The diagnosis of neurotoxic disease is not easy. The errors are usually of two types: either it is not recognized that a neurotoxic agent is the cause of neurological symptoms, or neurological (and especially neurobehavioural) symptoms are erroneously diagnosed as resulting from an occupational, neurotoxic exposure. Both of these errors can be hazardous since an early diagnosis is important in the case of neurotoxic disease, and the best treatment is avoiding further exposure for the individual case and the surveillance of the condition of other workers in order to prevent their exposure to the same danger. On the other hand, sometimes undue alarm can develop in the workplace if a worker claims to have serious symptoms and suspects a chemical exposure as the cause but in fact, either the worker is mistaken or the hazard is not actually present for others. There is practical reason for correct diagnostic procedures, as well, since in many countries, the diagnosis and treatment of occupational diseases and the loss of working capacity and invalidity caused by those diseases are covered by insurance; thus the financial compensation may be disputed, if the diagnostic criteria are not solid. An example of a decision tree for neurological assessment is given in Table 1.
Table 1. Decision tree for neurotoxic disease
I. Relevant exposure level, length and type
II. Appropriate symptoms insidiously increasing central (CNS) or peripheral (PNS) nervous system symptoms
III. Signs and additional tests CNS dysfunction: neurology, psychology tests PNS dysfunction: quantitative sensory test, nerve conduction studies
IV. Other diseases excluded in differential diagnosis
Exposure and Symptoms
Acute neurotoxic syndromes occur mainly in accidental situations, when workers are exposed short-term to very high levels of a chemical or to a mixture of chemicals generally through inhalation. The usual symptoms are vertigo, malaise and possible loss of consciousness as a result of depression of the central nervous system. When the subject is removed from the exposure, the symptoms disappear rather quickly, unless the exposure has been so intense that it is life-threatening, in which case coma and death may follow. In these situations recognition of the hazard must occur at the workplace, and the victim should be taken out into the fresh air immediately.
In general, neurotoxic symptoms arise after short-term or long-term exposures, and often at relatively low-level occupational exposure levels. In these cases acute symptoms may have occurred at work, but the presence of acute symptoms is not necessary for diagnosis of chronic toxic encephalopathy or toxic neuropathy to be made. However, patients do often report headache, light-headedness or mucosal irritation at the end of a working day, but these symptoms initially disappear during the night, weekend or vacation. A useful checklist can be found in Table 2.
Table 2. Consistent neuro-functional effects of worksite exposures to some leading neurotoxic substances
|
Mixed organic solvents |
Carbon disulphide |
Styrene |
Organophos- |
Lead |
Mercury |
|
|
Acquisition |
+ |
|
|
|
+ |
|
|
Affect |
+ |
|
+ |
|
+ |
|
|
Categorization |
+ |
|
|
|
|
|
|
Coding |
+ |
+ |
|
|
+ |
+ |
|
Colour vision |
+ |
|
+ |
|
|
|
|
Concept shifting |
+ |
|
|
|
|
|
|
Distractibility |
|
|
|
|
+ |
|
|
Intelligence |
+ |
+ |
|
+ |
+ |
+ |
|
Memory |
+ |
+ |
+ |
+ |
+ |
+ |
|
Motor coordination |
+ |
+ |
+ |
|
+ |
+ |
|
Motor speed |
+ |
+ |
+ |
|
+ |
+ |
|
Near visual contrast sensitivity |
+ |
|
|
|
|
|
|
Odour perception threshold |
+ |
|
|
|
|
|
|
Odour identification |
+ |
|
|
|
+ |
|
|
Personality |
+ |
+ |
|
|
|
+ |
|
Spatial relations |
+ |
+ |
|
|
+ |
|
|
Vibrotactile threshold |
+ |
|
|
+ |
|
+ |
|
Vigilance |
+ |
+ |
|
|
+ |
|
|
Visual field |
|
|
|
|
+ |
+ |
|
Vocabulary |
|
|
|
|
+ |
|
Source: Adapted from Anger 1990.
Assuming that the patient has been exposed to neurotoxic chemicals, the diagnosis of neurotoxic disease starts with symptoms. In 1985, a joint working group of the World Health Organization and the Nordic Council of Ministers discussed the matter of chronic organic solvent intoxication and found a set of core symptoms, which are found in most cases (WHO/Nordic Council 1985). The core symptoms are fatigability, memory loss, difficulties in concentration, and loss of initiative. These symptoms usually start after a basic change in personality, which develops gradually and affects energy, intellect, emotion and motivation. Among other symptoms of chronic toxic encephalopathy are depression, dysphoria, emotional lability, headache, irritability, sleep disturbances and dizziness (vertigo). If there is also involvement of the peripheral nervous system, numbness and possibly muscular weakness develop. Such chronic symptoms last for at least a year after the exposure itself has ended.
Clinical Examination and Testing
The clinical examination should include a neurological examination, where attention should be paid to impairment of higher nervous functions, such as memory, cognition, reasoning and emotion; to impaired cerebellar functions, like tremor, gait, station and coordination; and to peripheral nervous functions, especially vibration sensitivity and other tests of sensation. Psychological tests can provide objective measures of higher nervous system functions, including psychomotor, short-term memory, verbal and non-verbal reasoning and perceptual functions. In individual diagnosis the tests should include some tests that give a clue as to the person’s premorbid intellectual level. History of school performance and previous job performance as well as possible psychological tests administered previously, for example in connection with military service, can help in the evaluation of the person’s normal level of performance.
The peripheral nervous system can be studied with quantitative tests of sensory modalities, vibration and thermosensibility. Nerve conduction velocity studies and electromyography can often reveal neuropathy at an early stage. In these tests special emphasis should be on sensory nerve functions. The amplitude of the sensory action potential (SNAP) decreases more often than the sensory conduction velocity in axonal neuropathies, and most toxic neuropathies are axonal in character. Neuroradiological studies such as computed tomography (CT) and magnetic resonance imaging (MRI) usually do not reveal anything pertinent to chronic toxic encephalopathy, but they may be useful in the differential diagnosis.
In the differential diagnosis other neurological and psychiatric diseases should be considered. Dementia of other aetiology should be ruled out, as well as depression and stress symptoms of various causes. Psychiatric consultation may be necessary. Alcohol abuse is a relevant confounding factor; excessive use of alcohol causes symptoms similar to those of solvent exposure, and on the other hand there are papers indicating that solvent exposure may induce alcohol abuse. Other causes for neuropathy also have to be ruled out, especially entrapment neuropathies, diabetes and kidney disease; also alcohol causes neuropathy. The combination of encephalopathy and neuropathy is more likely of toxic origin than either of these alone.
In the final decision the exposure should be evaluated again. Was there relevant exposure, considering the level, length and quality of exposure? Solvents are more likely to induce psycho-organic syndrome or toxic encephalopathy; hexacarbons, however, usually first cause neuropathy. Lead and some other metals cause neuropathy, although CNS involvement can be detected later on.