Nervous-system health – Senses working overtime

31 August 2011

Because it is fed by and acts on information from all of the sensory organs – ears, eyes, nose and skin – the peripheral nervous system is vulnerable to the effects of myriad hazards in the workplace. Dr Chris Ide investigates.

I put down the telephone, having just agreed to produce an article for SHP on the nervous system, but was already wondering if it was, after all, a good idea. I turned to my recent clinic lists for inspiration: bone and joint pains, skin rashes, “My manager isn’t nice to me”, dodgy docs, naughty nurses, protracted convalescence, a few potentially life-threatening illnesses or injuries. . .

My heart sank as I envisaged a rather geeky article on rare toxins, the effects of which are confined to workers in niche industries, which attacked parts of the body with ponderous Latin names. . . Then, one of the nurses asked me to look at an audiogram, and the scales fell from my eyes.

The nervous system can be subdivided into two main constituents: the central nervous system, comprising the brain and the spinal cord, and the peripheral nervous system, which is what this article will focus on. The peripheral nervous system takes information from the various sensory organs – eyes, ears, nose and receptors in the skin, including those located in the viscera – transfers them to the brain, usually via the spinal cord, where they are processed at various levels of consciousness, while a response is formulated and transmitted to the parts of the body that can respond to the stimulus. In many instances, the whole process can occur with us being completely unaware.

Noise and vibration

Probably the part of the nervous system most likely to be affected by an adverse working (or leisure) environment is the auditory nerve, concerned with hearing. The hair cells in the cochlea – part of the inner ear – are damaged by high levels of noise, especially if protracted. This results in hearing loss, initially in the higher frequencies (4-6 kHz), which leads to problems with speech discrimination, particularly for sibilants, and when the listener is trying to hear what someone is saying against a noisy background. This may also be accompanied by a troublesome tinnitus.

It might seem reassuring that only 5 per cent of those exposed to noise levels exceeding 85dBA for eight hours per day over a 30-year period will progress to significant levels of hearing loss.1 However, even with properly conducted audiometric surveillance, it is not always possible to predict which individuals belong to this group. Furthermore, if large numbers of people are exposed to a hazard, then considerable casualties will occur.

The Health and Safety Executive estimates that about one million workers in the UK are exposed to significant work-related noise hazards.2 About 10 years ago, it was estimated that some 2 per cent of adults of working age had hearing difficulties that either required a hearing aid, or gave rise to great difficulties in hearing conversations in a quiet room. However, these were concentrated in the older age groups (55+), where the prevalence was 8 per cent, and more likely to occur where the person had worked in a noisy job.3 

For me, this was demonstrated quite spectacularly when, about four years ago, I attended a public lecture given by the Institute of Shipbuilding and Engineering in Scotland. As the lecture theatre filled up, it occurred to me that I was one of the younger members of the audience (I was in my late 50s then), and I lost count of the number wearing hearing aids. I suspected that a sizeable proportion had a noise-induced component to their hearing loss.

When visiting premises for ‘walk-through’ assessments, if I come across a potential noise hazard, I will also consider the possibility of exposure to vibration, since the two arise from exactly the same mechanism. Hand-Arm Vibration Syndrome (HAVS) is another frequently occurring disorder in which the peripheral nervous system is involved.

The most impressive colour changes in the fingers come from the constriction of the arterioles (small arteries) in the fingers when they are exposed to a combination of vibration and cold temperatures, giving rise to the vascular (V) stages of the syndrome, but other symptoms, such as numbness and parasthesiae (pins and needles) are often prominent. These are mediated through the digital nerves of the fingers and, if present, are staged separately as the sensori-neural (SN) component. (Interestingly, a group of Canadian researchers provided further evidence that workers with HAVS were more likely to suffer from cold-induced blanching in their toes.)4

Finally, the same population studied by Griffin et al3 was also the subject of a further study,5 which demonstrated that those with finger blanching also had worse hearing and that this was by no means also the result of concomitant noise exposure; instead, it was hypothesised that it was due to the digital vasospasm inducing a simultaneous reflex arterial constriction in the inner ear.

Noise is noise, and vibration is vibration, irrespective of whether they occur in the work or leisure environments, and there is no difference between the hearing loss and HAVS that occur as a result of angle-grinders, pneumatic hammers, lawnmowers and strimmers, etc. used by Bloggs and Co employees over the five-day working week, and the identical equipment deployed at the weekend, or on holidays by keen DIYers travelling on motor bikes, where the sound levels have been assessed as high as 105 dB!6

Thus, cumulative exposure is important. Even if exposure occurs solely in the workplace environment, it is essential to remember that exposure levels are set assuming an eight-hour day/40-hour week, so, if there is any significant degree of overtime working, a corresponding reduction in exposure levels is essential if harm is to be avoided. Also, domestic tools may not benefit from the same careful scheduled maintenance and repair programmes set up in the workplace.

