MRI findings in multiple sclerosis
In multiple sclerosis, optic nerve (nervus opticus) involvement typically affects relatively short segments. Tomography (in STIR and fat-suppressed T1-weighted imaging modes) reveals the following characteristic features:
- Presence of a hyperintense MR signal in the intraorbital or canalicular segments.
- Local edema of the nerve trunk.
- Pathological contrast enhancement.
The process is frequently unilateral. Additionally, these patients exhibit typical demyelinating plaques located in the white matter of the frontal lobes.
Features in neuromyelitis optica (NMO)
Neuromyelitis optica, or Devic's disease, has a more severe course and specific radiological markers. Antibodies against aquaporin-4 (anti-AQP4 / AQP-4) and myelin oligodendrocyte glycoprotein (anti-MOG) play a key role in the pathogenesis of this spectrum of disorders (NMOSD).
Differences between NMO and multiple sclerosis:
- Visual pathways: Long, extensive segments are affected. The process is often bilateral. Involvement of the optic chiasm (chiasma opticum), optic tract (tractus opticus), and concomitant hypothalamic inflammation are characteristic.
- Spinal cord: A longitudinally extensive transverse myelitis (LETM) develops. Demyelinating lesions span three or more vertebral segments.
- Brain: Pathological lesions are located circumventricularly, particularly in the area postrema and periependymal regions of the brainstem.
Granulomatous processes and non-specific changes
The anterior visual pathway may be involved in specific granulomatous diseases.
- Sarcoidosis reveals an inflammatory process accompanied by contrast enhancement in the pituitary stalk (pituitary infundibulum) and chiasm.
- Tuberculous basal meningitis is typically characterized by diffuse leptomeningeal enhancement involving the intracranial segments of the nerves and the chiasm.
In general, radiological signs of neuritis (nodular or uniform nerve enhancement, edema, basal contrast accumulation) are non-specific. It is extremely difficult to differentiate them from a neoplastic process based solely on neuroimaging data. Therefore, the differential diagnosis must always include metastatic involvement.
Chiasmal compression and the posterior visual pathway
External compression of the optic chiasm occurs much more frequently than intrinsic lesions. The main causes of compression include pituitary macroadenoma, meningioma, craniopharyngioma, chordoma, Rathke's cleft cyst, germinoma, aneurysms, and metastases.
The nature of the neurological deficit depends on the vector of compression:
- Midline compression: causes bitemporal hemianopia.
- Lateral compression: leads to a nasal visual field defect (due to compression of uncrossed fibers from the temporal half of the retina).
Pathologies of the posterior visual pathway (optic radiations, cortex, and lateral geniculate body — LGB) usually develop secondarily. Isolated LGB involvement is extremely rare and manifests as specific homonymous sector sparing/defects strictly along the horizontal meridian.
Briefly: answers to topical questions
What anatomical topographic segments is the optic nerve divided into when evaluated on MRI?
The optic nerve is anatomically divided into four segments:
- Intraocular (intraocular).
- Intraorbital (intraorbital).
- Intracanalicular (intracanalicular).
- Intracranial (intracranial).
Which MRI modes and pulse sequences are included in the standard diagnostic protocol for optic neuritis?
The described MRI protocols for optic neuritis utilize:
- STIR in the coronal plane.
- T2-weighted fast spin-echo with fat suppression.
- T1-weighted image after contrast administration, with fat suppression.
Coronal planes are specified for the study.
To memorize visual field defects in chiasmal compression: Midline pressure hits the crossed fibers (affecting the temporal fields — bitemporal hemianopia), while lateral pressure damages the uncrossed outer pathways (affecting the nasal fields).
Class and exam questions
What is the main MRI difference in optic nerve involvement between multiple sclerosis and NMO?
In multiple sclerosis, short segments of the nerve are affected (often unilaterally), whereas in neuromyelitis optica (NMO), long extensive areas are involved bilaterally, invariably including the optic chiasm.
Why is it necessary to rule out oncology in optic neuritis?
MRI signs of inflammation (nerve edema, nodular or uniform contrast enhancement) are non-specific. The same radiological picture can be observed in metastatic disease or other neoplasms.
What pathology causes homonymous sector visual defects?
Such specific defects along the horizontal meridian occur with lesions of the lateral geniculate body (LGB), which is typically involved in the pathological process secondarily.
What else to study on this topic
- Imaging features of longitudinally extensive transverse myelitis in NMOSD.
- MRI semiotics of pituitary macroadenoma and craniopharyngioma as causes of chiasmal compression.
- The role of fat-suppression techniques (STIR) in orbital neuroimaging.
- Differential diagnosis of tuberculous meningitis and neurosarcoidosis based on MRI findings.
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