Lesions in Mollaret’s triangle (Guillain–Mollaret triangle) classically cause palatal myoclonus, or “oculopalatal” myoclonus involving both the palate and eyes, as is described here. The nystagmus in oculopalatal myoclonus can be vertical, horizontal, or torsional, and is often pendular. The palatal tremor can often be noticed by patients as hearing a “clicking sound”.
Mollaret’s triangle consists of the inferior olivary nucleus in the medulla, the red nucleus in the tegmentum of the midbrain, and the contralateral dentate nucleus in the cerebellum. Pathways connecting the points of the triangle pass through the inferior and superior cerebellar peduncles.
A lesion anywhere in the triangle will often cause T2 hyperintensity seen in the inferior olive secondary to compensatory hypertrophy. These changes usually can be seen on MRI after 4 weeks of symptoms. These lesions are sometimes misread as glioma or demyelination, so it is important to use clinical correlation. Other MRI changes one might see early on are diffusion restriction in any region of the Guillain-Mollaret triangle, indicating stroke.
The red nucleus is located in the dorsal midbrain tegmentum near the substantia nigra, not in the superior colliculus of the midbrain.