Carpal tunnel syndrome

Carpal tunnel syndrome (CTS) is another surprisingly common disorder involving the nervous system of the hands. It affects more than one woman per thousand per year; the rate in men is slightly less than half that. The median nerve transmits sensory nerve impulses from the thumb and the first two fingers and, occasionally, the lateral aspect of the ring finger, as well as stimulating the muscles that move the thumb.

Between the hand and forearm, the nerve passes through a tunnel formed by the carpal bones of the wrist, the roof comprising a ligament called the flexor retinaculum, or the transverse carpal ligament. The carpal tunnel also contains the tendons that flex the fingers. All in all, the carpal tunnel forms a very cramped space, so anything that squashes the median nerve may produce symptoms.

According to the US Bureau of Labor Statistics Annual Survey of 2006, of 10 classes of injury, CTS gave rise to the second highest median days’ sickness absence – 27 days. The duration of the highest (fractures) was just one day longer!7 It’s also expensive – the average cost of a Workers’ Compensation Board (WCB) claim in Ontario, Canada was assessed at $13,700,8 and lost earnings of individual workers over six years in Washington state in the US varied between $45,000 and $89,000.9

CTS is more likely to occur when workers do jobs that require hand movements to be forceful, repetitive and the adoption of an exaggerated posture. It is also associated with the use of hand-held vibrating tools.

One of the reasons (work-related) CTS is so expensive is because the outlook seems worse. In a study of Alberta workers, Sperka et al showed that employees who lodged a Workers Compensation Board claim were three times as likely to take time off work, or change jobs, compared with employees with CTS who did not submit a WCB claim. Age, proportion of men, and duration of symptoms were similar in each group, and the difference persisted even when cases and controls were matched for treatment. The median estimate of loss of income was $4000 in the claimants, ranging from $0-$250k, compared to $0, and $0-45k in the non-claimants.10

The tendency for carpal tunnel syndrome to affect women (and pregnant ones, in particular) suggests that hormones and the associated fluid retention may also have something to do with the onset of CTS. Other metabolic disorders, such as diabetes, hypothyroidism, chronic kidney disease, and gout are also involved. As the carpal tunnel is so cramped anything that impinges on the tunnel’s lumen has the potential to bring on symptoms. Thus, rheumatoid or osteoarthritis – the latter perhaps arising from an old injury or other bone disease – may trigger the complaint. Benign or malignant tumours may also lodge in the carpal tunnel, though this is rare.

Toxins

So far, the disorders mentioned are all caused – at least in the workplace environment – by various physical and ergonomic hazards, but various chemical toxins can also damage nerves. Nerve fibres form junctions with each other called synapses, and the nerve impulse is transmitted from one fibre to another by neurotransmitters. Acetylcholine is one example, and constantly accumulates at the synapse. In turn, it is broken down by an enzyme – a biological catalyst – called cholinesterase.

Some pesticides, used in both human and veterinary medicine, as well as in agriculture – particularly organophosphates and carbamates – inhibit this enzyme, leading to acute toxicity, which can manifest itself in as little as four hours post-exposure, depending on the route of uptake.11

Mild poisoning gives rise to tiredness, nausea and blurred vision. Moderate poisoning causes drooling, watering eyes, headache and muscular twitching, while severe poisoning produces loss of consciousness, convulsions, and even death. The alarming symptoms occur mainly in third-world countries, whereas in the developed world, the interest has tended to shift to the possible chronic effects, which seem to present as a loss of sensory function in the limbs, together with disorders of gait and cognition (probably attributed to central nervous-system damage).12

Other substances in the workplace that have the potential to cause peripheral neuropathy include lead, mercury, arsenic and thallium, used in the manufacture of batteries, scientific instruments, dental amalgams, alloy, semiconductor manufacture, the pharmaceutical industry, and glass-making. Much more commonly-used compounds are organic solvents, such as toluene, xylene, n-hexane and chloroform, which are used in paints, coatings, as degreasing agents, etc.

The harmful effects of these substances have been known for many years, and attempts have been made to either eliminate or reduce their use, to lessen exposure by improving extraction/ventilation, and/or to provide appropriate personal protective equipment. Nonetheless, the hazard still remains and can trap the unwary or the naive. Over the past 30 years or so, there has been a phasing out of some of the halogenated solvents to protect the ozone layer. However, one of the substitute compounds – 1-bromopropane – was found to be a potent neurotoxin – a good example of the maxim: “Today’s solution = tomorrow’s problem!”13

Biological hazards

Having worked our way through the gamut of physical, ergonomic and chemical hazards to the nervous system it would be remiss to exclude biological hazards. Briefly, in the occupational context, a number of micro-organisms encountered in the working environment can give rise to diseases like tetanus (agricultural workers, gardeners) and diphtheria (microbiology laboratory workers). These diseases can be prevented by good working practices, and ensuring that immunisations are up to date.

