Pathogenic and clinical aspects of polyneuropathies, with reference to the hand-arm vibration syndrome.

TASKINEN H. Pathogenic and clinical aspects of polyneuropathies, with reference to the hand-arm vibrationsyndrome. Scand J Work Environ Health 13(1987)363-366. Alongwithattacks of whitefinger,symptoms suggesting peripheral sensorimotor neuropathy, ie, polyneuropathy or entrapment neuropathy, are veryimportant in the hand-arm vibration syndrome. Peripheral neuropathiesare probably associated with the occurrence of the syndrome becauseof a selectionmechanism. Polyneuropathy may be a contributing factor in the developmentof entrapment neuropathies in the upper extremities. It has multiple pathogenic mechanisms and numerous causative factors. However, peripheral nervescan react to pathological stimuli in a limited number of ways. Wallerian degeneration, segmentaldemyelination, and axonal degeneration are the classical neuropathological types of peripheral neuropathies, of which the first two are possibledirect consequences of vibration exposure. The clinical manifestations of poly neuropathy range from sensory to motor types, sometimeswithautonomic involvement. Wheneverpoly neuropathy is encountered in the hand-arm vibration syndrome, its etiologicpossibilitiesshould be con sidered. Regardless of the variablecriteria usedbydifferent authors, individualdiagnosisof the syndrome isalwaysa probability diagnosis, and adequate neurologicaldifferential diagnosticshaveto be employed.

Among the multiple symptoms and signs of the handarm vibration syndrome, those due to disturbance in the function of the nervous system and White-finger attacks are the most important. Paresthesias, numbness, muscular weakness, and atrophy may be signs of sensorimo to r peripheral neuropathy. Most clinical studies on the syndrome have primarily emphasized stru ctures and functions of the upper extremities. Regardless of the variable diagnostic criteria used by different authors for the hand-arm vibration syndrome, entrapment neuropathies of the upper extremities, notably the carpal tunnel syndrome, comprise an important differential diagnostic problem, as discussed by, eg, Dawson et al (I). It is also obvious that more proximal nerve entrapments may confuse the clinical picture (9). Moreover, patients with more generalized peripheral neuropathy or weakness of the peripheral nerves tend to be diagnosed more easily as having the synd rome (8). In neurological examinations of the se patients , other possible etiologies of neurological disturbances have to be carefully excluded. Some general aspects of polyneuropathy, which is very important when the hand-arm vibration syndrome is being considered, are discussed in this paper.

Polyneuropathy
Polyneuropathy refers to a diffuse symmetric involvement of the peripheral nervous system, and it may I Institute of Occupational Health, Helsinki, Finland. Reprint requests to: Dr J Juntunen , Department of Occupational Medicine, Institute of Occupational Health, Topeliuksenkatu 41 a A, SF-00250 Helsinki, Finland. occur in many diseases. Polyneuropathies are classified into the following four broad categories according to their etiology: nutritional polyneuropathy, metabolic polyneuropathy, toxic polyneuropathy, and polyneuropathy due to drugs (table 1).
In spite of the heterogeneity of the pathogenic mechanisms of peripheral neuropathies [see for instance, Dyck et al (2)] the peripheral nerve reacts only in a limited number of ways to pathological stimuli. From the neuropathological point of view, peripheral neuropathies can be classified into those following mechanical trauma (Wallerian degeneration), those due to primary involvement of the axon (axonal neuropathies), and those with primary involvement of the Schwann cells (segmental demyelination). For further details, see figure I. In the hand-arm vibration syndrome the two possible neuropathological mechanisms are classical Wallerian degeneration through direct trauma caused by vibration and segmental demyelination through ischemic damage to the nerve from vasospasms.

Clinical picture
The clinical manifestations of polyneuropathies range from pure sensory to sensorimotor or motor types. Autonomic polyneuropathy is also often present, although seldom without simultaneous sensorimotor polyneuropathy (7). In practice, acute, subacute, and chronic types of polyneuropathy can be distinguished. The progressive sensory disturbances of the stockingglove distribution, together with distal muscular weakness and diminished or absent muscle tendon reflexes, should be examined in each individual presenting symptoms typical for polyneuropathy. The symptoms  A 8 c are usually burning feet or other senso ry disturbances which occur symmetricall y and the degree of which usually increases at night. The spatial distr ibution of typical sensory deficits in vario us stages of polyneuropathy is shown in figure  2. Sensory deficit extends above the knee befo re the hand is affec ted (figure 2 B). When the deficit extends to the pro ximal parts of the extremities, a typical sensory defect, the tear-drop defect, is present over the abdom en (figure 2 C). In contrast to the border of hysteric sensory deficits, the border between normally and abnormally sensitive skin is not sha rp. Diminished muscle tendon reflexes are usually first observed in the gastrocnemius muscles and thereafter in more proximal muscles.
Th e characteristic development o f the signs of polyneuropathy is due to loss of function of the nerves according to their length , regardless of the und erlying neuro pathological mechani sm.

