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KEY FACTS

Terminology
- Inferior olivary nucleus (ION) degeneration - Unique type of transsynaptic neuronal degeneration - Olivary deafferentation thought to be source of ensuing hypertrophic olivary degeneration (HOD)
- Usually caused by primary lesions in dentato-rubro-olivary pathway (Guillain-Mollaret triangle)
- Triangle of Guillain-Mollaret defined by 3 anatomic structures - Red nucleus (RN) - ION ipsilateral to RN - Contralateral dentate nucleus (DN) of cerebellum
Imaging
- ION initially hypertrophies rather than atrophies
- 3 distinct MR stages in HOD - Hyperintense signal without hypertrophy of ION: Within first 6 months of ictus - ↑ signal + ION hypertrophy: Between 6 months & 3-4 years after ictus - Only ION hyperintensity: Begins when hypertrophy resolves (can persist indefinitely)
- MR also detects primary lesion located in ipsilateral CTT, SCP or contralateral DN
Top Differential Diagnoses
- Vertebrobasilar perforating artery infarct
- Demyelination (multiple sclerosis, microvascular disease)
- Amyotrophic lateral sclerosis
- HIV/AIDS
- Rhombencephalitis
Clinical Issues
- Palatal myoclonus (palatal "tremor"), ocular myoclonus
- Usually develops 10-11 months after primary lesion
- Clinical symptoms (tremors) rarely improve

TERMINOLOGY

Abbreviations
- Hypertrophic olivary degeneration (HOD)
Synonyms
- Pseudohypertrophy of inferior olivary nucleus
Definitions
- 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)

IMAGING

General Features

Best diagnostic clue

- T2-hyperintense, nonenhancing enlargement of ION

Location

- GMT is defined by 3 anatomic structures
        - Red nucleus (RN)
        - ION ipsilateral to RN
        - Contralateral dentate nucleus (DN) of cerebellum
- Central tegmental tract (CTT or rubro-olivary pathway) connects RN to ipsilateral ION
- Superior cerebellar peduncle (SCP, dentato-rubral tract) connects DN to contralateral RN
- Inferior cerebellar peduncle (olivo-cerebellar pathway) connects ION to contralateral cerebellar cortex & contralateral DN
- 4 patterns of HOD in relation to primary lesion
        - Ipsilateral HOD: Primary lesion is limited to brainstem (CTT)
        - Contralateral HOD: Primary lesion is in cerebellum (DN or SCP)
        - Bilateral HOD: Primary lesion involves midline/paramedian brainstem affecting brachium conjunctivum
        - Bilateral HOD: Primary lesion involves both unilateral brainstem & cerebellum

Size

- Variable (time-dependent) size of affected ION
        - Normal in acute stage
        - ↑ (hypertrophy) from 6 months to 3-4 years
        - ↓ (atrophy) in advanced stage (> 3-4 years)

Morphology

- Unique type of transsynaptic neuronal degeneration
        - ION initially hypertrophies rather than atrophies

CT Findings

NECT

- May show acute primary injury (e.g., hemorrhage) in tegmentum
- HOD typically not depicted on CT

MR Findings

T1WI

- Acute phase: Normal ION
        - Shows primary lesion in brainstem (cerebellum or tegmentum)
- After HOD ensues
        - Enlargement confined to ION, isointense to slightly hypointense to gray matter
        - Slightly ↑ olivary T1 signal also reported
        - ± residual primary lesion

T2WI

- 3 distinct MR stages in HOD
        - Hyperintense signal without hypertrophy of ION: Within first 6 months of ictus
        - Both ↑ signal & hypertrophy of ION: Between 6 months & 3-4 years after ictus
        - ↑ signal only in ION: Begins when hypertrophy resolves & can persist indefinitely
- Axial MR: Disappearance of pre- & postolivary sulci in hypertrophic stage
- MR also detects primary lesion located in ipsilateral central tegmental tract or contralateral DN
        - Old hematomas: Low-signal areas on T2WI revealing hemosiderin deposition
- ± ↓ size of contralateral ION with higher than normal signal intensity
- ± mild to severe atrophic changes of cerebellar cortex contralateral to HOD

PD/intermediate

- High signal intensity of ION better detected on PD images than on T2WI

FLAIR
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- Similar to T2WI
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T1WI C+

- No contrast enhancement of degenerated ION

DTI - ↑ radial diffusivity, ↑ mean diffusion & ↓ fractional anisotropy in GMT components reflecting demyelination - ↑ fractional anisotropy & ↑ axial diffusivity in ION reflect rearrangement of regenerating axons & shrunken neurons