The later manifestations of Lyme disease include Bell’s palsy, which affects the facial nerve, and other muscular weaknesses. This is why it is important for game-keepers, park rangers and other outdoor workers to wear appropriate clothing to cover legs and arms, to promptly remove ticks if found, and to seek treatment early. Lastly, vets and animal handlers in zoos and pet shops may run the risk of bites from the likes of snakes, spiders and scorpions, whose venom may have neurotoxic properties.

Diabetes, drink and drugs

Sensory neuropathies that involve the hands can result in loss of tactile and temperature sensation, and joint-position sense, leading to clumsiness and an increased likelihood of burns. In the lower limbs, these problems can manifest themselves as gait difficulties, resulting in more slips, trips, falls and difficulties moving up and down ladders.
Earlier, I mentioned the multiple causes of Raynaud’s phenomenon and carpal tunnel syndrome. All these conditions – and many others – may give rise to peripheral neuropathies elsewhere in the body, although sometimes no cause can be found. Diabetes and abuse of alcohol are probably the most common causes of peripheral neuropathy in the UK, as is chronic liver disease (often the result of alcohol excess). Several medicines include neuropathy as a side effect, particularly some drugs used in cancer treatment, antibiotics like metronidazole, and phenytoin, an antiepileptic.

Alternative therapies are not always blameless, either. There was a recent case report of an Asian patient who thought he would try to treat his diabetes using Ayurvedic medication. He subsequently developed a generalised peripheral neuropathy, and the compound he was taking was found to have sufficient lead to give him a blood-lead level of 74µg/dl! (In the UK this level in a lead worker would result in suspension from work.) Following treatment with chelation therapy, blood-lead levels returned to normal, and the symptoms remitted.14

The many and varied conditions that can afflict the nervous system demonstrate the importance of having access to advice from competent occupational physicians and nurse advisors, who are able to liaise, where necessary, with the employee’s GP in order that further appropriate treatment can be organised. There is also a role for appropriately targeted health-education campaigns.

As the working population ages, these disorders may well become more common, and challenge safety advisors in devising adjustments and accommodations to enable sufferers to attend and remain in the workplace, as well as good management practice and plain common sense. 

References

1    Dobie, RA (1998): ‘Noise-Induced Hearing Loss’, in: Bailey, BJ (Ed): ‘Head and neck surgery’, in Otolaryngology, 2nd Edn, Philadelphia:Lippincott-Raven, 1998:1782-1792
2    www.hse.gov.uk/noise/
3    Palmer KT, Griffin MJ, Syddall HE, Davis A, Pannett B, and Coggon D (2002): ‘Occupational exposure to noise and the attributable burden of hearing difficulties’, in Occup Environ Med 2002,59;9:634-39
4    House R, Jiang D, Thompson A, et al (2011): ‘Vasospasm in the feet in workers assessed for HAVS’, in Occup Med, 2011, 61;2:115-120
5    Palmer KT, Griffin MJ, Syddall HE, Pannett B, Cooper C, and Coggon D (2002): ‘Raynaud’s phenomenon, vibration-induced white finger and difficulties in hearing’, in Occup Environ Med, 2002,59:640-42
6    Ross, BC (1989): ‘Noise exposure of motorcyclists’, in Ann Occ Hyg 1989,33;1:123-27
7    www.bls.gov/iif/oshwc/osh/os/osh06_29.pdf
8    Manktelow RT, Binhammer P, Tomat LR, Bril V, Szalai JP (2004): ‘Carpal tunnel syndrome: Cross-sectional and outcome study in Ontario workers’, in J Hand Surgery 2004;28A:307-17
9    Foley M, Silverstein B, Polissar N (2007): ‘The ecomonic burden of carpal tunnel syndrome: long-term earnings of CTS claimants in Washington State’, in Am J Ind Med 2007;50:155-72
10    Sperka P, Cherry N, Burnham R, Beach J (2008): ‘Impact of compensation on work outcome of carpal tunnel syndrome’, in Occup Med 2008,58;7:490-95
11     http://extoxnet.orst.edu/tibs/cholines.htm
12    Jamal JA, Hansen S, Julu PO (2002): ‘Low-level exposures to organo-phosphorous esters may cause neurotoxicity’, in Toxicology 2002;181-182:23-33
13    Ichihara G, Li W, Shibata E et al (2004): ‘Neurological abnormalities in workers in a 1-bromopropane factory’, in Environ Health Perspect 2004,112;13:1319-25
14    Singh S, Mukherjee KH, Gill KD, Flora STS (2009): ‘Lead-induced peripheral neuropathy following Ayurvedic medication’, in Ind J Med Sci 2009,63;9:408-10

Dr Chris Ide is an occupational physician and regular contributor to SHP.

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