Cognitive functioning in patients with suspected chronic toxic encephalopathy: evidence for neuropsychological disturbances after controlling for insufficient effort

OBJECTIVES: Chronic toxic encephalopathy (CTE) caused by long term occupational exposure to organic solvents is still a controversial disorder. Neuropsychological testing is the cornerstone for diagnosing the syndrome, but can be negatively influenced by motivational problems. In this nationwide study, we investigated the neuropsychological functioning and psychological symptoms of a large group of patients with suspected CTE, and ruled out alternative explanations for their complaints, including suboptimal performance due to insufficient effort. METHODS: We studied participants with suspected CTE (n = 386) who were referred for further diagnosis to the Netherlands Centre of Occupational Diseases in the period 1998-2003 and who had completed the entire diagnostic protocol. Patients were excluded if there was the slightest suspicion that test performance had been negatively influenced by insufficient effort (n = 221), or if comprehensive assessment identified an alternative diagnosis (n = 80). Insufficient effort was defined by a combination of three indices. The neuropsychological test scores of the patient group (n = 85) were compared with those of a control group of building trade workers matched for sex, age, and educational level (n = 35). RESULTS: The patient group had significantly more psychological complaints and performed significantly worse than the control group on tests of speed of information processing and memory and learning. However, only a small percentage of the patients had clearly abnormal scores for cognitive speed (9%) or memory (8%). Attention, verbal abilities, and constructional functions were not disturbed. Exposure duration and cognitive complaints were significantly correlated, whereas the correlation between exposure duration and neuropsychological domain scores was not significant. CONCLUSIONS: Insufficient effort was present in a substantial part of the patient group. After minimising the likelihood that insufficient effort negatively influenced neuropsychological scores, we still found neuropsychological deficits in speed of cognitive processing and memory; however, these scores were clearly abnormal only in a minority of patients with suspected CTE. Screening instruments should focus on these domains.