Hypertrophic Inferior Olivary degeneration due to brainstem cavernoma
Hello friends.
This is the case of a 50 years old male presenting with right hemiparesis since a year, which had been mopre or less static
MRI showed a left sided pontine and middle cerebellar peduncular cavernoma and hypertrophied left inferior olivary nucleus.
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| Coronal T2WI |
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| Axial GRE showing left pontine cavernoma |
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| Axial T2WI showing left inferior olivary nucleus hypertrophy and altered signal |
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| Sagittal T2WI |
Hypertrophic olivary degeneration is a form of transsynaptic degeneration. It represents the end result of a lesion that damages the neuronal connections between the dentate nucleus of the cerebellum, the red nucleus, and the inferior olivary nucleus: the dentatorubral-olivary pathway (1,2). It is considered a unique type of degeneration because it is associated with enlargement, rather than atrophy, of the affected structure—the inferior olivary neurons (3).
The affected circuit connects the dentate nucleus of the cerebellum, the contralateral red nucleus, and the ipsilateral inferior olivary nucleus. The dentate nucleus and the contralateral red nucleus are connected by the superior cerebellar peduncle (dentatorubral tract), with fibers crossing in the decussation of the peduncle at the lower midbrain. This tract is part of a reflex arc that controls fine voluntary movements. The red nucleus and the ipsilateral inferior olivary nucleus are connected by the central tegmental tract. These dentatorubral-olivary connections were described by Guillain and Mollaret (2) in 1931 as the anatomic connections related to palatal myoclonus, which is a common clinical association, and this pathway is thus referred to as the “Guillain-Mollaret triangle.”
In 1935, Trelles (4) reported that isolated lesions of the inferior cerebellar peduncle never cause palatal myoclonus, since anatomically there are no direct connections between the inferior olivary nucleus and the contralateral dentate nucleus. Fibers from the inferior olivary nucleus instead project first to the cerebellar cortex (olivocerebellar tracts) and then to the dentate nucleus. He described these connections as the dentatorubral-olivary pathway (5).
Since hypertrophic olivary degeneration occurs owing to interruption of the pathways composing the Guillain-Mollaret triangle, it most commonly occurs following development of focal lesions of the brainstem. Focal brainstem insults that may lead to pathway interruption include ischemic infarction, demyelination, and hemorrhage, the latter often related to hypertensive disease or diffuse axonal injury following severe head trauma (6–8). Olivary hypertrophy is not seen immediately after the brainstem insult but typically appears in a delayed fashion, usually within 4–6 months. The pathologic process persists and is frequently visible after 10 months. Clinical symptoms such as abnormal movement rarely improve. Although olivary hypertrophy typically resolves in 10–16 months, olivary hyperintensity on T2-weighted images may persist for years after resolution of the hypertrophy (9,10).
The specific relationship between the primary lesion and the development of olivary degeneration results in the production of one of three patterns that are easily understood by reviewing a diagram of the involved neuronal pathway (Fig 3). When the primary lesion is limited to the central tegmental tract, olivary hypertrophy is ipsilateral, since only ipsilateral fibers are affected. When the primary lesion is in the dentate nucleus or in the superior cerebellar peduncle, olivary degeneration is contralateral; and when the lesion involves both the central tegmental tract and the superior cerebellar peduncle, olivary hypertrophy is bilateral. The patient in this case has only unilateral olivary hypertrophy, despite having some signal intensity abnormality in the superior cerebellar peduncle—hypointensity in the tegmental area extends into the left superior cerebellar peduncle.Transneuronal degeneration occurs only from a lesion that results in disconnection of the pathway. Abnormal changes in signal intensity on MR images do not necessarily define the severity of the injury. In this case, the hypointensity on T2-weighted images in the superior cerebellar peduncle does not represent disruption of fibers; instead, it most likely represents staining of adjacent tissues by blood products at the periphery of the pontine hemorrhage.
Olivary enlargement corresponds pathologically to an unusual vacuolar degeneration of cytoplasm that results in enlargement related in part to an increased number of astrocytes. Vacuolar degeneration of the cytoplasm occurs at 6–15 months, and gliosis follows 15–20 months after the onset of the primary lesion (10).
Clinical findings associated with olivary hypertrophy include the signature syndrome: palatal myoclonus, or cyclic jerk of the soft palate, and also dentatorubral tremor and ocular myoclonus. Palatal myoclonus, a form of “segmental” myoclonus, is a rhythmic involuntary movement of the oropharynx, similar to the accessory respiratory reflex seen in fish. The typical tremor consists of muscle contractions at one to three cycles per second and is not affected by voluntary controls. Severe myoclonus may also affect the cervical muscles and the diaphragm. The anatomic correlation is the central tegmental tract, which has several connections to the nucleus ambiguus—the vagus nerve is involved in control of palatal movement (5). A lesion in the dentatorubral pathway may cause the patient to lose inhibitory control transmitted through these structures, and palatal myoclonus or other types of abnormal movement may develop. Palatal myoclonus usually develops 10–11 months after the primary lesion, although palatal myoclonus does not always accompany olivary hypertrophy, as is evidenced in the test case (12–14). It should be noted that the clinical pattern of palatal myoclonus also may be seen with focal spinal cord insults (15).
Considering the differential diagnosis of the medullary lesion, high intensity in the anterolateral part of the medulla is not a specific imaging finding. It may be seen with a wide variety of pathologic processes, including infarction; demyelination related to multiple sclerosis; tumor (eg, astrocytoma, metastasis, and lymphoma); and infectious and other inflammatory processes such as tuberculosis, acquired immunodeficiency syndrome, sarcoidosis, and rhombencephalitis. However, if the lesion is strictly limited to one or both inferior olivary nuclei, with sparing of the surrounding medullary tissues, and particularly if there is associated focal olivary enlargement, hypertrophic olivary degeneration should be strongly suggested.
Most neoplasms and infectious processes would be expected to demonstrate intense enhancement. Most infarctions of the medulla are related to disease of the posteroinferior cerebellar artery and are posterolateral in location, or they are related to perforating branches from anterior spinal or vertebral arteries and have a paramedial location. Clinically, acute infarction of the inferior olivary nucleus would likely cause ataxia. Therefore, the diagnosis of infarction is unlikely. Wallerian degeneration, adrenoleukodystrophy, and amyotrophic lateral sclerosis also may demonstrate anterior medullary high intensity on T2-weighted images; however, in these cases, lesions generally are limited to the corticospinal tract and not to the inferior olivary nucleus.
The most important clue to the diagnosis is the association of a remote lesion. The presence of an olivary lesion in association with another lesion in the contralateral cerebellar dentate nucleus, the contralateral superior cerebellar peduncle, the ipsilateral dorsomedial red nucleus, or the ipsilateral pontine tegmentum (as in the test case) makes any diagnosis other than hypertrophic olivary degeneration highly unlikely.
Several authors (16,17) have noted that Wernicke-Korsakoff syndrome may be associated with pathologic change in the inferior olivary nucleus. Again, it is the combination of the inciting lesion in the proper anatomic location, and the observers' knowledge of the association, that permits the diagnosis to be made. MR imaging, due to its exquisite contrast sensitivity and lack of bone artifact in the low posterior fossa, is the most sensitive and specific tool for the diagnosis of olivary hypertrophic changes.
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| Diagram showing the anatomy involved |
References
He had thick hypertrophied soft palate with persistent myoclonus.......video will be posted next....
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