[A case of primary progressive multiple sclerosis with onset of memory impairment].
We report a 52-year-old woman with primary progressive multiple sclerosis (PPMS) presenting with chronic progressive memory impairment. From a couple of years prior to admission, she had developed impairment of her short-term memory. For example, she forgot her nephew's name, and spoke the same phrases again and again. She also sometimes forgot to turn off her gas stove and forgot things she bought in shops. Moreover, her mental activity gradually decreased and she became apathetic. However, she did not note her memory impairment, and had no hallucinations. She was admitted to our hospital on 20 May, 2003 because donepezil had been ineffective for treating her memory impairment. Neurologically, she showed bilateral horizontal gaze nystagmus, mild limb ataxia on the left and mildly ataxic gait. Neuropsychological examinations showed mildly impaired cognitive function, e.g., MMSE 25/30, WAIS-R full IQ 69 and especially in verbal short memory, which may represent temporal lobe dysfunction. Moreover, Benton's visual memory test revealed marked visual short-term memory impairment, while impaired performance on a Kana picking up test suggested mild to moderate attention impairment, which could have represented frontal lobe dysfunction. Brain MRI showed multiple T2-high plaque lesions close to the bilateral lateral ventricles, and bilateral optic nerve lesions enhanced by gadolinium. Also, spinal cord MRI showed a gadolinium enhanced lesion at Th5 on the left. Cerebral spinal fluid (CSF) examination showed normal cell count and protein level, and undetectable oligoclonal bands (OCB), but an elevated IgG index (1.1, normal < 0.85). Visual evoked potentials (VEPs) showed prolonged P100 latency bilaterally, indicating subclinical optic nerve lesions. She was thus diagnosed as having PPMS according to McDonald's diagnostic criteria for MS. 99mTc Single photon emission computed tomography (SPECT) showed a decreased cerebral blood flow (CBF) in the bilateral frontal and temporal lobes, which was consistent with her clinical features. PPMS patients generally present with chronic progressive spastic paraparesis and/or cerebellar ataxia. Cognitive impairments observed in PPMS are generally thought to be due to white matter lesions, i.e., subcortical dementia. However, some recent reports have shown MS patients with short-term memory impairment (antegrade amnesia) similar to cortical dementias such as Alzheimer's disease (AD). In such MS cases, visual short-term memory impairment seems characteristic of their cognitive impairment compared to AD cases. As well, the present case showed visual memory impairment as evaluated by Benton's memory test. Parietal and frontal lobes are reported to be important for verbal and visual working memory, respectively. Thus, in the present case, decreased CBF in the frontal and temporal lobes, which could have been due to a disconnection between cortices and subcortices caused by the white matter lesions, is consistent with the type of her cognitive dysfunction, i.e., notable visual memory impairment. PPMS may thus be an important disease as a differential diagnosis for chronic progressive dementia. Further neuropsychological and functional imaging studies will be necessary to achieve a better understanding of the mechanisms of cognitive impairment in PPMS.