n-HEXANE POLYNEUROPATHY
We read with great interest the case report of focal conduction block in n-hexane polyneuropathy by Chang et al. 1 They expressed the opinion that in n-hexane polyneuropathy focal conduction block is rare. [2][3][4] We evaluated the frequency of such block in our material of 35 patients (24 men and 11 women; age 18-49, mean 29.7 years) with subacute n-hexane polyneuropathy. In all cases the disease was a result of occupational exposure to n-hexane in small purse-makers' factories. Toxicological investigation revealed markedly increased concentration of n-hexane in the glue used in these factories at the time. Clinical and laboratory evaluation excluded other causes of polyneuropathy.
The clinical course was typical, with numbness and burning paresthesias, more pronounced in the feet than hands, as the first symptoms. Sensory symptoms were followed by weakness, sometimes severe, of the lower extremities. Worsening of symptoms after termination of the exposure was common.
The initial neurophysiological examination was done 4-6 weeks after onset of clinical symptoms. Motor nerve conduction studies of 127 nerves (70 peroneal, 36 median, 17 ulnar, and 4 tibial) were performed. Focal conduction block [defined as more than 50% reduction in both the compound muscle action potential (CMAP) amplitude and the negative-peak area] was found in 8 (22.9%) patients; in 5 (14.3%) cases it was present in more than one nerve. It was more frequent in the peroneal (8 persons) and tibial (4 persons) nerves than in nerves of the upper extremities (1 median and 1 ulnar nerve). Amplitudes of CMAPs were normal or only moderately reduced after distal stimulation of these nerves. The presence of block was associated with rather moderate slowing of the conduction velocity (usually to not less than 30 m/s in lower extremities) and prolonged distal latencies. The second examination revealed dramatically diminished CMAPs and focal conduction block was no longer observed. The worsening of neurophysiological parameters paralleled the progression of clinical impairment.
In our opinion, focal conduction block is a relatively frequent finding in early stage of n-hexane polyneuropathy and can be the main cause of the initial clinical symptoms, as suggested by Kuwabara et al. 2 However, it is rarely seen in more than one nerve, in contrast to inflammatory demyelinating polyneuropathy.