Mirror writing and cortical hypometabolism in Parkinson's disease

Abstract

Mirror writing (MW) is the production of individual letters, words, or word strings in the reverse direction. Parkinson's disease (PD) is a progressive neurodegenerative disorder, and high MW rates have been reported in patients with PD. Thus, the present study sought to identify the factors that cause MW in patients with PD. We examined the frequency of MW in patients with PD and investigated the area of the brain where such frequency inversely correlates with reduced regional cerebral metabolic rates of glucose (rCMRglc). We also examined whether this area satisfied the motor and visual monitoring hypotheses of MW that have been presented in previous studies. Thirty-six subjects with idiopathic PD and 23 healthy controls were included in the study. We asked the participants to write down words, numerals, and sentences from left to right using their dominant and non-dominant hands. Patients with PD underwent an 18F-fluorodeoxyglucose positron emission tomography scan to measure the rCMRglc. Neither the patients with PD nor the healthy subjects exhibited MW in the use of the right hand. In the use of the left hand, MW occurred in 15 of the 36 patients with PD, but in none of the healthy controls. The right intraparietal sulcus was identified as the area where rCMRglc was inversely correlated with the number of left-right reversed characters. Previous functional imaging studies have suggested that the right superior parietal cortex and intraparietal sulcus play an important role in recognizing left-right reversed letters. Therefore, dysfunction in the intraparietal sulcus may hinder the recognition of left-right reversed characters, resulting in MW. Consequently, our findings in patients with PD are consistent with the visual-monitoring hypothesis of MW.

Fig 1. Example of mirror writing in a PD patient.

(A)Task: Writing the word, “そら”. “ら” was reversed. (B) Task: Writing the sentence, “花がさく”. “く” was reversed. (C) Task: Writing the sentence, “今日はよいお天気です”. “今” was reversed.

Fig 2. Brain regions in which the resting CMRglc was negatively correlated with the number of left–right reversed characters.

The statistical threshold was set at an uncorrected p<0.001 at the voxel level and 20 voxels at the cluster level. Additional covariates were age, sex, levodopa equivalent daily dose, Unified Parkinson’s Disease Rating Scale Part III score, Mini–Mental State Examination score, forward and backward digit span, and forward and backward tapping spans. The results were overlaid on rendered brain (top row) or T1 single-subject template (bottom row) from SMP 12.