Teaching Case

A 63-year-old male patient presented with a chronic history of loss of coordination for 18 months, which was gradually progressive. Subsequently, diplopia appeared 10 months ago, which had worsened. Now, on examination, he had palatal myoclonus and dysarthric speech.

Non-contrast MRI acquired 18 months after symptom onset showed mild asymmetric expansion associated with high-signal intensity on axial T2 and FLAIR images, in the anterolateral aspect of left medulla (Fig 1,2). Axial and coronal T2 images at the level of pons, showed multiple discrete hyperintense foci in the left half of pons (Fig 3,4). Sagittal T1 and sagittal T1 images in magnified view showed multiple linear hypointensities, which were along the long axis of basilar artery perforators representing dilated Virchow-Robin spaces (dVRS) (Fig 5,6). These dVRS are in an area traversed by central tegmental tracts extending to superior cerebellar peduncle. The location of the T2 hyperintense lesion corresponding to the inferior olivary nucleus and dVRS on left side of pons suggests disruption of a part of dentato- rubro-olivary pathway causing hypertrophic olivary degeneration.

Hypertrophic olivary degeneration (HOD) is a rare transsynaptic neuronal degeneration affecting the dentato‐rubro‐olivary (DRO) pathway, also known as Guillain-Mollaret triangle (GMT) that connects the ipsilateral red nucleus and inferior olivary nucleus with the contralateral dentate nucleus [1].Characteristic hypertrophic changes are because of transsynaptic degeneration, because of disruption of afferent signals into the inferior olivary nucleus (ION) of medulla [1,2]. Although HOD due to pathological conditions has been described, HOD secondary to physiological cause such as prominent Virchow-Robin (VR) spaces has not been reported and extensive pontine involvement of our patient’s dVRS is also unusual.

Many aetiological factors can lead to HOD, like vascular malformations, tumours, haemorrhage, infarction, trauma, demyelination or infection. [3]

Specific presentations include palatal tremor, involuntary movements (dentato-rubral or Holmes’ tremor and myoclonus) and ocular symptoms (ocular myoclonus, nystagmus). Vague neurologic deficits can also be present [4].

HOD is unilateral and ipsilateral, if primary lesion is in the brainstem, and contralateral if the lesion is in dentate nucleus or cerebellar peduncle. If the lesion is in both brainstem and cerebellar peduncle, bilateral involvement is seen [1,2]. Several gene mutations can also cause bilateral HOD [4].

HOD usually appears at around 3 weeks after insult to the DRO pathway [1]. MRI shows T2 lengthening with or without hypertrophy of the ION [5]. On T1WI, iso- to hypointensity is seen with no restricted diffusion, blooming on T2∗ or contrast enhancement [6]. The first of three stages on MR includes the initial stage (within first 6 months of ictus), showing T2 hyperintensity in ION. The second stage shows hypertrophy and T2 hyperintensity (~ 6 months- 3 years). Resolution of hypertrophy and persistent increased signal intensity is seen in the last stage [7,8]. Increased radial and axial diffusivity in the inferior olives is seen on diffusion tensor imaging [9].

VRS are pial‐lined, fluid‐filled structures found in characteristic locations throughout the brain. In rare cases, dilated VRS (> 2 mm) in brainstem can cause obstructive hydrocephalus [10]. dVRS can mimic cystic neoplasms such as dysembryoplastic neuroepithelial tumour, multinodular and vacuolating neuronal tumour, cystic infections [10,11] or even arteriovenous malformations [12]. Signal intensity characteristics (similar to CSF on all pulse sequences) and locations of VRS helps differentiate them from pathologic conditions [12,13].   

The treatment options for palatal myoclonus include carbamazepine, clonazepam, and/or botox injection to the tensor veli palatini muscle [14]. dVR spaces in pons was the imaging cause of HOD in our patient. When palatal tremor or when lesions are found within the GMT along with T2-weighted hyperintensity and hypertrophy in ION, it is diagnostic of HOD [14].

Written informed patient consent for publication has been obtained.