Nuclear Medicine Findings

PET

- Focal glucose hypermetabolism in medulla of patients with HOD

Imaging Recommendations
- Best imaging tool
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- MR
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- Protocol advice
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- T2WI (include coronal or sagittal sections)
```

DIFFERENTIAL DIAGNOSIS

Other Causes of High T2 Signal Intensity in Anterior Part of Medulla
- Demyelination related to multiple sclerosis
- Tumor (astrocytoma, metastasis, lymphoma)
- Lesions involving corticospinal tract - Wallerian degeneration, adrenoleukodystrophy - Amyotrophic lateral sclerosis
- Vertebrobasilar perforating artery infarct - Most medullary infarctions occur in posteroinferior cerebellar artery territory & involve posterolateral medulla (e.g., vertebral artery dissection) - Alternatively, medullary infarcts could be related to perforating branches of anterior spinal or vertebral arteries & have paramedial location
- Infectious/inflammatory processes - Tuberculosis - Sarcoidosis - HIV/AIDS - Rhombencephalitis

PATHOLOGY

General Features

Etiology

- Transsynaptic degeneration caused by interruption of pathways composing GMT
- Olivary deafferentation thought to be source of ensuing HOD
- Primary lesions usually located in contralateral DN or ipsilateral CTT
- Focal brainstem insults that may lead to dentato-rubral-olivary pathway interruption
        - Ischemic infarction, demyelination
        - Hemorrhage (related to hypertensive disease, occult cerebrovascular malformation, or diffuse axonal injury following severe head trauma)
        - Cavernous malformation

Associated abnormalities

- Primary brainstem insult
        - Most commonly pontine hemorrhage from trauma (including surgery), hypertension, tumor, & infarction

Olivary enlargement: Histologically unusual vacuolar cytoplasmic degeneration → hypertrophy related in part to ↑ number of astrocytes
After onset of primary lesion - Vacuolar cytoplasmic degeneration in 6-15 months - Gliosis follows at 15-20 months

Staging, Grading, & Classification
- 6 phases of pathologic change - No olivary changes within first 24 hours - Degeneration of olivary amiculum (white matter capsule at olive periphery) at ≥ 2-7 days - Olivary hypertrophy (mild enlargement with neuronal hypertrophy, no glial reaction) at 3 weeks - Maximal olivary enlargement (hypertrophy of neurons & astrocytes) at 8.5 months - Olivary pseudohypertrophy (neuronal dissolution with prevailing large gemistocytic astrocytes) after 9.5 months - Olivary atrophy (neuronal disappearance with olivary atrophy & prominent degeneration of amiculum olivae) after 3-5 years of primary lesion
Gross Pathologic & Surgical Features
- Focal swelling of ION
- Unilateral HOD - Asymmetric enlargement of anterior medulla - "Pallor" in contralateral DN - Atrophy of contralateral cerebellar cortex
- Bilateral HOD: More difficult to observe - No left-right asymmetry
Microscopic Features
- Changes in hypertrophic degenerated ION - Hypertrophic, thickened neurites - Vacuolation of neurons - Fibrillary gliosis - Demyelination & astrocytic proliferation of WM
- In contralateral cerebellar cortex - ↓ number of Purkinje cells
- Contralateral DN reduced in size, possibly due to - Iron depletion secondary to axonal iron transport block - Loss of cells in nucleus

CLINICAL ISSUES

Presentation

Most common signs/symptoms

- Symptomatic palatal tremor/myoclonus
        - Rhythmic involuntary movement of soft palate, uvula, pharynx, & larynx
- Severe myoclonus may also affect cervical muscles & diaphragm
- ± dentato-rubral tremor (Holmes tremor)
        - 2-5 Hz rest, postural, & kinetic tremor of upper extremity
        - May occur before onset of palatal tremor
- Symptoms of cerebellar or brainstem dysfunction
        - Associated with acute lesion within triangle of Guillain-Mollaret
- Ocular myoclonus & nystagmus

Clinical profile

- Palatal myoclonus (palatal "tremor")
        - Usually develops 10-11 months after primary lesion
        - Virtually all patients who develop palatal myoclonus after brain insult will have HOD
        - Not all HOD patients develop palatal myoclonus
        - May result from hypermetabolism of ION

Demographics

Age

- Rare; reported in all ages, both sexes

Natural History & Prognosis
- After primary brainstem injury, olivary hypertrophy typically appears in delayed fashion - May occur between 3 weeks to 11 months (usually within 4-6 months)
- Maximum hypertrophy at 5-15 months
- Olivary hypertrophy typically resolves in 10-16 months
- Olivary hyperintensity on T2WI may persist for years after resolution of hypertrophy
- Finally ION undergoes atrophy
- Clinical symptoms (tremors) rarely improve
- Self-limiting disease & can be managed by symptomatic treatment

DIAGNOSTIC CHECKLIST

Image Interpretation Pearls
- Avoid misdiagnosis of tumor or multiple sclerosis
- Bilateral & symmetrical lesions in ION argue against subacute infarct & vertebral artery dissection

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References

Selected References

Images

Selected Images

Axial graphic of the upper medulla shows the medullary pyramids on each side of the ventral median fissure. The olives lie just posterior to the preolivary sulci .

Coronal graphic of the midbrain, pons, and medulla is sectioned to depict the Guillain-Mollaret triangle (GMT). The GMT is composed of the ipsilateral inferior olivary nucleus (green), dentate nucleus (blue) of the contralateral cerebellum, and the ipsilateral red nucleus (RN, red).

