- 1200 patients who presented clinically with pontine symptoms were evaluated using MRI of the brain and the following cases were encountered:
- Metabolic - congenital disorders: Wernicke’s encephalopathy,
Wilson’s disease,
Central pontine myelinolysis,
Spinocerebellar ataxia,
Multisystem atrophy,
X-linked adrenoleukodystrophy.
- Infectious - inflammatory conditions : Multiple sclerosis,
acute disseminated encephalomyelitis,
neurocysticercosis,
tuberculomas.
- Degenerative conditions : Progressive supranuclear palsy,
Wallerian degeneration of the corticospinal tracts.
- Infiltrating diseases: Sarcoidosis.
- Vascular conditions: Cavernoma,
AVM,
infarct.
- Tumor: Glioma,
lymphoma and metastatic lesions.
The Brainstem is divided into three parts namely midbrain,
pons and medulla.
The pons ("bridge of Varolius") is the middle bulbous portion which acts as a relay between the cerebellum and cerebral hemispheres to conduct signals from the brain.
It develops from metencephalon.
Gross anatomy
It has two parts: The basis pontis (ventral part) and the pontine tegmentum (dorsal part).
The basis pontis consists of white matter tracts (anterior and lateral corticospinal,
corticobulbar and corticopontine tracts) with transverse fibers contributing to the bulk of the pons.
The pontine tegmentum is in continuity with tegmentum.
It consists of white matter tracts mainly medial longitudinal fasciculus,
medial lemniscus,
lateral lemniscus and grey matter nuclei for cranial nerves.
Posteriorly,
the pons is connected to the cerebellum by the middle cerebellar peduncle [1].
Within the dorsal tegmentum lie four cranial nerve nuclei [1]: Refer to figure 1.
- Trigeminal nerve (CN V): motor,
sensory and mesencephalic nuclei extending from the pons to the upper cervical cord
- Abducens nerve (CN VI): motor nucleus
- Facial nerve (CN VII): including superior salivary,
motor and solitary tract nuclei
- Vestibulocochlear nerve (CN VIII): including vestibular and cochlear nuclei
Blood supply:
Vertebrobasilar circulation: Medial branches of the superior cerebellar artery,
pontine branches of the basilar artery (refer figure 2 & 3 for tracts,
nuclei,
and basilar artery supply),
thalamoperforator arteries.
The primary function of pons: Sleep,
respiration,
swallowing,
bladder control,
hearing,
equilibrium,
taste,
eye movement,
facial expressions,
facial sensation,
and posture – refer figure 4 for a brief review of structure and function of the pons.
Syndromes associated with pontine pathologies: Refer to table 1.
Pontine pathologies:
1. Metabolic - congenital disorders:
a) Wernicke’s encephalopathy:
Aka Wernicke-Korsakoff syndrome occurs due to chronic thiamine i.e.,
vitamin B1 deficiency.
It typically occurs in alcoholics,
gastric malignancies,
and nutritional deficiencies.
Often present as amnesia,
ataxia,
and ophthalmoplegia.
Imaging pearl: (Figure 5)
Symmetrical T2/FLAIR hyperintensities in mammillary bodies,
dorsomedial thalami,
tectal plate,
periaqueductal area,
and around the third ventricle +/- restricted diffusion and contrast enhancement [2].
Utilization of advanced imaging technique: On MRS there is decreased NAA and increased lactate.
b) Wilson’s disease:
Wilson's disease (WD) is an autosomal recessive inherited copper metabolism disorder.
Mutations in the ATP7B gene disturb copper transport and lead to pathological copper accumulation especially in the liver and brain.
Imaging pearls (figure 6):
- T2/ FLAIR Hyperintensities around the substantia nigra and red nuclei produce the classic panda and mini panda sign – double panda sign of Wilson’s disease.
- High-signal-intensity lesions in the basal ganglia on T1.
- T2 hyperintensities reflect cerebral involvement and showed a good correlation with neurologic symptoms and for follow up to assess treatment response.
- Neurologic symptoms are reversible if treated early.
