Objective: This is a case report from the Portland VA Medical Center.

Background: PSP is a parkinsonian neurodegenerative disorder characterized by deposition of tau in the basal ganglia, brainstem, cerebellum, spinal cord, and cortex. Features include supranuclear gaze palsy, postural instability, cognitive impairment, wide-eyed stare, axial rigidity, and reduced eye blink frequency. Although there are a variety of PSP presentations, there are only three reported cases of myoclonus and no reported cases of axial myoclonus.

Methods: A 72-year-old man with PSP presented with acute onset jerking. He denied any other new symptoms or recent medication changes, injury, or infection. The diagnosis of PSP had been made two years earlier following the development of falls and cognitive decline, and MRI three months ago showed midbrain atrophy. Exam on presentation revealed a fully oriented man with mild dysarthria and no tremor or myoclonic-like quality to voice. Cranial nerve exam was significant for restricted upward and downward gaze, mild right eye dysconjugate lateral gaze, and mild hypomimia. Motor exam demonstrated mild axial rigidity, no tremor in the bilateral upper extremity, and positional, non-rhythmic myoclonic jerk movements involving the cervical, axial thoracic, and abdominal musculature that completely remitted with standing and returned after eight minutes of sitting. Reflexes were 1-2+ and symmetric throughout and sensory exam was normal.

Results: He was admitted for full spine and brain MRI without contrast to rule out spinal cord compression, which was found to be unremarkable. Per the recommendation of his outpatient neurologist, the inpatient team discontinued his levodopa/carbidopa and started amantadine. Hospital course was remarkable for one observed episode of myoclonus. The leading diagnoses were propriospinal myoclonus (PSM), specifically the slow, flexion truncal jerks exacerbated while supine and relieved while standing, and atypical presentation of PSP. The non-distractible and reproducible nature of the myoclonus lowered suspicion for a functional component.

Conclusions: The etiology of axial myoclonus in a patient with PSP is unclear. Clinically, this presentation most closely resembles PSM. Recent work on PSM using DTI with fiber tracking found microstructural abnormalities in the lemniscal posterior and corticospinal posterolateral tracts, which suggests a possible connection between PSP and PSM. This may explain this unusual presentation.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/axial-myoclonus-in-a-patient-with-psp/