Volumetric measures of mesial temporal lobe structures on magnetic resonance imaging scans recently have been explored as potential biomarkers of dementia in patients with Parkinson's disease, with investigations primarily focused on hippocampal volume. Both in vivo magnetic resonance imaging and post-mortem tissue studies in Alzheimer's disease, however, demonstrate that the entorhinal cortex is involved earlier in disease-related pathology than the hippocampus. The entorhinal cortex, a region integral in declarative memory function, projects multimodal sensory information to the hippocampus via the perforant path. In Parkinson's disease, entorhinal cortex atrophy as measured on magnetic resonance imaging, however, has received less attention compared to hippocampal atrophy.

We compared entorhinal cortex and hippocampal atrophy in 12 subjects with Parkinson's disease dementia including memory impairment, 14 Parkinson's disease subjects with normal cognition, and 14 healthy controls with normal cognition, using manual segmentation methods on magnetic resonance imaging scans.

While hippocampal volumes were similar in the two Parkinson's disease cognitive groups, entorhinal cortex volumes were substantially smaller in the demented Parkinson's disease subjects compared the cognitively normal Parkinson's disease subjects (p<0.05). In addition, normalized entorhinal cortex and hippocampal volumes for right and left hemispheres were significantly lower in the demented Parkinson's disease group compared to healthy controls.

Our findings suggest that entorhinal cortex atrophy differentiates demented and cognitively normal Parkinson's disease subjects, in contrast to hippocampal atrophy. Thus, entorhinal cortex atrophy on magnetic resonance imaging may be a potential biomarker for dementia in Parkinson's disease, particularly in the setting of memory impairment.