Dopamine transporter scan: A helpful yet underutilized tool to distinguish various subtypes of dementia

Sir,

A 70-year-old female presented with gradual cognitive decline, early onset visual hallucination, and poor attention span for the last 8 months. Her cognitive decline was initially fluctuating but later became persistent and progressive.

Although provisionally, she was diagnosed as Alzheimer's dementia (AD) as it is the most common cause of gradual cognitive decline and she had no extrapyramidal signs the early onset of visual hallucination prompted the neurologist to request a dopamine transporter single photon emission computed tomogram scan (DaT-SPECT or DaT). DaT imaging using ioflupane 123 tracer showed reduced/abnormal tracer uptake in both basal ganglia (lentiform and caudate nucleus). The distribution volume ratio (DVR), a marker of tracer uptake was 1.92 on both basal ganglia (lowest range of normal DVR was 2.6) [Figure 1]. DVR images were obtained by scaling voxel intensities adopting the whole brain without corpus striatum as the reference. She was diagnosed as probable dementia with Lewy bodies (DLB) based on the cognitive decline, poor attention span (central features), visual hallucination (core feature), and positive DaT scan (suggestive feature).[1] As DaT scan helped the diagnosis fluorodeoxyglucose-positron, emission tomography scan was not performed.

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Figure 1 Dopamine transporter scan showing reduced tracer uptake in both basal ganglia

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DLB is one of the most common varieties of dementia after the AD.[2] The lack of parkinsonism at early stages often makes the clinical diagnosis of DLB difficult like in our patient. DLB can be diagnosed as clinically probable or possible on the basis of the criteria proposed by McKeith et al. The diagnosis of DLB is probable if a combination of one central feature and two core features or one central, one core, and one supportive feature are present.[1] DaT scan is an important noninvasive supportive tool in DLB diagnosis. DaT imaging is abnormal in DLB because of presynaptic dopaminergic receptors degenerates but in AD, the DaT scan is normal as dopaminergic pathway is not involved.[3] Although only histopathological studies can confirm the exact nature of dementia but the lack of availability of tissue material makes DaT-SPECT a very important noninvasive tool to distinguish AD from DLB. Prognosis and management vary considerably among various subtypes of dementia, so accurate diagnosis is very important.[1] In many centers of the UK, the supportive diagnosis of DLB constitutes a major indication for the DaT imaging.[4]