Sleep disorder in occupational exposure to Hg vapours

J. A. Scopoli presented his knowledge on occupational Hg° exposure of miners and smelters in the Idrija Mercury Mine in his book entitled DE HYDRARGYRO IDRIENSI TENTAMINA Phisico - Chimico - Medica, which was printed in Venice in 1761, and reprinted in 1771. In the third part of this book, De Morbis Fossorum Hydrargyri, he presents an in-depth description of the symptoms of mercury intoxication - mercurialism among pit and smeltery workers. He classifies mercurialism according to those symptoms that are the most pronounced in the disease pattern. Scopoli describes acute, sub-acute and chronic Hg intoxication appearing during work in the smelting plant and in the pit, in poorly ventilated sites with native ore where, according to our present-day knowledge (Kobal, 1994), mercury vapour concentrations were extremely high. Among the symptoms accompanying chronic intoxication, Scopoli mentions changes in some personality traits, such as bad temper, irritability and sadness, as well as sleep disorder. "...somnus inquietus, somnia terrífica, artuum agitatio..." are the key words which Scopoli uses (1771, p. 80). He finds that mercury intoxication is accompanied by restless sleep, terrible dreams with nightmares, sleep terrors, and strange, periodic contractile movements of the legs (Kobal & Kobal-Grum, 2010). The reputed clinical toxicologist, Adolph Kussmaul, presented in his book (1861, p. 227) an occupational clinical picture of mercurial intoxication in miners. Among the symptoms of eretism-increased irritability, he also mentions "restless sleep, terrible dreams and frighten awakenings".

Our observations are based on data collected from the program of health surveillance of workers exposed to Hg° in the Idrija Mercury Mine. In the first 20 years following the Second World War, the number of Hg° intoxicated workers was very high (ranging from 10 to 14% of workers in the mine and smelting plant). After 1975, no new cases of intoxication were observed thanks to the introduction of preventive-target medical examinations, which, after 1968, also included biological monitoring of exposure. Subjective descriptions of sleep disturbances and other potential, known, subjective troubles associated with Hg° exposure were always evaluated directly by the physician during contact with intoxicated or exposed workers in the course of preventive target examinations. No polysomnography recording was used to define the stages of sleep in intoxicated workers, or in workers with increased Hg° absorption. Some disordered sleep, such as fragmentation of sleep accompanied with dreaming and awakening, as well as periodic leg contractile movements, were often observed as some important early symptoms that announced the critical absorption of Hg° vapours in miners working in the pit where native Hg ore was mined, with substantially elevated air Hg° vapour levels. During the target medical surveillance and biological monitoring of miners intermittently exposed to native Hg, the previously mentioned sleep disorder appeared in 30% of exposed miners, associated with increased urinary Hg excretion. In these miners, the urine Hg concentrations were usually within a range of 100400 |g/L, which is, at intermittent type of exposure, associated with blood Hg levels from 60 to 260 |g/L (Kobal, 1975a, 1991), which are substantially above the blood Hg level of 35 |g/L usually accompanied with the earliest nonspecific symptoms (WHO, 1976). In cases of subacute mercurialism with classical signs of intoxication, such as stomatitis, limb tremor, and other known symptoms and signs, the sleep disorders were much more pronounced, and the urinary Hg excretions were very high, in some cases even over 700 |g/L (Kobal, 1975b, 1991). The periodic leg movement index was not evaluated in these miners (calculated by dividing the total number of periodic leg movements by sleep time in hours). In the cases of increased Hg° absorption, the sleep disorder decreased usually in one to two months after the interruption of exposure associated with decreased urine Hg level. In the cases of Hg° intoxication, sleep disorders with terrible dreams and and periodic leg movements were much more obstinate and disappeared very slowly in association with other symptoms and clinical signs of mercurialism; the urine Hg level decreased after 3 to 6 months. A subclinical peripheral nerve function with lower motor conduction velocities of the median nerve and lower sensory conduction velocities of the ulnar nerve was observed in the subgroup of miners with long-term intermittent exposure and increased Hg° absorption (urine Hg excretion > 100|g/L). In contrast to sleep disorder, these subclinical pripheral nerve function changes usually persist many years after the cessation of exposure (Gabrovec-Nahlik et al., 1977; Kobal et al., 2004), which is also in agreement with some other observations (Albers et al., 1982).

As already mentioned above, sleep disorders were also mentioned in the monographs on inorganic-elemental mercury published by WHO (1976, 1991) and ATSDR (1999), which place them among the symptoms of erethism. However, no disorders of sleep structure or any possible neurobiological or biochemical mechanisms and EEG changes that could accompany sleep disorders in intoxicated subjects exposed to Hg° are described in these monographs.

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