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---
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title: "Hypertrophic Olivary Degeneration"
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docid: "78257543-6d52-4879-84b1-445f3611d996"
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breadcrumbs:
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- "Brain"
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- "Diagnosis"
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- "Pathology-Based Diagnoses"
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- "Acquired Toxic/Metabolic/Degenerative Disorders"
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- "Dementias and Degenerative Disorders"
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- "Hypertrophic Olivary Degeneration"
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---
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# KEY FACTS
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- ## Terminology
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- Inferior olivary nucleus (ION) degeneration
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- Unique type of transsynaptic neuronal degeneration
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- Olivary deafferentation thought to be source of ensuing hypertrophic olivary degeneration (HOD)
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- Usually caused by primary lesions in dentato-rubro-olivary pathway (Guillain-Mollaret triangle)
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- Triangle of Guillain-Mollaret defined by 3 anatomic structures
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- Red nucleus (RN)
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- ION ipsilateral to RN
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- Contralateral dentate nucleus (DN) of cerebellum
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- ## Imaging
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- ION initially hypertrophies rather than atrophies
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- 3 distinct MR stages in HOD
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- Hyperintense signal without hypertrophy of ION: Within first 6 months of ictus
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- ↑ signal + ION hypertrophy: Between 6 months & 3-4 years after ictus
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- Only ION hyperintensity: Begins when hypertrophy resolves (can persist indefinitely)
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- MR also detects primary lesion located in ipsilateral CTT, SCP or contralateral DN
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- ## Top Differential Diagnoses
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- Vertebrobasilar perforating artery infarct
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- Demyelination (multiple sclerosis, microvascular disease)
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- Amyotrophic lateral sclerosis
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- HIV/AIDS
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- Rhombencephalitis
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- ## Clinical Issues
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- Palatal myoclonus (palatal "tremor"), ocular myoclonus
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- Usually develops 10-11 months after primary lesion
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- Clinical symptoms (tremors) rarely improve
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# TERMINOLOGY
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- ## Abbreviations
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- Hypertrophic olivary degeneration (HOD)
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- ## Synonyms
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- Pseudohypertrophy of inferior olivary nucleus
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- ## Definitions
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- Transsynaptic degeneration of inferior olivary nucleus (ION), usually caused by primary lesions in dentato-rubro-olivary pathway (DROP) also called anatomic triangle of Guillain & Mollaret (GMT)
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# IMAGING
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- ## General Features
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- ### Best diagnostic clue
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- T2-hyperintense, nonenhancing enlargement of ION
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- ### Location
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- GMT is defined by 3 anatomic structures
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- Red nucleus (RN)
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- ION ipsilateral to RN
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- Contralateral dentate nucleus (DN) of cerebellum
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- Central tegmental tract (CTT or rubro-olivary pathway) connects RN to ipsilateral ION
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- Superior cerebellar peduncle (SCP, dentato-rubral tract) connects DN to contralateral RN
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- Inferior cerebellar peduncle (olivo-cerebellar pathway) connects ION to contralateral cerebellar cortex & contralateral DN
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- 4 patterns of HOD in relation to primary lesion
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- Ipsilateral HOD: Primary lesion is limited to brainstem (CTT)
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- Contralateral HOD: Primary lesion is in cerebellum (DN or SCP)
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- Bilateral HOD: Primary lesion involves midline/paramedian brainstem affecting brachium conjunctivum
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- Bilateral HOD: Primary lesion involves both unilateral brainstem & cerebellum
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- ### Size
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- Variable (time-dependent) size of affected ION
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- Normal in acute stage
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- ↑ (hypertrophy) from 6 months to 3-4 years
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- ↓ (atrophy) in advanced stage (> 3-4 years)
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- ### Morphology
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- Unique type of transsynaptic neuronal degeneration
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- ION initially hypertrophies rather than atrophies
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- ## CT Findings
