Epidemiological study of the relationships between exposure to organophosphate pesticides and indices of chronic peripheral neuropathy, and neuropsychological abnormalities in sheep farmers and dippers. Phase 3. Clinical neurological

a. Summary of aims: In 1994, the then Minister of Agriculture, Fisheries and Food announced that the Government had accepted advice from the Medical and Scientific Panel of the Veterinary Products Committee that there should be further research into the effects on health of organophosphate (OP) sheep dips. Subsequently, the Health and Safely Executive (HSE), the Department of Health (DoH) and the Ministry of Agriculture, Fisheries and Food (MAFF) jointly commissioned a major epidemiological study into the effects of long-term exposure to OP sheep dips. This study was carried out between November 1995 and April 1999 by the Institute of Occupational Medicine in Edinburgh and the Institute of Neurological Sciences in Glasgow. The conduct and results of the research are being reported in three companion volumes, of which this report is the third.The broad aim of the study as a whole was to investigate whether cumulative exposure to sheep dip OPs is related to clinically detectable measures of polyneuropathy. The specific objectives of Phase 3 were to: classify in terms of clinical disease the subjects with abnormal indices of peripheral neuropathy identified in the Phase 2 field studies; describe any associations between neurological and neuropsychological abnormalities; and examine any evidence for a relationship between neuropsychological status and estimated cumulative OP exposure.This was achieved by a nested case-control clinical study of selected subjects with neurological abnormalities identified in the field studies, and of control subjects. This investigation aimed to provide additional information about the nature of any neuropathies found, and included a neuropsychological component which had not been practicable in the field investigation.A recent report by the Royal Colleges of Physicians and Psychiatrists (1998) suggests that reports of chronic low dose effects of OPs are limited by small numbers of cases, selection bias and inadequate controls. It also considered that some cases may be the result of undocumented episodes of acute exposures. A review by ECETOC (1998) concluded that animal experiments confirm acute and protracted effects on cognitive function, but have not demonstrated effects of prolonged low exposure. Both these reports suggest that the issue of OP exposure and health remains important and controversial.b. Methods: A subset of subjects involved in the Phase 2 field study were invited to participate in the clinical studies at the Institute of Neurological Sciences (INS) in Glasgow. Recruitment was carried out by the IOM. To ensure a wide representation of disease status in the clinical study, the objective was to select individuals in equal numbers from the ‘no’, ‘possible’ and ‘probable/definite’ groups as derived from the field survey data, using the original neuropathy score. Seventy nine subjects attended assessments at INS, and 76 were included in the study group, comprising 17, 36 and 23 subjects respectively from the ‘no’, ‘possible’ and ‘probable/definite’ categories in the field study. All 79 were sheep farmers. No ceramics workers were invited to attend the clinical study, and of the few non-exposed farmers invited, none in fact participated.The symptoms questionnaire used in Phase 3 was the same as that used during Phase 2 epidemiological survey, but excluded details of occupation or details of relevant occupational exposure, and was administered by a neurologist. This was followed by a clinical assessment based on the Mayo Clinic criteria. The same range of sensory tests (QST) were performed in Phase 3 as in the Phase 2 studies. Additional tests included nerve conduction and electromyography. During the clinical assessments none of the INS personnel were aware of the subjects’ occupational exposure or the neuropathy classification derived from the results of the Phase 2 field study.A battery of neuropsychological tests was performed to assess the following functions; General Intelligence; Psychomotor Function; Attention ; Memory; Mood and Affect. General Intelligence was used as a control variable and measured using the National Adult Reading Test. A verbal IQ estimate was derived based on the number of words read correctly from a standard list.The CANTAB battery was developed for the assessment of cognitive defects in humans with degenerative brain disease. CANTAB has been used in early identification of progressive neurological disorders, and in toxicological assessments for acquired disorders such as alcoholism. Standardised data are therefore available on normal volunteers for all the tests (Robbins et al. 1994).A questionnaire on recent exposure to OPs was administered to all Phase 3 participants by an independent nurse. The format used was similar to that of the Phase 2 exposure-history questionnaire. Five individuals had handled concentrate in the two weeks prior to the commencement of Phase 3.c. Main findings:Reproducibility of symptoms and QST: The study was designed from the outset to include a detailed clinical examination of selected subjects who had earlier participated in the field surveys. On comparing the field (Phase 2) and clinic (Phase 3) classifications using the neuropathy scoring system, although agreement was better than chance, it was not considered reproducible enough as a basis for exposure-response modelling of disease status in the field. The symptom score component proved reproducible. The sensory thresholds were less reproducible, although this was felt to be due to comparison of thresholds measured