studies, the prevalence data from different studies were pooled for separate age groups with a random-effects model. In healthy persons older than 60 years the prevalence of absent vibration sense at the big toes (29 % [95 % CI 18 % to 38%]) or ankles (15 % [95 % CI 11 % to 20%]), and absent ankle reflexes (23 % [95 % CI 16 % to 30 %]) was increased.

CONCLUSIONS

Self-declared healthy adult persons younger than 60 years do not have neurological signs. After the age of 60 absent vibration sense at the big toes or ankles, and absent ankle reflexes are more prevalent, although the majority does not have these neurological signs. It seems more appropriate to apply different diagnostic criteria for polyneuropathy in adult persons younger and older than 60 years.

Comparison of occupational exposure to carbon disulphide in a viscose rayon factory before and after technical adjustments

The objective of this follow-up study was to verify the efficacy of the technical adjustments gradually introduced in departments of a viscose rayon factory from 1989 onward. Personal exposure to carbon disulphide was assessed by means of personal monitoring through active sampling. Six job titles in three departments of the factory were sampled. Geometric means were calculated and used as estimates of time-weighted average (TWA) concentrations. The results from the present study were compared with similar measurements from a previous study in the same factory. Due to organizational changes, only three job titles (spinner, first spinner, and viscose preparator) could be compared directly. Two new job titles were identified, although tasks performed in these two job titles already existed. The measurements from one job title could not be compared, due to a substantial reorganization and automation of the tasks carried out in the department. The comparison before and after technical improvements shows that personal exposure of spinner and first spinner has been substantially reduced. Even the geometric means of measurements outside the fresh air mask are below the TWA-TLV (Threshold Limit Value). Despite the difficulties in comparing the results from the two studies, it is concluded that the technical measures reduced up to tenfold personal exposure to carbon disulphide and personal protection reduced it further by a factor two.

Study on relationship between carbon disulfide exposure and symptoms and signs of nervous system in workers

The relationship between carbon disulfide (CS2) and the symptoms and signs of nervous system in rayon workers was investigated. In a cross sectional study of CS2-exposed workers from the viscose industry and controls, data on neurological symptoms and clinical neurological examination were evaluated. The exposed subjects includes 326 workers and 105 workers from the same factory, without exposure to CS2 or to any other neurotoxic agents, served as controls. The median exposure to CS2(8 h time weighted) in the low-level exposure group was 4.6 mg/m3, and 18.3 mg/m3 in high-level exposure group. Non-conditional multiple logistic regression analysis was employed for evaluating statistical differences by using SAS software. In the analysis, age, sex, work time, body mass index (BMI), smoking and drinking habits, educational background and marriage status were controlled. The odds ratio (OR) significantly increased of some symptoms in workers exposed to CS2 at levels below 10 mg/m3, as compared to controls. The dose-effect relationship between the exposure to CS2 and the symptoms of nervous system was demonstrated. In terms of neurological parameters, no other abnormal change but that in temperature sensitivity was observed in workers exposed to CS2. In conclusion, the influence of low level CS2 on nervous system of workers should not be ignored.