MR Imaging Approach for Dementia

Volume 3 Issue 3 March, 2013

Consultation Liaison Psychiatry Focus: Radiology

Many guidelines recommend neuroimaging in patients with dementia. The indication has progressed from exclusion of a rare finding of treatable pathology to making a probable antemortem diagnosis, with the rationale that patients are better managed when the diagnosis is known. It is important that treatable diseases like subdural hematomas, tumors and hydrocephalus needs to be excluded. The diagnosis of a specific cause for dementia can still only be confirmed by brain biopsy or at postmortem, and imaging is not specific.

Alzheimer Disease. Atrophy beginning in the entorhinal cortex and hippocampi is recognized as a structural imaging biomarker of AD. The rate of hippocampal atrophy is more accurate than cross‐sectional measurement. Methods of assessing hippocampal volume loss on structural MR imaging have progressed from visual assessment to manual hippocampal volumetry, to automated voxel‐based methods.

Vascular dementia. This is regarded as the second most common cause of dementia and is ubiquitous in the elderly, with most people older than of 60 years having some evidence of Cerebro Vascular Disease (CVD) on brain imaging. CVD can have differing appearances, depending on the site and size of the vessel involved, ranging from large cortical infarcts, lacunar infarcts, and macro‐ and microhemorrhage to white matter ischemia. White matter ischemia can be measured by using semiquantitative visual rating scales or quantitative voxel‐based methods. White matter hyperintensities (WMH) are associated with vascular risk factors, particularly hypertension and diabetes. They are most common in the frontal white matter, increase linearly with age. WMH are accepted as an imaging biomarker of vascular disease. Lacunar infarcts (or lacunes) occur in the subcortical white matter and basal ganglia and are also an imaging correlate of CVD. Enlarged perivascular spaces (VR spaces) have been associated with vascular risk factors, such as hypertension, and are regarded as likely markers of cerebral small vessel disease. Cerebral hemorrhage is also evidence of CVD, with lobar macro hemorrhages having been traditionally associated with cerebral amyloid angiopathy.

Frontotemporal Dementia (FTD) FTD is a heterogeneous group of diseases that result in degeneration of the frontal and/or anterior temporal lobes and insula. Routine structural imaging with MR will show characteristic atrophy in the frontal lobes involving the ventromedial, orbitofrontal, anterior cingulate, anterior insula, and amygdala. The most useful role of imaging is to suggest the diagnosis when the clinical picture is not clear, and quantitative analysis of 3D T1 MR imaging have good specificity in distinguishing FTD from AD based on identification of predominantly anterior‐versus‐posterior patterns of atrophy.

Dementia with Lewy Bodies (DLB) Structural MR imaging studies by using voxel based morphometry have shown differences between DLB and AD, with greater atrophy in DLB in the striatum,midbrain, and hypothalamus, but the common finding is of relative preservation of the hippocampus in DLB compared with prominent atrophy in AD. MR imaging is also useful in Parkinson Plus syndromes, some of which cause dementia and are associated with specific structural MR imaging findings; for example, progressive supranuclear palsy results in characteristic atrophy of the midbrain giving rise to the “hummingbird” sign on sagittal images and the “Mickey Mouse” sign on axial images.

Other Diseases Causing Dementia
Limbic encephalitis, may present with cognitive impairment, delirium, seizures; and most important, approximately 50% are responsive to steroids. MR imaging may show high signal intensity on T2‐weighted images in involved brain areas.
Rapidly progressive dementia is the typical clinical presentation of both sporadic Creutzfeldt‐Jacob disease (sCJD) and variant Creutzfeldt‐Jacob disease (vCJD), and earlier age of onset is typical in vCJD. MR imaging in CJD shows typical high signal intensity on T2 and FLAIR in the pulvinar of the thalami in vCJD and in the caudate heads and cortex in sCJD, which can be asymmetric. Pulvinar sign is virtually pathognomonic of vCJD. These abnormalities are best demonstrated with diffusion weighted imaging (DWI).

HIV‐associated dementia is the most severe HIV‐associated neurocognitive disorder, and the most common imaging manifestation of this is diffuse cerebral atrophy. MR spectroscopy is effective in distinguishing those with HIV infection, characterized by an increased choline factor (consistent with glial proliferation), from those with HIV‐associated dementia, characterized by a reduced N‐acetylaspartate factor (consistent with neuronal cell dysfunction and death), and demonstrates the importance of white matter involvement in HIV.

However it needs to be cautioned that often diagnosis of dementia is clinical and it is commonly associated with physical and psychiatric disorders. This calls for multidisciplinary team approach.