Vibrotactile threshold and pin-prick sensitivity as indicators of subclinical changes in somatosensory function: effects of environmental exposure to arsenic in drinking water

DA Otto1, HK Hudnell1, Y-H Li2, YJ Xia2
1US Environmental Protection Agency, Research Triangle Park, NC 27711 USA
2Institute of Endemic Disease, Huhhot, Inner Mongolia

Introduction. Peripheral neuropathy results from exposure to a variety of neurotoxic compounds. Clinical assessment of peripheral neuropathy is commonly based on electrophysio-logical measures of nerve conduction velocity (NCV). However, data on early, subclinical changes in neurologic function resulting from exposure to neurotoxicants are needed for human health risk assessment. Behavioral measures of vibrotactile thresholds (VTT) and pin-prick sensitivity (PPS) provide non-invasive methods for assessing somatosensory function that are suitable for use in field studies. VTT measures have previously identified subclinical effects from exposure to organic solvents, jet fuel, elemental mercury and organophosphate pesticides (1). The current study investigated the effects of environmental exposure to arsenic in drinking water on somatosensory function using VTT and PPS techniques.

Methods. A population of farm families living in the Bamen region of Inner Mongolia chronically exposed to arsenic in well water were screened for potentially confounding factors. 309 residents (mean age 35.8 +/- 12.8 years) qualified for participation. Subjects were assigned to three exposure groups based on arsenic levels in individual wells as follows: low (<20 ug/L; N=97), medium (100-300 ug/L; N=109) and high (400-700 ug/L; N=103). VTT was measured on the dorsal surface of digit 2 and the ventral surface of digit 5 on each hand. PPS was assessed on both arms and both legs.

Results. VTT was significantly elevated and PPS was significantly reduced in the high exposure group relative to both the medium and low exposure groups. The data suggested a larger exposure effect on PPS than VTT.

Conclusion. These results indicate that subclinical effects of chronic arsenic exposure on somatosensory function are observable only at exposure levels well above those associated with increased risk for cancer. Differences between the VTT and PSS results suggest that small diameter unmyelinated axons with free nerve endings in superficial skin layers may be more susceptible to exposure than the larger diameter and myelinated axons with Pacinian corpuscle receptors located in deeper skin layers. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.

(1) D Mergler. Behavioral Neurophysiology: Quantitative Measures of Sensory Toxicity. In: LW Chang & W Slikker (eds). Neurotoxicology: Approaches and Methods, New York, Academic Press, 1995, ch47 pp727-736.