Increased 18F-THK5351 Uptake at Bilateral Primary Motor Cortex in Patients with Progressive Pseudobulbar Palsy

Background. Although patients can present with progressive pseudobulbar palsy due to neurodegenerative diseases, detection of the precise location of radiological abnormalities can be difficult. 18F-THK5351 was initially developed as a tau positron emission tomography (PET) tracer. Later, it was found to sensitively detect astrogliosis associated with neurodegeneration. Therefore, it has been used in diagnosis of various diseases. However, its utility in progressive pseudobulbar palsy was unknown. Methods. 18F-THK5351 PET results of two patients presenting with progressive pseudobulbar palsy are reported. Results. Patient 1 was a 77-year-old man with a two-year history, and Patient 2 was a 61-year-old woman with a 1-year history. Both patients presented with gradually progressive spastic dysarthria, suggesting pseudobulbar palsy without clinical lower motor neuron signs. Facial asymmetry was detected in both patients, while left-dominant pyramidal signs in the extremities were detected only in Patient 2. Brain magnetic resonance imaging did not show signal abnormality explaining pseudobulbar palsy. However, 18F-THK5351 PET clearly visualized bilateral increased uptake in limited areas of the posterior portion of the precentral gyrus, corresponding to the midportion of the primary motor cortex. Laterality of increased 18F-THK5351 uptake corresponded to the symptom laterality and was higher on the left and right side in patients 1 and 2, respectively. After one year, Patient 1 was unable to vocalize and could only produce grunts; concomitant apraxia of speech was suspected. Conclusions. 18F-THK5351 PET is a useful method to detect bilateral primary motor cortex involvement in patients presenting with progressive pseudobulbar palsy, likely by imaging astrogliosis.