Utilization of advanced imaging technique: Paramagnetic mineralization deposition of both copper and iron in brain gray nuclei causes blooming on SWI.
c) Central pontine myelinolysis: aka osmotic demyelination syndrome is acute demyelination which occurs due to rapid correction of hyponatremia.
If extrapontine structures are affected then it is called extrapontine myelinolysis.
Often present as spastic quadriparesis and pseudobulbar palsy [3].
Imaging pearl (figure 7):
- T2 and FLAIR hyperintensities in the pons within 24 hours of onset of symptoms with a classic trident shaped appearance.
- The ventrolateral longitudinal fibers,
periventricular and subpial regions,
are typically spared.
- On axial FLAIR images it appears as piglet sign.
Utilization of advanced imaging technique: The earliest change is seen on DWI with restriction in the lower pons.
There can be contrast enhancement.
d) Spinocerebellar ataxia: Autosomal dominant inheritance with degeneration of cerebellum.
Abnormal mutation on the 12th chromosome results in extra copies of nucleotides C-A-G.
Imaging pearl (figure 8):
- Brainstem and cerebellar volume loss are the attractive surrogate markers of disease severity in SCA3 and SCA6.
- Differences from MSA includes atrophy of superior cerebellar peduncles,
frontal and temporal lobes and linear hyperintensities along the medial margins of globus pallidi.
e) Multisystem atrophy: A sporadic neurodegenerative synucleinopathies presenting as cerebellar ataxia,
autonomic dysfunction,
parkinsonism and corticospinal dysfunction.
Three patterns - Shy-Drager syndrome,
striatonigral degeneration (MSA-P),
olivopontocerebellar atrophy (MSA-C) [4].
Imaging pearl (figure 9):
- Hot cross bun sign (MSA-C): T2 hyperintensities in the pontocerebellar tracts
- Reduced volume and GRE signal in putamen,
globis pallidus and red nucleus (MSA-P).
- Disproportionate atrophy of the cerebellum and brainstem.
Utilization of advanced imaging technique: ADC values are higher and Fractional anisotropy values are lower in the pons,
cerebellum,
and putamen.
f) X-linked adrenoleukodystrophy: It occurs due to deficiency of peroxisomal enzyme Acyl Coa Synthetase. Abnormal accumulation of the very long fatty acids into myelin leads to dysmyelination.
Imaging pearl (figure 10):
- White matter dysmyelination predominantly in the parieto occipital lobes,
corpus callosum and pontomedullary corticospinal tract with relative sparing of the frontal lobe white matter and subcortical U fibers.
- ALD can be diagnosed from typical history and biochemical changes as well as from MRI findings.
Utilization of advanced imaging technique: Schaumberg zones,
enhancement in serpiginous,
garland-shaped pattern,
decrease in the NAA peak and an elevation in the lactate peak.
2. Infectious - inflammatory conditions:
a) Multiple sclerosis: Chronic autoimmune disease of the central nervous system in which inflammation,
demyelination,
and axonal loss occur.
It mainly affects young people with a female predominance.
Imaging pearl (figure 11):
- Hypo- or isointense on T1WI ("black holes") correlates with axonal destruction.
- Multiple T2/FLAIR hyperintense linear,
round,
or ovoid lesions surrounding the medullary veins,
ependymal dot-dash sign,
and Dawson’s fingers.
- Incomplete rim (horseshoe) of enhancement with the open nonenhancing segment facing the cortex.
- Latest criteria for diagnosing MS – refer table 2-4.
Utilization of advanced imaging technique:
Occasionally acute MS plaques can demonstrate restriction on DWI and should not be considered a reliable biomarker of plaque activity.
On MRS there is decreased NAA.
b) Acute disseminated encephalomyelitis: It is monophasic,
immune-mediated demyelinating disease which often presents in children following an infection or vaccination.
Imaging pearl (figure 12):
- High T2 signal,
with surrounding oedema in thalami and brainstem.
Less commonly in spine.
- Punctuate,
ring or arc enhancement along the leading edge of inflammation.
- The centre of the lesion does not have increased restriction on DWI.