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- ### NECT
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- May show acute primary injury (e.g., hemorrhage) in tegmentum
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- HOD typically not depicted on CT
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- ## MR Findings
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- ### T1WI
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- Acute phase: Normal ION
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- Shows primary lesion in brainstem (cerebellum or tegmentum)
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- After HOD ensues
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- Enlargement confined to ION, isointense to slightly hypointense to gray matter
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- Slightly ↑ olivary T1 signal also reported
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- ± residual primary lesion
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- ### T2WI
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- 3 distinct MR stages in HOD
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- Hyperintense signal without hypertrophy of ION: Within first 6 months of ictus
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- Both ↑ signal & hypertrophy of ION: Between 6 months & 3-4 years after ictus
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- ↑ signal only in ION: Begins when hypertrophy resolves & can persist indefinitely
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- Axial MR: Disappearance of pre- & postolivary sulci in hypertrophic stage
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- MR also detects primary lesion located in ipsilateral central tegmental tract or contralateral DN
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- Old hematomas: Low-signal areas on T2WI revealing hemosiderin deposition
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- ± ↓ size of contralateral ION with higher than normal signal intensity
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- ± mild to severe atrophic changes of cerebellar cortex contralateral to HOD
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- ### PD/intermediate
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- High signal intensity of ION better detected on PD images than on T2WI
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- ### FLAIR
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- Similar to T2WI
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- ### T1WI C+
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- No contrast enhancement of degenerated ION
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- DTI
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- ↑ radial diffusivity, ↑ mean diffusion & ↓ fractional anisotropy in GMT components reflecting demyelination
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- ↑ fractional anisotropy & ↑ axial diffusivity in ION reflect rearrangement of regenerating axons & shrunken neurons
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- ## Nuclear Medicine Findings
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- ### PET
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- Focal glucose hypermetabolism in medulla of patients with HOD
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- ## Imaging Recommendations
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- ### Best imaging tool
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- MR
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- ### Protocol advice
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- T2WI (include coronal or sagittal sections)
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# DIFFERENTIAL DIAGNOSIS
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- ## Other Causes of High T2 Signal Intensity in Anterior Part of Medulla
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- [Demyelination related to multiple sclerosis](/document/multiple-sclerosis/7892b2a2-f52a-4d7f-9858-a326f2b7ab04)
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- Tumor (astrocytoma, metastasis, lymphoma)
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- Lesions involving corticospinal tract
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- Wallerian degeneration, adrenoleukodystrophy
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- [Amyotrophic lateral sclerosis](/document/amyotrophic-lateral-sclerosis-als/23de52b7-d9bd-441c-a18c-95c8afccb470)
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- Vertebrobasilar perforating artery infarct
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- Most medullary infarctions occur in posteroinferior cerebellar artery territory & involve posterolateral medulla (e.g., vertebral artery dissection)
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- Alternatively, medullary infarcts could be related to perforating branches of anterior spinal or vertebral arteries & have paramedial location
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- Infectious/inflammatory processes
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- [Tuberculosis](/document/tuberculosis/6e389773-2150-4299-9ce2-0b83b13c2119)
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- [Sarcoidosis](/document/neurosarcoid/fef69139-0019-4be3-9bdc-e26bc3644251)
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- HIV/AIDS
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- [Rhombencephalitis](/document/miscellaneous-encephalitis/1c3c0881-4046-46a1-90fc-371941c0cf2c)
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# PATHOLOGY
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- ## General Features
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- ### Etiology
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- Transsynaptic degeneration caused by interruption of pathways composing GMT
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- Olivary deafferentation thought to be source of ensuing HOD
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- Primary lesions usually located in contralateral DN or ipsilateral CTT
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- Focal brainstem insults that may lead to dentato-rubral-olivary pathway interruption
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- Ischemic infarction, demyelination
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- Hemorrhage (related to hypertensive disease, occult cerebrovascular malformation, or diffuse axonal injury following severe head trauma)
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- Cavernous malformation
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- ### Associated abnormalities
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- Primary brainstem insult
