Diversity of dopamine transporter images in frontotemporal lobar degeneration‐motor neuron disease

Dear Editor, We read with great interest Arafuka et al.’s case report on frontotemporal lobar degeneration (FTLD) with trans-activation response DNA protein 43 (TDP43) type B published in this journal. Their patient exhibited a mildly decreased dopamine transporter (DAT) availability on DAT imaging 1.5 years before the onset of dysphagia. In contrast, severely decreased DAT availability was observed in a patient with a similar disease course, based on our previously reported case without confirmatory autopsy results. The authors associated the discrepancy in the severity of the DAT findings between their patient and ours with the differences in disease stage. Our recent report discussed the reduced striatal DAT availability in six patients with FTLD with motor neuron disease (MND). It included three cases of pathologically confirmed FTLD with TDP-43 type B. Moderate to severe reduction in DAT availability was observed among patients with FTLD-MND. On pathological examination, the degeneration of the substantia nigra was apparent. Meanwhile, the pathological findings for the caudate and putamen varied among patients. The patient in Arafuka et al.’s report had mild degeneration of the substantia nigra and caudate, but there was no apparent putamen damage. The mild DAT reduction in their case was likely due to the mild or minimal degeneration of the substantia nigra, caudate, and putamen, which influenced the DAT imaging results. Based on the findings of our cases and Arafuka et al.’s case, the striatal DAT imaging findings of FTLD with TDP-43 type B varies, depending on the degree of degeneration in the substantia nigra, caudate, and putamen. Another remarkable finding in the case report by Arafuka et al. was the DAT reduction, observed prior to the onset of MND symptoms in their patient. This was consistent with the clinical course of the previously reported patients. Therefore, the development of MND should be considered in FTLD patients with abnormal findings on DAT imaging. More studies on the application of DAT imaging in patients with FTLD are needed.