Utilization of advanced imaging technique: Magnetisation transfer in normal-appearing brain has normal MT ratio and normal diffusivity in ADME,
whereas in multiple sclerosis both measurements are significantly decreased.
c) Neurocysticercosis: It is caused by the pork tapeworm Taenia solium,
which is endemic in developing countries where pigs are raised,
caused by eating food or drinking water contaminated by human faeces containing Taenia solium eggs.
Imaging pearl (figure 13):
•Escobars four pathological stage: Vesicular,
Colloidal vesicular,
Granular nodular,
Nodular calcified. The signal intensity of the cyst fluid depends on the viability of the parasite.
•Live cysts have a signal intensity similar to those of CSF.
Cyst with dot sign can be seen in vesicular stage
•With degeneration or after antihelminthic therapy,
the cyst fluid becomes more proteinaceous and gelatinous.
This change manifests as increased signal intensity on T1-weighted MR image.
Utilization of advanced imaging technique:
Three-dimensional (3D) constructive interference in steady state (CISS) is a gradient-echo MRI sequence helps in identification of the scolex is essential for definitive diagnosis of NCC.
d) Tuberculomas: Well defined focal masses that result from Mycobacterium tuberculosis infection,
and are more severe morphological forms.
Imaging pearl (figure 14):
- Central T2 hypointensity in a single/conglomerate rim /nodular enhancing lesion +/- parenchymal enhancement favors tuberculoma.
- Caseating tuberculomas with a solid center are isointense to hypointense on both T1- and T2-weighted MR images.
- Caseating tuberculomas with a liquid center are hypointense on T1-weighted images and centrally hyperintense on T2-weighted images,
with a peripheral hypointense rim on T2-weighted images.
- Other infectious or neoplastic diseases may present with similar findings.
The diagnosis of intracranial tuberculoma should rest on the proper integration of data from clinical manifestations,
cerebrospinal fluid analysis,
and neuroimaging studies.
Utilization of advanced imaging technique: show lipid resonance at 1.3 ppm,
2.02 ppm,
and 3.7 ppm at in vivo MR spectroscopy.
Lipid peak on MRS indicates caseating necrosis and decreased perfusion within tuberculoma differentiates it from necrotic neoplasm.
3.
Degenerative conditions :
Progressive supranuclear palsy: Aka Steele-Richardson-Olszewski syndrome,
is a tauopathy and considered a neurodegenerative disease.
Characterized by downward gaze palsy,
dysarthria,
parkinsonism,
and dementia.
Imaging pearl (figure 15):
- Flattening or concavity of the normally convex superior profile of the midbrain (hummingbird sign),
dilation of the Sylvian aqueduct and relative increase in the length of the interpeduncular fossa.
- Midbrain: pons ratio - reduced area ratio on the midline sagittal plane to approximately 0.12 (normal ~ 0.24).
Wallerian degeneration of the corticospinal tracts: Process of progressive demyelination and disintegration of the distal axonal segment following damage to the neuron.
It occurs due to the establishment of a microenvironment supportive of axonal regeneration.
Imaging pearl (figure 16): Chronic infarction in supratentorial cortico-spinal tracts from corona radiata to internal capsule and cerebral peduncle.
Atrophy with FLAIR hyperintensities in the brainstem.
4. Infiltrating diseases:
Sarcoidosis: It is a multisystem disease of unknown etiology characterized by the formation of inflammatory non-caseating granulomas within affected tissues.
Neurosarcoidosis potentially involves the leptomeninges,
pituitary and neuroparenchyma,
presenting as cranial nerve or endocrine manifestations.
Imaging pearl (figure 17):
- Heterogenous T2/FLAIR with contrast enhancement with smooth or nodular pachymeningeal enhancement along the perivascular space,
involvement of vessels results in vasculitis.
- Cranial nerves especially facial and optic nerve involvement are common.
Utilization of advanced imaging technique: Gallium-67 citrate scan is insensitive to central nervous system involvement and is helpful in confirming the presence of a systemic disease.
