Step 1 topics → Neurology and Special Senses → Spinal Cord Syndromes

Spinal Cord Syndromes

USMLE Step 1 trap: Confuses syringomyelia's dissociated sensory loss with complete sensory loss at the affected level. Syringomyelia destroys the anterior white commissure, causing bilateral loss of pain and temperature only (dissociated sensory loss), sparing dorsal column modalities.

Spinal cord syndromes are one of the highest-yield topics on USMLE Step 1 because they reward students who actually understand tract anatomy rather than just memorizing buzzwords. Each syndrome knocks out a specific anatomical region, producing a predictable deficit pattern — and the exam exploits the fact that most students confuse which side and which modality is affected. You'll see these tested as clinical vignettes describing a trauma patient, a syphilitic patient, or someone with gradual proprioception loss, and you need to reverse-engineer the lesion location from the deficit pattern.

The tricky part isn't learning the syndromes in isolation — it's keeping them straight under pressure. Syringomyelia, anterior cord syndrome, and central cord syndrome all cause bilateral sensory loss, but they spare completely different modalities. Brown-Séquard is notoriously reversed by students under stress. Subacute combined degeneration (B12 deficiency) and tabes dorsalis both hit the dorsal columns, which causes students to lump them together — but their additional tract involvement is completely different, and that difference drives the clinical picture. USMLE Step 1 will also distinguish conus medullaris from cauda equina, a classic 'same region, different structure' trap.

To master this topic, stop thinking in terms of syndrome names and start thinking in terms of tract anatomy: where does this pathway decussate, which cord region does it run through, and what does losing it look like clinically? Every spinal cord syndrome question is really a tract anatomy question in disguise. Build that map once and these vignettes become straightforward.

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Common misconceptions

Common mistake
Wrong: Syringomyelia causes loss of all sensory modalities at the affected level.
Right: Syringomyelia destroys the anterior white commissure, causing bilateral loss of pain and temperature only (dissociated sensory loss), sparing dorsal column modalities.
Syringomyelia expands within the central cord and specifically destroys the anterior white commissure — the site where second-order spinothalamic fibers cross from one side to the other. This selectively eliminates pain and temperature bilaterally at the affected levels while leaving dorsal column modalities (vibration, proprioception, fine touch) completely intact because those fibers travel ipsilaterally in the posterior cord without crossing through that region. The result is 'dissociated sensory loss' — a classic cape-distribution pattern where the student must recognize that only some modalities are lost, not all of them.
Common mistake
Wrong: Brown-Séquard syndrome causes contralateral motor loss and ipsilateral pain/temperature loss.
Right: Brown-Séquard causes ipsilateral UMN motor loss and ipsilateral dorsal column loss, with contralateral pain and temperature loss below the lesion.
Brown-Séquard is a spinal cord hemisection, and the key is knowing where each tract decussates. The corticospinal tract has already crossed in the medulla, so damage to the left cord = left-sided (ipsilateral) UMN motor loss below the lesion. The dorsal columns cross in the medulla as well, so they too are lost ipsilaterally below the lesion. The spinothalamic tract crosses within 1-2 spinal levels of entry, so it's already on the opposite side — meaning pain and temperature are lost contralaterally. Motor and proprioception = ipsilateral; pain and temperature = contralateral. That's the rule.
Common mistake
Wrong: Anterior cord syndrome spares motor function because the posterior cord is intact.
Right: Anterior cord syndrome spares dorsal column function (vibration and proprioception) while causing bilateral motor loss and bilateral pain/temperature loss.
Anterior cord syndrome (most commonly from anterior spinal artery occlusion) destroys the anterior two-thirds of the cord, which includes the corticospinal tracts and the spinothalamic tracts. What it spares is the posterior third — the dorsal columns. This means patients lose bilateral motor function and bilateral pain/temperature sensation, but retain vibration and proprioception. Students who say 'motor is spared because the posterior cord is intact' have it exactly backwards — it's the posterior column sensory modalities that are spared, not motor.
Common mistake
Wrong: Subacute combined degeneration and tabes dorsalis both affect the same spinal cord tracts.
Right: SCD (B12 deficiency) affects dorsal columns AND lateral corticospinal tracts, while tabes dorsalis (syphilis) affects dorsal columns and dorsal roots only, without corticospinal involvement.
Both conditions damage dorsal columns, which is why students conflate them — but the additional tract involvement is the critical distinguishing feature. SCD from B12 deficiency also destroys the lateral corticospinal tracts, producing UMN signs (spasticity, hyperreflexia) on top of dorsal column loss. Tabes dorsalis from tertiary syphilis involves the dorsal columns and dorsal nerve roots but spares the corticospinal tracts entirely, giving you hyporeflexia, Argyll Robertson pupils, and lancinating pain without UMN findings. If you see both dorsal column loss AND hyperreflexia, think SCD; if you see dorsal column loss WITH areflexia and syphilis history, think tabes.
Common mistake
Wrong: Conus medullaris syndrome presents with pure LMN signs because it is at the end of the cord.
Right: Conus medullaris syndrome produces mixed UMN and LMN signs, whereas cauda equina syndrome produces pure LMN signs because it involves nerve roots below the cord.
The conus medullaris is the terminal end of the spinal cord itself, containing sacral cord segments S2-S4. Because it's still cord tissue, it still has UMN components — so a conus lesion produces a mixed picture: UMN signs from cord involvement plus LMN signs from destruction of the anterior horn cells at that level. The cauda equina, on the other hand, is made entirely of peripheral nerve roots floating below the cord. There's no cord tissue there, only LMN fibers — so cauda equina syndrome gives pure LMN signs: areflexia, flaccidity, saddle anesthesia, and bowel/bladder dysfunction. The rule: if it's cord, expect mixed; if it's roots only, expect pure LMN.
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What the exam tests

