A 68-year-old man was referred to our hospital with tension-type headaches and a 1-year history of dementia. On neurologic examination, he had ideomotor apraxia and incomplete Gerstmann syndrome that was characterized by acalculia, agraphia, and finger agnosia. On imaging, multiple cystic lesions reported as "unusually dilated perivascular spaces" were observed along the medullary arteries in the left hemisphere; some of them had adjacent hyperintense areas in fluid attenuated inversion recovery images. We assumed that the multiple cystic lesions caused his higher cerebral dysfunction, because ideomotor apraxia and Gerstmann syndrome are usually indicative of a left parietal lobe lesion. MR spectroscopy in the lesion site revealed increased lactate. On MR angiography, the left middle cerebral artery and the left posterior cerebral artery were poorly visualized without localized stenosis. Technetium-99 bicisate single-photon emission computed tomography showed severely decreased cerebral blood flow in the left hemisphere. Electroencephalography showed slow waves in the left hemisphere.