Effects of lead on the vestibular system: Preliminary findings

Abstract

Twelve adult squirrel monkeys were subjected to chronic lead exposure for a period of five to 24 weeks. Poisoning was confirmed by elevated blood lead levels, a consistent drop in body weight and by neurological signs of paresis and convulsions. Five animals died as a direct result of lead poisoning, and the remaining seven were sacrificed either when death seemed imminent (two animals) or at critical times during the experimental schedule. Bilateral cold and warm water irrigations as well as body rotational tests (upright, right side down and left side down, clockwise and counterclockwise), were given prior to lead poisoning and at various times during the experimental period. Positional and induced nystagmus (horizontal for caloric stimulation and both horizontal and vertical for rotatory stimulation) were recorded with eyes closed, by electronystagmography. Auditory detection thresholds for bone‐conducted pure tones were measured in two monkeys by a conditioned avoidance response technique. There was great intra‐ and inter‐subject variability in nystagmus duration and intensity (total number of eye beats and beat rate) for pre‐lead and lead intoxication conditions. Analysis of records was made difficult by the occurrence of positional and provocation nystagmus. Four of the 12 monkeys exhibited some positional nystagmus (mostly vertical when placed on either side) during the pre‐lead phase. Six of the group showed enhanced positional nystagmus or developed it during the lead intake phase. In general, both caloric‐ and rotational‐induced nystagmus decreased slightly in median duration and intensity under lead poisoned conditions, but the differences were not statistically reliable. Control measures of right‐left labyrinthine sensitivity and directional preponderance did not differ significantly from measures obtained under chronic lead intake conditions. After 20 weeks of lead intoxication, three animals showed a directional preponderance opposite that seen at the beginning of the study. Examination of the temporal bones of three monkeys revealed neither hair cell damage nor demyelination of VIIIth nerve fibers. Bone conduction detection thresholds remained within normal limits. A generalized state of muscular weakness due to chronic lead absorption could explain the unsteadiness which may have been mistaken for vestibular ataxia, reported by others in the literature.

Effects of Lifetime Lead Exposure in Monkeys on Detection of Pure Tones

“…Consistent with these results, high-dose lead exposure resulted in segmental demyelination of the VHIth nerve with no effect on cochlear morphology in developing guinea pigs (Gozdzik-2olnierkiewicz and Moszyriski, 1969). In contrast, no damage to the VHIth nerve or hair cells was observed in adult monkeys exposed from 5 to 24 weeks to doses of lead sufficient to produce weight loss and muscle weakness (Wilpizeski, 1974). Minimal changes were observed in vestibular function; two monkeys in which bone conduction detection thresholds were determined between 125 and 16,000 Hz showed no impairment in function.…”

Experimental Lead Paint Poisoning in Nonhuman Primates

“…Experimental lead intoxication of nonhuman primates has not, despite these facts, been thoroughly studied. In several experimental poisonings, simian primates were given inorganic lead com pounds intravenously [7-9, 18, 29], intratracheally [19], by inhalation [5,16], 1 or by a combination of methods [30]. Two reports describe infant to juve nile baboons fed small amounts of organic lead in paint [9,27], Oral admi nistration of inorganic lead has been reported in 16 adult [15], 20 juvenile [9,21,27], four infant [6], an unspecified number of infant, juvenile, and adult rhesus monkeys [2,11,24], and in eight infant [9] and one juvenile baboon [8].…”

Experimental Lead Paint Poisoning in Nonhuman Primates II. Clinical Pathologie Findings and Behavioral Effects

“…Symptoms of lead poisoning in children may occur with blood lead levels of as little as 50 μg/dl, but generally, the blood contains over 100 μg/dl (24). Blood levels of 200 μg/dl or more are reported when clinical encephalopathy has occurred in monkeys accidentally (18, 36, 38) or experimentally (2, 10, 17, 34) poisoned by lead. Blood lead values in this study were related to various factors which include dose of lead, form of lead (different lead‐containing paints or lead acetate), age of monkey (37), and, as might be expected, blood lead levels were related to physical condition at termination.…”