  1. Given a patient with bilateral loss of pain and temperature at a specific dermatome level but intact vibration and proprioception, identify syringomyelia as the cause and explain why the anterior white commissure is the culprit.
  2. Given a hemisection injury of the spinal cord, correctly assign ipsilateral vs. contralateral deficits for each tract — UMN motor loss, dorsal column loss, and spinothalamic loss — and identify this as Brown-Séquard syndrome.
  3. Identify which sensory modality is preserved in anterior cord syndrome (vibration and proprioception via dorsal columns) and which deficits are present (bilateral motor loss and bilateral pain/temperature loss).
  4. Distinguish subacute combined degeneration (dorsal columns + lateral corticospinal tracts, B12 deficiency) from tabes dorsalis (dorsal columns + dorsal roots only, syphilis) based on tract involvement and associated clinical signs.
  5. Differentiate conus medullaris syndrome (mixed UMN + LMN signs, involves the cord itself) from cauda equina syndrome (pure LMN signs, involves nerve roots below the cord) given a clinical vignette with lower sacral deficits.

Can you avoid these mistakes?

A 30-year-old man with a known Arnold-Chiari malformation develops progressive bilateral loss of pain and temperature over his shoulders and upper arms, but vibration sense and proprioception are intact in the same region. What structure is being destroyed, and why are only these modalities affected?
A patient sustains a knife wound causing complete left-sided hemisection of the spinal cord at T6. On examination, which deficits are ipsilateral and which are contralateral — and for each deficit, name the specific tract responsible?
A 55-year-old vegan presents with gradually worsening gait ataxia and lower extremity weakness. Exam shows loss of vibration sense in the feet, hyperreflexia, and a positive Babinski sign. What two spinal cord tracts are affected, what is the unifying diagnosis, and how does this differ from what you'd expect in tabes dorsalis?
Two patients both present with saddle anesthesia and urinary retention. Patient A has flaccid lower extremity weakness with areflexia. Patient B has lower extremity spasticity with hyperreflexia and extensor plantar responses alongside LMN bowel/bladder signs. Which syndrome does each patient have, and what anatomical difference explains the distinction?

Related topics

Spinal Cord Tracts (DCML, Spinothalamic, Corticospinal) Neural Tube Formation and Derivatives Neural Crest Derivatives Neural Tube Defects Other Developmental Malformations

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