Hypertrophic olivary degeneration

Findings

Figure 1 and Figure 2: Noncontrast head CT shows acute hemorrhage in the superior and middle cerebellar peduncles.
Figure 3, Figure 4, Figure 6 and Figure 7: FLAIR and T2 show chronic blood products in the superior and middle cerebellar peduncles.
Figure 5 and Figure 8: Axial FLAIR and T2 show T2 prolongation and mild enlargement
of the inferior olivary nucleus of the medulla.


Diagnosis: Hypertrophic olivary degeneration


Hypertrophic olivary degeneration is a unique type of transynaptic degeneration which results in hypertrophy rather than atrophy of the affected structure, the inferior olivary nucleus. The affected circuit involves dentrorubral-olivary connections which were described by Guillain and Mollaret in 1931 as the anatomic connections related to palatal myoclonus, and is commonly referred to as the "Guillain-Mollaret triangle."

The "triangle" consists of 3 nuclei: 1) ipsilateral inferior olivary nucleus (medulla) 2) ipsilateral red nucleus (midbrain) and 3) contralateral dentate nucleus (cerebellum). The ipsilateral red nucleus and contralateral dentate nucleus are connected by the superior cerebellar peduncle. The ipsilateral red nucleus and ipsilateral inferior olivary nucleus are connected by the central tegmental tract. There are no direct anatomic connections between the inferior olivary nucleus and the contralteral dentate nucleus.

Olivary degeneration is typically seen several months after the insult. The side of olivary degeneration depends on the location of original insult and can be predicted by familiarity with the "Guillain-Mollaret triangle." When the primary lesion is in the central tegmental tract, olivary hypertrophy is ipsilateral. When the primary lesion is in the superior cerebellar peduncle or dentate nucleus, the olivary hypertrophy is contralateral. When the primary insult involves both the central tegmental tract and superior cerebellar peduncle, the olivary hypertrophy is bilateral.

The classic clinical finding associated with hypertrophic olivary degeneration is palatal myoclonus, a cyclic jerk of the soft palate.