The initial study cohort comprised 38 consecutive patients with ET who were examined at the Movement Disorder Clinic of Kangnam St. Mary's Hospital between January and December 2007. Four of these patients were excluded from the study after comprehensive neuropsychological testing revealed that they had mild dementia. Thus, a final total of 34 patients who had no history or symptoms of memory impairment or other cognitive disorders with functional impairment (as revealed by a dementia-screening questionnaire) were enrolled in this study. The evaluation procedure comprised a detailed medical history, a physical and neurological examination, a neuropsychological assessment, and brain magnetic resonance imaging. The patient's history of medical and neurological problems was obtained from the patients and family members, or from other caregivers. All patients were diagnosed as having either definite or probable ET based on National Institutes of Health diagnostic criteria,12 and all were tested for mutations of the FMR gene of fragile X tremor and ataxia syndrome. Patients were excluded from the study if 1) they had neurological abnormalities related to systemic or other neurological diseases, 2) they were taking medications reported to influence cognition, such as anticholinergics or beta-blocking agents, or 3) they were found to have an FMR gene mutation.

The age-matched controls (n=33) did not have a history or symptoms of tremor, memory impairment, or other cognitive dysfunctions, as revealed by a dementia screening questionnaire, and they had no history of other neurological diseases, such as head trauma, epilepsy, and stroke, or brain surgery. This study was approved by the local ethics committee, and each patient provided written informed consent to participate.

The patient's general cognitive status and severity of dementia were evaluated by the Korean version of Mini-Mental State Examination (K-MMSE), Clinical Dementia Rating (CDR), and the sum of box (SOB) of CDR.13-15 Several cognitive domains were assessed by conducting a detailed battery of neuropsychological tests. The tests included an attention and working memory test (forward digit span, backward digit span, and letter cancellation), a language and related-functions test (comprehension, repetition, Boston Naming Test, writing, calculation, finger naming, right-left orientation, body part identification, and praxis), a visuospatial function test (drawing an interlocking pentagons and Rey-Osterrieth Complex Figure Copy Test), a verbal memory test [registration, recall, recognition of three words, Hopkins Verbal Learning Test (HVLT), immediate recall, delayed recall, and recognition], a nonverbal (visual) memory test (immediate recall, delayed recall, and recognition of a Rey's complex figure), a frontal executive function test (motor impersistence, contrasting program, go-no-go test, fist-edge-palm, alternating hand movement, alternating square and triangle, and Luria Loop Test), the word fluency test [Controlled Oral Word Association Test (COWAT) animal, supermarket, and Korean letters], and the Stroop Color and Word Test (word reading and color reading).15-17

The statistical analyses were performed using SPSS, version 13.0. Statistical comparisons of some of the demographic data between the groups, and neuropsychological test results were performed using an independent-samples t-test. Other group differences (data for nominal variables) were assessed with the chi-square test. The relationships between the general mental functions and the age at examination, symptom duration, educational status, and Barthel index were analyzed using Spearman rank correlation coefficients. The level of statistical significance was set at p<0.05.