Diagnostic criteria for polyneuropathy
The diagnostic criteria for polyneuropathy are the following (7): I. The pat ient has subjective symptoms comp atibl e with polyneuropathy.
2. The clinical examin ation reveals diminished or absent muscle tendon reflexes , mu scular atrophy and weakne ss, and typical sensory deficits.
3. Electroneuromyography shows at least one abnormally slow nerve conduction velocity in both the upper and the lower extremitie s or two abnormally slow nerve conduction velocities in the lower extremities. Evidence of neurogenic muscle damage is also observed.
If a case fulfills two of these diagnostic criteria , the diagnosis of pol yneuropathy can be considered established. The importanc e of polyneuropathy in the handarm vibration syndro me lies in the fact that diseased or "weak" peripheral nerves, regardless of the etiology, are less resistant to additional exogenou s factor s such as vibration exposure. The same hold s tru e with regard to entrapment neuropathies, which are often early manifestations of underlying polyneuropathies.
Therefore, patients with mild periph eral neuropathy are likely to be found among those patients with suspected hand-arm vibration syndrome (8). It should be pointed out that knowl edge on the epidem iology of pol yneuropathies is rather limited . For some types of polyneuropathies, data are available on the prevalence or incidence, but the figures vary greatly. In a norm al population, signs suggesting peripheral neuropathy can be detected in less than 5 0/0, depending, of course , on the diagnostic criteria. The pre valenc e of these finding s increa ses with aging. In a recent field study on forest workers in Finland (3), the prevalence of peripheral neuropathy of the polyneurop athic type was 4 %.

Diagnosis of occupationally related hand-arm vibration syndrome in Finland
The diagnostic criteria for occupationally related handarm vibration syndrome in Finland are the followin g: I . Verified relevant vibration exposure.
2. Typical clinical picture which includes typical subjecti ve symptoms (attacks of white finger, numbness, paresthesias, loss of dexterity) and objective findin gs in a clinical examination on peripheral vasospasm and /or neuropathy dom inant in the uppe r extrem ity.
3. Primary Raynaud's disease, idiopathic Raynaud' s syndrome, and peripheral neuropathies of other etiologies reasonably well excluded.
The first requisite for the diagno sis of the hand -arm vibration syndrome is "sufficient" exposure to handarm vibration from a vibrating tool. Among lumberjack s the mean latency for the development of vibration-induced white finger was 5 600 workhours, corresponding to four to five years of professional use of chain saws (II). In a special type of pedestal grinding at a foundry, however, the mean laten cy for white finger symptoms was onl y 10.3 months (14). The frequency, acceleration, and impulsiveness of the vibration determine the vibration hazard (13).
The typical symptoms of vibration disease are Raynaud's phenomenon, numbness and paresthesias in the hand s, and , in advanced cases, muscle fatigue and loss of manual dexterity.
Objec tive signs of the vasospastic attacks or defects in neurological functions are required for the diagnosis of the syndrome. Various cold provocation tests have been used to verify Raynaud's phenomenon. If Raynaud's phenomenon is detected by inspection only , a negati ve result does not exclude the possibilit y of the syndrome (10). With plethysmographic methods , better sensitivity in cold provocation tests has been achieved (10). The role played by the autonomic nervous system in vasospasm is unclear, but interesting inferences have been made on disturbances in autonomic functions (4).
Epidemiologic studi es in Finland have shown that the prevalence of the most prominent symptom of the hand-arm vibration syndrome, ie, attacks of white finger, decreased clearl y with a decrease in vibration exposure (12). However, numbing symptoms did not correlate with vibration exposu re (5). In this respect, the possibilit y of ner ve entrapments (9) or polyneuropathy (8) may expla in the continuing symptoms. It is conceivable that the polyneuropathy associated with the hand-arm vibration syndrome is more severe in the upper extremities. In fact, this phenomenon should be one of the diagnostic criteria of the syndrome in cases in which polyneuropathy is encountered.
In conclu sion, neurological differential diagnostics of the hand-arm vibration syndrome consists of the exclusion of entrapment neuropathies and polyneuropathies of other origin. The occurrence of these disorders does not exclude the possibility of a simul taneous occurrence of the hand-arm vibration syndrome. If a polyneuropathy is dominant in the upper extrem ities, the diagnosis of the syndro me is more obvious.