Axial T2 MR of a 40-year-old woman with brainstem glioma and secondary hypertrophic olivary degeneration (HOD) shows a heterogeneous mass lesion involving midbrain invading the RN (R > L). RN is a component of GMT.

Axial T2 MR at the level of medulla in the same patient shows enlarged right inferior olivary nucleus with hyperintense signal indicating HOD. Also note normal-appearing left olivary nucleus and preolivary sulcus .

Axial FLAIR MR of a 58-year-old woman presenting with palatal myoclonus and a history of treated CNS lymphoma shows volume loss and hyperintense signal in left dentate nucleus (DN) due to encephalomalacia (DN is a component of GMT).

Axial T2 MR in the same patient at the level of medulla shows mild hypertrophy and increased signal involving bilateral inferior olivary nuclei indicating HOD.

Axial T2 of a 67-year-old man with left para median pontine cavernous malformation (CM) involving central tegmental tract resulting in ipsilateral HOD shows hyperintense popcorn lesion with rim of hemosiderin in left para median pons due to CM.

Axial FLAIR MR in the same patient shows enlarged left inferior olivary nucleus with hyperintense signal due to HOD.

Axial T2 MR at 1 day (top left), 4 months (top right), and 7 months (bottom left) postoperative follow-up show edema in left DN and normal right olive . Note light enlargement and ↑ signal in right olive , progressive enlargement and ↑ signal in olive , and lack of enhancement in olive on postcontrast T1WI (bottom right).

Axial graphic of the midbrain at the level of the hypoglossal nuclei shows the distinct wavy pattern of the olives corresponding to the FLAIR hyperintensity in the previous image.

Additional Images

Axial T2WI MR demonstrates hypertrophy of both inferior olivary nuclei, which are also hyperintense , secondary to HOD.

Sagittal FLAIR MR shows abnormally ↑ signal intensity in an anterior medullary area that corresponds to the inferior olivary nucleus.

Axial FLAIR MR in the same patient who suffered midbrain hemorrhage (not shown) depicts bilateral hyperintense and hypertrophied inferior olivary nuclei .

Axial FLAIR MR shows high signal intensity and asymmetric enlargement of right anterior medulla corresponding to the region of hypertrophic degeneration of the right inferior olivary nucleus .

Axial T2WI MR in the same patient shows a right pontine infarct, the primary lesion that led to right HOD.

Axial T2WI MR shows bilateral symmetric hypertrophy with ↑ signal intensity confined to inferior olivary nuclei, with loss of pre- and postolivary sulci .

Axial T2WI MR in the same patient shows the primary midbrain lesion that caused the occurrence of bilateral HOD.

Axial T2WI MR in a patient who developed onset of dysarthria and upper extremity dysmetria 15 months following stereotaxic XRT for midbrain arteriovenous malformation shows mixed hyper-/hypointensity in the residual vascular malformation .

Axial T2WI MR in the same patient shows bilateral inferior olivary hyperintensity and hypertrophy .

Axial T2WI MR (CISS) shows the normal shape of the medullary olives .

Axial T2WI MR in a patient who developed palatal myoclonus ~ 6 months after resection of a midbrain CM shows hyperintensity and enlargement of both olives . This pattern is typical in the subacute stage of HOD, which typically appears between 6 months and 3-4 years after injury to the dentato-rubro-olivary pathway.

Axial SWI MR demonstrates hemosiderin staining in the dorsal aspect of the brainstem in the midline and to the right due to an old hemorrhage.

Axial FLAIR MR in the same patient at the level of the medulla shows mild hypertrophy with hyperintensity in the region of the right inferior olivary nucleus . Findings are typical for HOD caused by primary lesions in dentato-rubro-olivary pathway (anatomical GMT).

Axial T2WI MR through the medulla shows that the ipsilateral olive is atrophic and hyperintense . This patient also has crossed cerebellar atrophy due to interruption of the ponto-cerebellar pathway.

Axial T2WI MR in a patient who developed palatal myoclonus several months following midbrain surgery for CM. Imaging obtained 1 year later shows residual CM .

Axial FLAIR MR in the same patient delineates the somewhat wavy appearance of the hyperintensity conforming to the configuration of the olives . The pyramids are spared, helping differentiate HOD from perforating artery infarction.

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KEY FACTS

Terminology

Imaging

Top Differential Diagnoses

Clinical Issues

TERMINOLOGY

Abbreviations

Synonyms

Definitions

IMAGING

General Features

Best diagnostic clue

Location

Size

Morphology

CT Findings

NECT

MR Findings

T1WI

T2WI

PD/intermediate

FLAIR

T1WI C+

Nuclear Medicine Findings

PET

Imaging Recommendations

Best imaging tool

Protocol advice

DIFFERENTIAL DIAGNOSIS

Other Causes of High T2 Signal Intensity in Anterior Part of Medulla

PATHOLOGY

General Features

Etiology

Associated abnormalities

Staging, Grading, & Classification

Gross Pathologic & Surgical Features

Microscopic Features

CLINICAL ISSUES

Presentation

Most common signs/symptoms

Clinical profile

Demographics

Age

Natural History & Prognosis

DIAGNOSTIC CHECKLIST

Image Interpretation Pearls

References

Selected References

Images

Selected Images

Additional Images

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