5. Vascular conditions:
a) Cavernoma:
Cavernomas are defined as vascular malformations consisting of abnormal,
dilated vessels within intervening neural tissue.
Imaging pearl (figure 18): A characteristic popcorn appearance with a rim of signal loss due to haemosiderin,
which demonstrates prominent blooming on gradient imaging.
Utilization of advanced imaging technique: Cavernomas show blooming on SW images (due to blood degradation products).
b) AVM: Malformations characterized by a nidus forming the transition between the feeding artery and draining vein.
When multiple,
syndromic associations such as Osler-Weber-Rendu syndrome and Wyburn-Mason syndrome should be considered.
It is rarely seen in the brainstem.
Imaging pearl (figure 19):
- Flow void in T2 images due to fast flow.
- Presence of hemorrhage/Aneurysms should be looked for.
- Spetzler-Martin arteriovenous malformation grading system: Size of nidus: small (<3 cm) = 1,
medium (3-6 cm) = 2,
large (>6 cm) = 3,
Eloquence of adjacent brain: non-eloquent = 0,
eloquent = 1,
Venous drainage: superficial veins only = 0,
deep veins = 1
Utilization of advanced imaging technique: Phase contrast MR angiography is often useful for detecting feeders,
aneurysms and draining vessels and subtracting the hematoma components.
c) Infarct:
Clinico-radiological spectrum in infarct – refer figure 5.
Most common presentation is anteromedial pontine syndrome and anterolateral pontine syndrome.
The main etiology was vertebrobasilar arteries disease [5].
Imaging pearl (figure 20): Restricted diffusion with low ADC values in a definitive clinical scenario.
Utilization of advanced imaging technique:
DWI is the most sensitive sequence for stroke imaging.
DWI is sensitive to the restriction of Brownian motion of extracellular water due to the imbalance caused by cytotoxic edema.
6.
Tumor:
a) Gliomas:
The WHO grading for gliomas ranges from I-IV.
I being low grade,
II being intermediate grade and III,
IV is high grade.
Primary glioma involving pons are rare,
usually seen in children and young adults and present with behavioral impairment.
Most common location in children for diffuse pontine glioma,
focal dorsally exophytic brainstem glioma is an uncommon variant and has a much better prognosis.
Imaging pearl (figure 21):
- Non enhancing/mildly enhancing T2 hyperintense lesion with enlargement of pons suggests low grade glioma in pediatric age group.
- Intra-axial heterogenous irregular peripherally enhancing masses with necrosis and perilesional edema causing mass effect, blooming on SWI,
restricted diffusion suggests high grade glioma.
Utilization of advanced imaging technique:
•MRS- increased choline and reduced NAA,
increased lipids/lactate,
decreased myoinositol indicates glioblastoma multiformae.
•At perfusion MR imaging,
significant increased cerebral blood volume favours high grade lesion.
•Significant blooming in SWI indicates increased neovascularity and suggests high grade neoplasm.
b) Metastatic lesions: Less commonly seen in brainstem.
Most common primary to metastasize are lung cancer,
renal cell carcinoma,
breast cancer,
melanoma and gastrointestinal tract adenocarcinomas.
Imaging pearl (figure 22):
Most are Hypointense on T1 and hyperintense on T2,
hyperintense on T1 if hemorrhagic with uniform,
punctate,
or ring-enhancement with restricted diffusion.
Utilization of advanced imaging technique: MR spectroscopy show intratumoural choline peak with no choline elevation in the peritumoural edema.
c) Lymphoma: Primary CNS lymphoma is uncommon tumors,
accounting for only 1% of malignant CNS tumors.
It commonly affects the periventricular location.
Imaging pearl (figure 23):
- T1-weighted MR imaging,
lesions are typically hypo- or isointense.
- T2-hypointensity and restricted diffusion with low ADC values suggest increased cellularity.
T2 hyperintensity (like in our case) is rare.
- Homogeneous enhancement.
Utilization of advanced imaging technique: ADC is particularly useful in assessing response to chemotherapy,
with increases in ADC values to above those of normal brain predictive of complete response