パーキンソン病の視覚性認知障害

Impairment in Visual Cognition in Patients with Parkinson Disease

はじめに

　パーキンソン病（Parkinson disease: PD）の視覚性認知については，さまざまな障害が報告されている。その中にはかなり確立したものもあるが，真に視覚の問題なのか見極めにくいものも含まれている。それらの障害の原因となる病巣については，不明な部分がさらに多い。しかし，過去の研究の積み重ねにより，ある程度の見通しは語りうる段階にある。以下，PDの視覚認知障害について，

　Ⅰ．視覚経路とパーキンソン病の病理変化，

　Ⅱ．要素的視覚の障害，

　Ⅲ．複雑な視覚認知の障害，

　Ⅳ．複雑な視覚認知障害と脳機能画像，

　Ⅴ．中間段階の視覚認知障害，

　Ⅵ．中間段階の視覚認知障害と脳機能画像，

の順で述べる。

　近年，レビー小体型痴呆（dementia with Lewy bodies: DLB）と呼ばれる疾患が注目されている。これは，痴呆に加え，動揺する認知障害，パーキンソニズム，幻視，転倒，失神，一過性の意識消失，抗精神病薬に対する過敏性，系統的妄想，うつ，レム睡眠行動異常などを特徴とする疾患である。一方，PDも，経過とともに認知障害を併発し，痴呆と呼べる段階に達する例の少なくないことが明らかになっており，痴呆を伴うパーキンソン病（Parkinson disease with dementia: PDD）と呼ぶ。DLBとPDDの特徴は共通^1）で，病理学的にも共通性があり，同じ疾患の表現型の違いにすぎないという考え方が優勢である^2）。以下の記述では，痴呆のないPDについてを中心に述べるが，必要に応じてPDDやDLBでの研究結果を援用することにする。

Abstract

　Neural pathway for visual information processing involves retina, lateral geniculate body, primary visual cortex, and higher visual cortical areas, all of which have been reported to be disordered either functionally or pathologically in Parkinson disease (PD).　As elementary visual disorders, there have been studies that reported reduced contrast sensitivity for middle to high spatial frequencies and impaired blue color perception.　Most of those studies suggested retina as the damaged cite that is responsible for the impairments, whereas some studies pointed to the possible cortical involvement.　Impairments of higher visual functions also have been reported.　In the dorsal stream, impairments of object localization, depth perception, and mental rotation have been reported.　In the ventral stream, object perception and visual integration of objects have been found to be impaired.　A meta-analysis study, however, concluded that although there may be impairments in higher order functions like attention and problem solving capacity there is no firm evidence for the impairments of higher visual functions.　Neuroimaging studies have found a relationship between reduced metabolism centered in the parietal lobe and impaired performance in higher visual functions.　Impaired identification of overlapping figures has been reported in dementia with Lewy bodies a disease that is akin to PD.　Capacity to discriminate textured areas has been found to be damaged in PD.　We conducted a FDG-PET study to explore the relationship between brain metabolism and perception of overlapping figures, perception of shapes defined by texture differences and perception of subjective contours in PD.　It revealed that there is a correlation between reduced activation in lateral occipital complex and impaired performance for these tasks, suggesting some compromised ventral rout functions.