Congenital & Spinal Cord Neuroradiology

A high-difficulty intersection of embryology, anatomy and MRI signal characteristics. NEET PG loves single-best-answer image-based stems here: a sagittal spine MRI, a posterior fossa anomaly, or a "finger-like" white-matter lesion. Master the named signs, the cut-off levels and the signal behaviour and these become free marks.

How to read this topic

Three big buckets are tested: (1) spinal dysraphism, (2) posterior fossa / hindbrain malformations (Chiari, Dandy-Walker), and (3) demyelinating disease of brain and cord (MS, NMO). Each has a pathognomonic MRI sign. The exam rarely asks pathology in isolation — it shows the picture/landmark and asks the diagnosis, or asks the signal/level.

High-yield: MRI is the investigation of choice (IOC) for all spinal cord and congenital CNS lesions. CT is used only for acute trauma, bony detail or when MRI is contraindicated. Antenatally, USG screens (raised maternal serum AFP triggers it) and fetal MRI confirms.

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1. Spinal Dysraphism

Failure of normal neurulation/closure of the neural tube. Classified by whether the neural tissue is covered by intact skin.

Classification

Type	Skin cover	Subtypes	Key clue
Open (aperta)	Absent — exposed neural tissue, leaks CSF	Myelomeningocoele, myelocoele	Almost always with Chiari II; raised AFP
Closed (occulta)	Intact skin	With subcutaneous mass: lipomyelomeningocoele, meningocoele. Without mass: tethered cord, dermal sinus, diastematomyelia, filum lipoma	Cutaneous stigma: tuft of hair, dimple, naevus, dermal sinus

Myelomeningocoele — commonest open dysraphism; neural placode + meninges herniate through a posterior bony defect. Lumbosacral region most common. Virtually 100% associated with Chiari II.

Diastematomyelia (split cord malformation) — sagittal cleft splitting the cord into two hemicords.

• Type I: two dural sacs separated by a bony/cartilaginous septum (osseous spur on CT) — usually symptomatic, surgical.
• Type II: single dural sac, fibrous septum — milder.

Tethered cord syndrome

Abnormally low, tethered conus that cannot ascend.

High-yield: Normal conus medullaris ends at L1–L2 (lower border of L2 by adulthood). A conus below L2–L3 with a thickened filum terminale (>2 mm) = tethered cord. Often a fatty (T1-hyperintense) filum.

Clinical: progressive lower-limb weakness, bladder/bowel dysfunction, foot deformity, scoliosis; worsens with growth spurts. MRI is IOC. Treatment is surgical untethering.

Approach to a midline back lesion: Cutaneous stigma seen → MRI whole spine → conus level & filum thickness → look for lipoma/sinus/split cord → screen brain for Chiari.

High-yield: A dermal sinus tract is a recurrent-meningitis cause in children — it is a conduit for infection and may end in a dermoid/epidermoid. Always image the brain too.

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2. Chiari Malformations

Hindbrain herniation through the foramen magnum. Chiari I and II dominate the exam.

Chiari I

Caudal descent of the cerebellar tonsils through the foramen magnum, with a normally positioned fourth ventricle.

High-yield: Diagnostic cut-off — tonsils >5 mm below the foramen magnum (McRae line, basion–opisthion). 3–5 mm is borderline. Pointed/"peg-like" tonsils favour true Chiari I over benign ectopia.

• Association: syringomyelia in 30–70% — the single most tested association.
• NOT associated with myelomeningocoele or hydrocephalus (that's Chiari II).
• Presents later (adolescence/adult): suboccipital cough headache, central-cord sensory loss.

Chiari II (Arnold-Chiari)

Small posterior fossa with downward herniation of vermis, brainstem AND fourth ventricle through foramen magnum.

High-yield: Chiari II is almost always associated with myelomeningocoele and hydrocephalus. This near-100% link is a classic one-liner.

Imaging features (memorise the cluster):

• Beaked tectum (tectal beaking)
• Towering cerebellum, cerebellar peg
• Large massa intermedia
• Callosal dysgenesis
• Luckenschadel skull (lacunar skull / craniolacunia)

Feature	Chiari I	Chiari II
Herniated structure	Tonsils only	Vermis + brainstem + 4th ventricle
Posterior fossa	Normal/small	Small, crowded
Myelomeningocoele	Absent	Present (~100%)
Hydrocephalus	Usually absent	Present
Syrinx	Common	Can occur
Presentation	Adolescent/adult	Neonate/infant

Chiari III: Chiari II features + occipital/high-cervical encephalocoele — rare, severe.

Syringomyelia / Syrinx

A fluid-filled cavity within the cord (CSF-like signal: T1 hypo, T2 hyper), often cervical, expanding the cord.

High-yield: Classic clinical sign — "cape/shawl" dissociated sensory loss (loss of pain & temperature, preserved touch/proprioception) from decussating spinothalamic fibres at the central canal. Hands show wasting. Causes: Chiari I (commonest), post-traumatic, post-infective, intramedullary tumour. Always gadolinium to exclude an underlying tumour-associated syrinx.