in the field with an inappropriate reference population.There were major inconsistencies between field study QST measurements and the clinical reference values. Measurements had been taken during training by all technicians. Some of these had been in the clinic and others outwith the clinic. None had indicated any likelihood of the problems found. However, none had been taken under the kinds of temperature extremes which occurred later during the actual field survey. There are strong grounds for believing that QST results may be sensitive to individuals’ limb temperature and related core temperature.Additional neurological tests: Three individuals had the following conditions recorded on their Phase 2 neuropathy questionnaires: rheumatoid arthritis, family history of high arches, and hypertension requiring medication which was associated with hypotension. It was decided to err on the side of caution and exclude them from remaining Phase 3 analyses to limit possible confounding. A further four subjects were found to have a profile of abnormalities of neurophysiological parameters not consistent with generalised neuropathy. Three had carpal tunnel syndrome and one subject had findings consistent with radiculopathy. These subjects, two each from of the ‘no neuropathy’ and ‘possible neuropathy’ groups, were excluded from further analysis. This left a total of 72 subjects for analysis.The groupings described in the following paragraphs are all based on the Phase 3 neuropathy score for symptoms and QST. Twenty three (32%) out the 72 subjects had confirmation of their neuropathy by neurological signs or nerve conduction abnormality. One (7%) of the 15 subjects from the ‘no neuropathy’ group had abnormal nerve conduction but no clinical (signs and symptoms) or QST evidence of neuropathy. Three subjects from this group had abnormal EMG.Ten (29%) of the 34 individuals classified as having ‘possible neuropathy’ had evidence of neuropathy. Three (9%) of these showed only clinical evidence including neurological signs and symptoms/abnormal QST. One of the three also had abnormal EMG. The remaining seven (21%) showed symptoms/abnormal QST suggestive of neuropathy together with evidence of abnormal nerve conduction. A further six had abnormal EMG in distal muscles without neurological signs or abnormal nerve conduction.Of the 23 subjects classified as having ‘probable/definite neuropathy’, twelve (52%) showed evidence of peripheral neuropathy. Four (17% of 23) of the twelve had neurological signs and symptoms/abnormal QST and two of these also had abnormal EMG. Eight (35%) had of abnormal nerve conduction and symptoms/abnormal QST. Six of the eight had abnormal EMG. A further three had abnormal EMG without neurological signs or abnormal nerve conduction.Thirteen (18%) of the 72 subjects had sensory abnormalities defined as abnormal sural conduction and one or more abnormal QST values while only two subjects (3% of 72) had abnormal motor nerve conduction and both were in the definite neuropathy group. Forty seven subjects (65% of 72) had abnormal small nerve fibre function, assessed by hot or cold sensation threshold, while only 15 (21% of 72) had abnormal large fibre function, assessed by vibration threshold or sural nerve function. Thus, small fibre dysfunction was three times more common than large fibre dysfunction. In defining the relative involvement of small versus large and sensory versus motor fibre populations, one must remember that methods used to assess them may not have identical sensitivity.Autonomic nervous system (ANS) symptoms were reported more commonly than peripheral nervous system (PNS) symptoms in the phase 2 study. This is also the case in the phase 3 study for the ‘no neuropathy’ and ‘possible neuropathy’ groups. Sensory symptoms were more commonly reported than motor symptoms.Neuropsychologicalfindings Subjects classified in the clinic as being ‘probable/definite’ cases of neuropathy had poorer self-reported general mental health and experienced greater self-reported anxiety and depression than other subjects less likely to be diagnosed as having neuropathy.Allowing for age and IQ, there was some evidence of slower processing times among ‘probable/definite’ cases of neuropathy. However, the results, across a variety of such tests, were not consistent and did not provide clear evidence of an overall slowing of processing time.Also, allowing for age and for general IQ, there was no evidence of a difference in memory capability between probable cases of neuropathy and ‘no neuropathy’ controls.The results did not show that the neuropsychological findings were related to cumulative exposure to OPs, but it was acknowledged that the study design would have limited power to examine such a relationship.d. Key findings: The neuropathy described in Phase 3 is predominantly of a sensory type both clinically and neurophysiologically and is characteristic of distal, chronic neuropathy with no acute features. Small fibre populations are affected more than large fibre populations. The results of the additional tests (clinical examination and nerve conduction) therefore corroborate the other aspects of the Mayo methods in detecting a possible toxic neuropathy in the clinical studies.Increasing severity of neuropathy, as based on symptoms and sensory tests in the clinic was associated with anxiety and depression as measured in the neuropsychological tests. The results did not show that the neuropsychological findings were related to cumulative exposure to OPs, but it was acknowledged that the study design would have limited power to examine such a relationship.The implications of these findings are considered in more detail in a summary of all three Phases which can be found in this Phase 3 report. “”