Teaching Case

A 63 year old male patient presented with left upper extremity rhythmic tremor and palatal myoclonus 8 months after a medullary stroke.

The 63 year old male patient suffered an acute stroke with left hemiparesis, vertigo, nausea and vomiting two years ago. The diagnosis of acute ischemic stroke of the medulla was made based on an MRI scan. The patient was hospitalized in an intensive care unit and he underwent rehabilitation treatment one month later. 8 months after this event the patient returned with a left upper extremity rhythmic tremor (dentatorubral tremor) and palatal myoclonus. Physical examination revealed spastic hemiparesis of the left side of his body, dysphagia and difficulty in swallowing plus a 2-5 Hz rest, postural and kinetic tremor of his left upper extremity. He underwent an MRI study of the brain which included T1 SE, T2 TSE and FLAIR sequences.

Hypertrophic olivary degeneration (ΗΟD) ιs a form of transsynaptic degeneration. It represents the end result of a lesion that damages the neuronal connection between the dentate nucleus of the cerebellum, the contralateral red nucleus and the ipsilateral inferior olivary nucleus (ΙΟΝ), a functional system also known as the Guillain-Mollaret triangle. Efferent fibers from the dentate nucleus travel over the contralateral superior cerebellar peduncle, cross the midline trough the inferior colliculi, enter the ipsilateral red nucleus and then transverse through the central tegnental tract to the inferior olivary nucleus. The triangle is completed by ION efferents crossing the midline, entering the inferior cerebellar peduncle and terminating on the original dentate nucleus. HOD most commonly occurs following the development of focal lesions of the brainstem. Such brainstem insults include ischemic infarction, hemorrhage, demyelination, trauma( DAI ). It represents a unique type of transsynaptic neuronal degeneration since it leads to hypertrophy rather than atrophy of the affected structure, the inferior olivary nucleus. Pathologically, cell body enlargement, vacuolation of the cytoplasm, astrocytic hyperplasia and proliferation and fibrillary gliosis are seen. These histopathological changes are reflected in the typical MRI findings which consist of a non -enhancing T1 isointense, T2 hyperintense enlargement confined to the olivary nucleus plus an inciting lesion in the Guillain-Mollaret triangle. Olivary hypertrophy typically developes around six months after the event and resolves after three to four years while signal hyperintensity typically appears early(around one month after the initial lesion) and persists infinitely. Clinically, HOD is expressed with Holmes tremor, a 3-5 MHz rhythmic rest, postural and kinetic tremor of an upper extremity, as well as with palatal myoclonus, which is characterized by rhythmic involuntary movements of the oropharynx. The differential diagnosis of signal hyperintensity and enlargement of the lateral pontomedullary junction includes tumor, infarct, demyelination and inflammatory conditions. However, if the lesion is strictly limited to one or both olivary nuclei, with sparing of the surrounding medullary tissues, and particularly if there is associated focal olivary enlargement, HOD should be strongly suggested. The most important clue though to this specific diagnosis is the concomitant presence of a remote lesion in the region of the Guillain-Mollaret triangle.