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- Most commonly pontine hemorrhage from trauma (including surgery), hypertension, tumor, & infarction
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- Olivary enlargement: Histologically unusual vacuolar cytoplasmic degeneration → hypertrophy related in part to ↑ number of astrocytes
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- After onset of primary lesion
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- Vacuolar cytoplasmic degeneration in 6-15 months
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- Gliosis follows at 15-20 months
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- ## Staging, Grading, & Classification
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- 6 phases of pathologic change
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- No olivary changes within first 24 hours
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- Degeneration of olivary amiculum (white matter capsule at olive periphery) at ≥ 2-7 days
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- Olivary hypertrophy (mild enlargement with neuronal hypertrophy, no glial reaction) at 3 weeks
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- Maximal olivary enlargement (hypertrophy of neurons & astrocytes) at 8.5 months
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- Olivary pseudohypertrophy (neuronal dissolution with prevailing large gemistocytic astrocytes) after 9.5 months
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- Olivary atrophy (neuronal disappearance with olivary atrophy & prominent degeneration of amiculum olivae) after 3-5 years of primary lesion
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- ## Gross Pathologic & Surgical Features
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- Focal swelling of ION
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- Unilateral HOD
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- Asymmetric enlargement of anterior medulla
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- "Pallor" in contralateral DN
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- Atrophy of contralateral cerebellar cortex
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- Bilateral HOD: More difficult to observe
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- No left-right asymmetry
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- ## Microscopic Features
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- Changes in hypertrophic degenerated ION
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- Hypertrophic, thickened neurites
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- Vacuolation of neurons
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- Fibrillary gliosis
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- Demyelination & astrocytic proliferation of WM
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- In contralateral cerebellar cortex
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- ↓ number of Purkinje cells
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- Contralateral DN reduced in size, possibly due to
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- Iron depletion secondary to axonal iron transport block
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- Loss of cells in nucleus
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# CLINICAL ISSUES
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- ## Presentation
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- ### Most common signs/symptoms
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- Symptomatic palatal tremor/myoclonus
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- Rhythmic involuntary movement of soft palate, uvula, pharynx, & larynx
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- Severe myoclonus may also affect cervical muscles & diaphragm
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- ± dentato-rubral tremor (Holmes tremor)
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- 2-5 Hz rest, postural, & kinetic tremor of upper extremity
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- May occur before onset of palatal tremor
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- Symptoms of cerebellar or brainstem dysfunction
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- Associated with acute lesion within triangle of Guillain-Mollaret
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- Ocular myoclonus & nystagmus
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- ### Clinical profile
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- Palatal myoclonus (palatal "tremor")
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- Usually develops 10-11 months after primary lesion
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- Virtually all patients who develop palatal myoclonus after brain insult will have HOD
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- Not all HOD patients develop palatal myoclonus
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- May result from hypermetabolism of ION
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- ## Demographics
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- ### Age
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- Rare; reported in all ages, both sexes
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- ## Natural History & Prognosis
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- After primary brainstem injury, olivary hypertrophy typically appears in delayed fashion
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- May occur between 3 weeks to 11 months (usually within 4-6 months)
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- Maximum hypertrophy at 5-15 months
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- Olivary hypertrophy typically resolves in 10-16 months
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- Olivary hyperintensity on T2WI may persist for years after resolution of hypertrophy
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- Finally ION undergoes atrophy
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- Clinical symptoms (tremors) rarely improve
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- Self-limiting disease & can be managed by symptomatic treatment
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# DIAGNOSTIC CHECKLIST
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- ## Image Interpretation Pearls
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- Avoid misdiagnosis of tumor or multiple sclerosis
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- Bilateral & symmetrical lesions in ION argue against subacute infarct & vertebral artery dissection
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16a22041-bb55-47a8-b601-8617efdb98ad
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