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3. Corpus Callosum Agenesis (ACC)

Failure of callosal formation. The callosum forms front-to-back (genu → body → splenium → rostrum); the rostrum forms last, so in partial agenesis the splenium and rostrum are absent.

Classic MRI signs:

• "Racing-car" / Viking-helmet sign — parallel, widely separated lateral ventricles on axial.
• Colpocephaly — disproportionate dilatation of the occipital horns (atria).
• High-riding/everted third ventricle between the lateral ventricles.
• Radiating ("sunburst") medial gyri pointing to the third ventricle on sagittal.
• Probst bundles — longitudinal white-matter tracts (un-crossed callosal fibres) indenting the medial ventricular walls.

High-yield: ACC is associated with interhemispheric lipomas and Dandy-Walker and Aicardi syndrome (ACC + chorioretinal lacunae + infantile spasms; female, X-linked). Colpocephaly + parallel ventricles = ACC until proven otherwise.

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4. Dandy-Walker Malformation

A posterior fossa cystic malformation. Triad:

1. Agenesis/hypoplasia of the cerebellar vermis (vermian)
2. Cystic dilatation of the fourth ventricle filling the posterior fossa
3. Enlarged posterior fossa with high torcula (torcular-lambdoid inversion) — elevated tentorium/torcula

High-yield: Hydrocephalus develops in ~80%. The hallmark is the upward rotation of a hypoplastic vermis with the 4th ventricle open to a large retrocerebellar cyst.

Entity	Vermis	4th ventricle / cyst	Posterior fossa size	Mass effect
Dandy-Walker malformation	Hypoplastic/absent	4th ventricle continuous with large cyst	Enlarged, high torcula	Yes, ± hydrocephalus
Mega cisterna magna	Normal	Normal 4th ventricle	Enlarged CSF space	None
Arachnoid cyst (post. fossa)	Normal	Normal, displaced	Variable	Compresses, displaces 4th ventricle

The differential of a "posterior fossa cyst" is a favourite — the state of the vermis and 4th ventricle distinguishes them.

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5. Demyelinating Diseases

Multiple Sclerosis (MS)

Autoimmune demyelination disseminated in space and time. MRI is far more sensitive than CT.

High-yield: Dawson fingers — ovoid demyelinating plaques oriented perpendicular to the ventricles / callososeptal interface, best seen on sagittal FLAIR. They follow the deep medullary veins. This is the single most-asked MS image finding.

Lesion locations (typical): periventricular, juxtacortical, infratentorial (brainstem/cerebellum), and spinal cord. These four sites form the McDonald "dissemination in space" criteria.

Feature	MS plaque appearance
T1	Iso/hypointense; chronic axonal loss = "black holes"
T2/FLAIR	Hyperintense ovoid lesions
Gadolinium	Enhancing = active lesion; ring/open-ring enhancement (open ring toward grey matter is fairly specific for demyelination)
Cord lesions	Short-segment (< 2 vertebral bodies), peripheral, dorsolateral

High-yield: Active (enhancing) and non-enhancing lesions seen together satisfy "dissemination in time" on a single scan (2017 McDonald criteria). CSF oligoclonal bands can substitute for dissemination in time.

Diagnosis: Revised McDonald criteria (clinical + MRI dissemination in space & time + CSF oligoclonal bands). MRI brain + cord with contrast is the key investigation.

Neuromyelitis Optica (NMO / Devic disease)

Anti-aquaporin-4 (AQP4) antibody astrocytopathy. Distinguishing it from MS is heavily tested.

High-yield: NMO causes longitudinally extensive transverse myelitis (LETM) — a cord lesion spanning ≥ 3 vertebral segments, typically central cord. Plus optic neuritis. Serum AQP4-IgG (NMO-IgG) is the diagnostic antibody.

Feature	Multiple Sclerosis	NMO Spectrum Disorder
Antibody	None specific	AQP4-IgG (± MOG)
Cord lesion length	Short (<2 segments)	Long ≥3 segments (LETM)
Cord lesion position	Peripheral/dorsolateral	Central
Brain	Dawson fingers, periventricular	Often spares brain early; lesions around 3rd/4th ventricle (AQP4-rich)
Optic neuritis	Unilateral, anterior	Bilateral, posterior (chiasmal), severe
CSF oligoclonal bands	Positive (~85%)	Usually negative
Treatment	Interferon-β, etc. (interferon worsens NMO)	Eculizumab, rituximab, azathioprine

Stepwise cord-lesion approach: Transverse myelitis on MRI → measure segment length → ≥3 segments & central → think NMO → send AQP4-IgG; <2 segments & peripheral → think MS → brain MRI + oligoclonal bands.

ADEM (contrast point)

Acute disseminated encephalomyelitis — monophasic, post-infectious/post-vaccination, children. Large, bilateral, asymmetric subcortical and deep grey (thalami/basal ganglia) lesions, often enhancing simultaneously (same age). Differs from MS where lesions are of different ages.

