Central pontine myelinolysis (CPM) is one of those Step questions that rewards pattern recognition: a patient with chronic hyponatremia gets “fixed” too fast—and a few days later, they can’t talk, swallow, or move normally. If you can draw the pons and label what’s spared, you’ll remember CPM forever.
The one-liner (what CPM is)
Central pontine myelinolysis = osmotic demyelination of the central pons after overly rapid correction of chronic hyponatremia, causing acute dysarthria, dysphagia, and quadriparesis (± “locked-in” syndrome).
Draw-it-out method (quick sketch + visual)
Grab a pen and draw this in 10 seconds:
Step 1: Draw the “bagel pons”
- Draw an oval/circle (the pons in cross-section).
- Shade the center as the “danger zone.”
Step 2: Add the mnemonic
Write this across the shaded center:
“RAPID Na⁺ UP → PONS LAYS DOWN MYELIN”
Step 3: Label what’s spared
Draw a thin rim around the edge and label:
- Peripheral pons spared
- Corticospinal/corticobulbar tracts hit → motor + bulbar symptoms
What you should visualize
- Central lesion in the pons = big-time motor pathway disruption
- But classically no primary sensory loss (motor-heavy picture)
High-yield trigger: the timeline + the setup
Classic vignette:
- Chronic hyponatremia (often in alcohol use disorder, malnutrition, liver disease, post–liver transplant)
- Hospital corrects sodium aggressively
- Neurologic deterioration 2–6 days later (often not immediate)
Key idea: chronic hyponatremia → brain adapts by dumping osmolytes; rapid correction → extracellular fluid becomes relatively hypertonic → water shifts out of brain cells → oligodendrocyte injury + demyelination.
Clinical features you should memorize (Step 1/2 gold)
Think “pons = bulbar + bilateral motor.”
Common findings:
- Dysarthria
- Dysphagia
- Quadriparesis (UMN signs)
- Pseudobulbar affect can show up (emotional lability)
- Severe cases: locked-in syndrome (ventral pons/corticospinal + corticobulbar tracts)
What’s often preserved in locked-in:
- Consciousness (reticular activating system spared)
- Vertical eye movements (midbrain control) ± blinking
The rule you’ll be tested on: safe sodium correction
For chronic hyponatremia, avoid rapid correction.
Typical exam-safe targets:
- (many sources allow 8–10; boards love 8 as the conservative ceiling)
High-risk patients (alcohol use disorder, malnutrition, advanced liver disease) → keep it even slower (often mEq/L/day in practice).
CPM vs similar look-alikes (fast table)
| Condition | Key trigger | Key localization | Hallmark clues |
|---|---|---|---|
| Central pontine myelinolysis | Rapid correction of chronic hyponatremia | Central pons | Dysarthria/dysphagia + quadriparesis, delayed onset (days) |
| Wernicke encephalopathy | Thiamine deficiency (often alcohol use disorder) | Mammillary bodies, periaqueductal gray | Confusion, ataxia, ophthalmoplegia; treat thiamine before glucose |
| Guillain-Barré syndrome | Post-infectious autoimmune | Peripheral nerves | Ascending weakness + areflexia, autonomic instability |
| Myasthenia gravis | AChR (or MuSK) antibodies | NMJ | Fatigable weakness, ptosis; normal sensation/reflexes |
Imaging & diagnosis (what to know for exams)
- MRI (especially diffusion-weighted imaging) is most sensitive early.
- Lesion is classically central pontine, and can look “trident-shaped” on axial images.
- CPM is part of osmotic demyelination syndrome (ODS), which can also include extrapontine lesions (basal ganglia, thalamus).
Management (Step 2-level framing)
- Prevention is the treatment: correct sodium slowly and thoughtfully.
- If overcorrection occurs: clinicians may re-lower Na⁺ (e.g., desmopressin + free water) depending on scenario—this is a common “what should you do now?” twist.
- Supportive care/rehab for established deficits.
Quick recall capsule (shareable)
CPM = “Correct chronic Na⁺ too fast → central pons demyelinates → can’t speak/swallow + quadriparesis (days later).”
Safety rule: aim mEq/L per 24 h correction for chronic hyponatremia.