Imaging Features of Hypertrophic Olivary Degeneration

Abstract

Hypertrophic olivary degeneration (HOD) is a unique form of transneuronal degeneration caused by a disruption of the dentato-rubro-olivary pathway, also known as the triangle of Guillain-Mollaret. The triangle of Guillain-Mollaret is involved in fine voluntary motor control and consists of both the inferior olivary nucleus and the red nucleus on one side and the contralateral dentate nucleus. Clinically, patients classically present with symptomatic palatal myoclonus. Typical magnetic resonance imaging findings include T2-hyperintensity and enlargement of the inferior olivary nucleus evolving over time to atrophy with residual T2-hyperintensity. In this article, we provide a case-based illustration of the anatomy of the Guillain-Mollaret-triangle and the typical imaging findings of hypertrophic olivary degeneration.

Keywords: Hypertrophic olivary degeneration.

Figure 1

Axial T2-weighted image at the level of the medulla shows symmetric hyperintensity and expansion of the medullary olives (A). Axial T2-weighted image at the same level in the same patient but eight years earlier showed no abnormality (B).

Figure 2

Initial MRI demonstrates a mass in the fossa posterior and fourth ventricle with heterogeneous high signal intensity on axial T2-weighted images and only moderate enhancement after injection of intravenous gadolinium (not shown) (A). MRI obtained seven months postoperatively. The axial T2-weighted image on the left shows a postoperative defect in the right dentate nucleus (white arrow) (B) and a new expansile T2- hyperintense lesion in the left inferior olivary nucleus (white arrow) (C).

Figure 3

Unenhanced CT shows an acute hemorrhage in the left dorsal pontine tegmentum (A). Follow-up MRI four months later shows residual hemosiderin deposits in the area of the previous hemorrhage (B) as well as a new expansile T2-hyperintense lesion in the left inferior olivary nucleus (C).

Figures 4 and 5

Schematic representations of the Guillain-Mollaret triangle. The contralateral dentate nucleus sends efferent fibers to the red nucleus via the superior cerebellar peduncle (dentato-rubral pathway). The red nucleus in turn sends efferent fibers to the inferior olivary nucleus via the central tegmental tract (rubro-olivary pathway). Finally, the inferior olivary nucleus sends efferent fibers the contralateral cerebellar cortex via the inferior cerebellar peduncle (olivo-dentate pathway), which in turn connects to the contralateral dentate nucleus, hence completing the triangle.

Figure 6

Axial FLAIR images at the level of the medulla oblongata (A) and the lateral ventricles (B) in a 36-year-old woman with established multiple sclerosis show a discrete hyperintense lesion ventrally and right-sided in the medulla oblongata (white arrow) (A). Although this lesion could reflect a hypertrophic degeneration of the right-sided inferior olivary nucleus, there were no other lesions along the Guillain-Mollaret triangle, and the patient did not exhibit symptoms typical for HOD. Due to the simultaneous presence of multiple periventricular and juxtacortical supratentoriell white matter lesions (B), a diagnosis of a demyelinating lesion was made.