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Key signal characteristics (rapid table)

Tissue / lesion	T1	T2 / FLAIR	Note
CSF / syrinx	Hypo (dark)	Hyper (bright); FLAIR suppresses pure CSF	Syrinx follows CSF
Fat / lipoma / fatty filum	Hyper (bright)	Variable; suppresses on fat-sat	Chemical-shift artefact
MS plaque (chronic)	Iso/hypo (black hole)	Hyper	Enhances if active
Demyelination active	—	Hyper	Gadolinium ring/open-ring
Subacute blood (methaemoglobin)	Hyper	Hyper	Helps date haemorrhage

High-yield: A T1-bright intraspinal structure = fat (lipoma/fatty filum) or subacute blood. In a tethered cord context, T1-bright filum = fatty filum.

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Complications to remember

• Tethered cord: progressive neuro-urological deficit, scoliosis, foot deformity.
• Chiari I: syringomyelia → central cord syndrome; hydrocephalus if obstructive.
• Chiari II / myelomeningocoele: hydrocephalus, neurogenic bladder, recurrent meningitis (open defect/dermal sinus), latex allergy.
• Dandy-Walker: hydrocephalus, developmental delay.
• MS: progressive disability (black holes = axonal loss), optic atrophy, bladder dysfunction.
• NMO: blindness, respiratory failure from high cervical cord lesions.

Key differentials

• Posterior fossa cyst: Dandy-Walker vs mega cisterna magna vs arachnoid cyst (judge by vermis + 4th ventricle).
• Long cord lesion (LETM): NMO vs cord infarct (anterior, "owl-eye"/"snake-eyes") vs SACD (dorsal column "inverted-V") vs intramedullary tumour.
• White-matter lesions: MS vs ADEM vs small-vessel ischaemia (older, basal ganglia, no Dawson fingers, no enhancement).
• Low conus + thick filum: tethered cord vs normal variant (use the L2 and 2-mm cut-offs).

Recently asked / exam angle

• "Dawson fingers seen on sagittal FLAIR — diagnosis?" → Multiple sclerosis.
• "Tonsillar herniation >5 mm below foramen magnum" → Chiari I; "associated with?" → syringomyelia.
• "Myelomeningocoele with hindbrain herniation of 4th ventricle" → Chiari II.
• "Posterior fossa cyst + vermian agenesis + enlarged posterior fossa" → Dandy-Walker.
• "Parallel lateral ventricles + colpocephaly (racing-car sign)" → corpus callosum agenesis.
• "LETM ≥3 segments + optic neuritis + AQP4 antibody" → NMO; "which drug worsens it?" → interferon-β.
• "Normal conus level in adult?" → lower border of L2.
• "Recurrent meningitis in a child with a back dimple" → dermal sinus tract → MRI spine.
• IOC for spinal cord lesions / congenital CNS anomalies → MRI.
• Open-ring enhancement → demyelination (differentiates from abscess/tumour ring).

Mnemonics:

• Chiari II "CHIARI": Cerebellar peg, Hydrocephalus, Inferior vermis herniation, Aqueduct issues, Rostral (tectal) beaking, I = myelomeningocoele invariably.
• Dandy-Walker triad: vermis gone, 4th ventricle blown, posterior fossa grown.
• NMO vs MS cord: NMO = Numerous segments (Long, ≥3, central); MS = Minimal/short, peripheral.

Rapid revision

1. MRI = IOC for all spinal cord and congenital CNS lesions; CT only for bone/acute trauma.
2. Conus normally ends at lower L2; below L2–L3 with filum >2 mm = tethered cord.
3. Chiari I = tonsils >5 mm below foramen magnum (McRae line); associated with syringomyelia, not myelomeningocoele.
4. Chiari II = herniated vermis + brainstem + 4th ventricle; ~100% with myelomeningocoele + hydrocephalus; tectal beaking, Luckenschadel skull.
5. Syrinx = T1-dark/T2-bright cavity; "cape" dissociated sensory loss; always give gadolinium to exclude tumour.
6. Dawson fingers on sagittal FLAIR = MS; lesions periventricular, juxtacortical, infratentorial, cord (DIS).
7. MS cord lesions are short (<2 segments), peripheral; black holes on T1 = axonal loss; open-ring enhancement is specific.
8. NMO = AQP4-IgG, LETM ≥3 central segments + optic neuritis; interferon worsens it; treat with rituximab/eculizumab.
9. Corpus callosum agenesis: parallel ventricles (racing-car sign), colpocephaly, high third ventricle, Probst bundles; linked to lipomas and Aicardi syndrome.
10. Dandy-Walker: vermian hypoplasia + cystic 4th ventricle + enlarged posterior fossa with high torcula; 80% hydrocephalus.
11. Posterior fossa cyst differential turns on vermis + 4th ventricle (DW vs mega cisterna magna vs arachnoid cyst).
12. Diastematomyelia Type I = bony septum + two dural sacs (CT spur); Type II = fibrous septum, single sac.