Imaging findings:

MRI is the imaging modality of choice for diagnosing HOD. HOD is a dynamic and evolving process that may take months to years to progress. 

The pathological changes have been shown to correlate with the imaging features of HOD. Likewise, three evolutionary stages of the inferior olivary nucleus were established in the MRI sequences [1, 2, 9].

1. the first stage, approximately one month after onset, shows ION T2WI hyperintensity without volume alterations
2. the second, 4-6 months post-injury, features both hyperintense T2WI signals and an increase in ION volume
3. while in the third, 3-4 years post-injury, the ION returns to its normal volume or exhibits atrophy, while the hyperintense T2WI signals can persist and remain detectable for several years.

Table 1 summarizes the evolution of pathological changes and the associated imaging features of HOD. 

The differential diagnosis of HOD includes conditions causing T2 hyperintensity in the pontomedullary region, such as tumors, ischemia, demyelination, and infections. Wallerian degeneration, amyotrophic lateral sclerosis, and adrenoleukodystrophy are further radiological conditions to be considered. A lesion in the contralateral cerebellum or ipsilateral brainstem strongly suggests HOD. Unlike tumors, inflammation, or infections, HOD does not show gadolinium enhancement. While some infarctions can enlarge the olive, most affect the posterolateral medulla. A decrease in inferior olivary nucleus size over time helps rule out other diseases. Persistent olivary enlargement distinguishes HOD from chronic multiple sclerosis or ischemic injury, while the absence of diffusion restriction helps rule out acute infarction [1, 2].

The management primarily aims to address the underlying cause. Typically, HOD is a self-limiting condition managed with symptomatic treatment. Many patients either remain asymptomatic or experience clinical symptoms without significant discomfort, often making treatment unnecessary [1]. PT pharmacological treatment includes valproic acid, carbamazepine, and clonazepam, while some patients may also respond to tryptophan, carbamazepine, or trihexyphenidyl. In refractory cases of palatal myoclonus, botulinum toxin injections into the tensor veli palatini muscle may be utilized [1, 8].

Clinical history: 

A 64-year-old man presented at our hospital's Emergency Department with an abrupt onset of numbness in his left limbs and dysarthric speech. An urgent neurological assessment was performed, followed by an emergency brain computed tomography (CT) scan. CT revealed an acute right pontine hemorrhage (Figure 3). The patient was discharged one month later, alert and oriented, with mild dysarthria, and significant hearing impairment. Neurological examination revealed right gaze palsy, and partial left gaze palsy with dissociative nystagmus.

On a follow-up MRI performed five months later, a well-defined area of T2-hypointensity and blooming artifact on GRE images was seen in the right dorsal tegmentum of the pons, compatible with hemosiderin deposits (Figure 4). Furthermore, a nodular T2-hyperintense lesion was noted in the left anterolateral part of the ventral medulla oblongata, showing no restriction and no enhancement, respectively, on diffusion and postcontrast imaging (Figure 5). Based on the typical location of the lesion, a diagnosis of HOD was made. Subjective symptoms and objective signs of  palatal myoclonus further confirmed the diagnosis.