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DOPAMINE TRANSPORTER (DAT) IMAGING CAN BE NORMAL WITH NEUROPATHOLOGICALLY CONFIRMED CORTIOBASAL DEGENERATION

Objective To report 3 cases of Corticobasal Degeneration with normal DAT scans. Background Corticobasal degeneration (CBD) is a progressive neurodegenerative condition associated with neuronal and glial tau pathology (including astrocytic plaques) in gray and white matter of the cortex, basal ganglia, diencephalon and rostral brainstem.1 It presents with a highly variable combination of progressive asymmetric akinetic–rigidity, apraxia, limb dystonia, myoclonus, cognitive impairment, dementia or alien limb phenomena. The complex and highly variable clinical presentation of CBD makes accurate diagnosis difficult. Misdiagnosis is so frequent–as Alzheimer's disease, Parkinson's disease, cerebrovascular disease or progressive supranuclear palsy–that the term corticobasal syndrome (CBS) is often used in the absence of neuropathological confirmation. CBD/CBS are commonly associated with striatal dopaminergic deficiency. Dopamine transporter (DAT) imaging is considered to be a sensitive method to detect presynaptic dopamine neuronal dysfunction, the hallmark of neurodegenerative parkinsonism including CBD.2 But, does a normal DAT scan exclude a neurodegenerative aetiology in the context of CBS or suspected CBD? Methods We describe 3 case studies clinically diagnosed with CBD including one case with a pathological diagnosis (See case 2) and normal DAT scan on initial investigation. Results Case 1: a 58 year old right handed man presented with 5 years' progressive left alien limb phenomena, dressing apraxia and visual hallucinations. On examination there was dysgraphesthesia, left finger agnosis and left arm drift. Pursuit eye movements were broken but saccades and antisaccades were fast and accurate, with no restriction. On cognitive tests, ACE–R98/100 MMSE 29/30. VOSP cubes 9/10. Frontal assessment battery 17/18. Investigations: DAT scan normal. MRI brain demonstrated right parietal atrophy. EEG one non–specific burst of slow wave with hyperventilation. Case 2: a 76 year old man presented with asymmetric rigidity and rest tremor on the left, with a partial response to levodopa. Initial diagnosis Parkinson's disease. He failed to respond to increases in levodopa as akinetic rigidity progresses, with deteriorating dexterity. After two years, he developed severe dyspraxia and myoclonus, with impaired graphaesthesia and later fixed dystonic posturing of the left arm and dysarthria. Cognitive function remained good, with ACE–R 91/100 and MMSE 29/30. Cardiac pacemaker fitted due to bradyarrhythmias without atrial fibrillation. Investigations DAT scan slight asymmetry but no loss of tracer uptake against reference norms. Initial CT brain was normal. Pathological confirmation of CBD. Case 3 a 67 year old right handed man presented with increasing unsteadiness, speech difficulty, bilateral but asymmetrical apraxia and features of alien limb in his right hand. Examination of limbs revealed asymmetrical apraxia worse on the right and stimulus sensitive myoclonic jerks. Primitive reflexes were present and graphesthesia was impaired bilaterally. Cognitive function was impaired with MMSE 23/30 ACE–R 64/100. Investigations: DAT scan normal, MRI brain shows hyper intensity throughout the corticospinal tracts. Conclusion The differential diagnosis of parkinsonism, including CBS, CBD and PD, is important for patient management. The disease course, therapy and prognosis differ substantially from non–degenerative diseases, and between CBD and PD. Where there is diagnostic uncertainty, DAT scan can provide valuable additional information, and in CBS/CBD unilateral balanced reduction in tracer uptake has been observed in caudate and putamen (2) and suggested to be reliably abnormal in CBD. However, our three cases llustrate that a normal DAT scan can occur in CBS, and even in pathologically confirmed CBD. This study highlights the importance of clinical follow up of patients and suggest that DAT scan cannot be relied upon to exclude CBD or a progressive CBS.