Step 1 topics → Neurology and Special Senses → Other Dementias (FTD, DLB, MSA, PSP, CBD, CJD, NPH, Vascular)

Other Dementias (FTD, DLB, MSA, PSP, CBD, CJD, NPH, Vascular)

USMLE Step 1 trap: Knows the NPH triad but is unaware that gait disturbance characteristically appears first. NPH classically presents with gait disturbance first (magnetic gait), followed by urinary incontinence, then dementia — the order of onset is diagnostically important.

The 'other dementias' category covers a dense cluster of conditions that USMLE Step 1 loves to test through clinical vignettes — a patient walks in with a specific constellation of findings and you have to identify which rare dementia fits. The high-yield ones are FTD, DLB, CJD, NPH, PSP, CBD, and MSA. The exam rarely asks you to name a disease outright; instead it gives you a stem with 2-3 defining features and expects you to work backward to the diagnosis, mechanism, or next best step. Most questions hinge on recognizing what makes each condition uniquely identifiable versus Alzheimer's, which is the implicit foil in nearly every vignette.

The trickiest part is that students memorize facts about each condition in isolation but can't distinguish them under pressure. FTD gets confused with Alzheimer's because both cause cognitive decline — but the initial symptoms are completely different. CJD gets confused with a slowly progressive dementia because students don't appreciate how fast it kills. NPH is notorious for the 'wet, wobbly, wacky' mnemonic — but the mnemonic doesn't tell you that gait comes first, which is exactly what the exam tests. DLB trips students up because antipsychotics (which seem like a reasonable treatment for hallucinations) are actually dangerous in this population.

For USMLE Step 1, the winning strategy is to anchor each condition to its single most distinctive feature: FTD = frontal behavioral change under 65, DLB = fluctuating cognition + visual hallucinations + REM sleep behavior disorder + antipsychotic sensitivity, CJD = rapidly progressive dementia + myoclonus + months to death, NPH = gait first then incontinence then dementia + normal opening pressure on LP, PSP = vertical gaze palsy + axial rigidity, CBD = alien limb, MSA = autonomic failure + parkinsonism/cerebellar. Build the matrix in your head and the questions become pattern recognition.

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Common misconceptions

Common mistake
Wrong: NPH presents with the triad of dementia, ataxia, and incontinence in any order of onset.
Right: NPH classically presents with gait disturbance first (magnetic gait), followed by urinary incontinence, then dementia — the order of onset is diagnostically important.
The NPH triad mnemonic ('wet, wobbly, wacky') lists three symptoms but hides the clinically critical fact that gait disturbance comes first, urinary incontinence second, and dementia last. On the exam, a vignette may describe a patient whose memory is still relatively preserved but who has started shuffling with a magnetic gait and leaking urine — that sequence is your diagnostic signal. If you just memorize the triad without the order, you'll miss stems that emphasize early gait findings and late cognitive changes.
Common mistake
Wrong: CJD progresses over years like Alzheimer disease.
Right: CJD causes rapidly progressive dementia leading to death within months (median ~4–6 months), with characteristic periodic sharp-wave complexes on EEG and 14-3-3 protein in CSF.
CJD is a prion disease, not a neurodegenerative disease in the Alzheimer's sense — it causes catastrophic, rapidly progressive dementia that kills within months, not years. The key distinguishing features the exam pairs with this rapid course are myoclonus, periodic sharp-wave complexes on EEG, and 14-3-3 protein in the CSF (a marker of rapid neuronal death). If a vignette shows a patient who went from normal to severely demented and is near death within 6 months, think CJD immediately — Alzheimer's doesn't do that.
Common mistake
Wrong: Frontotemporal dementia presents first with memory loss like Alzheimer disease.
Right: FTD presents first with personality changes, disinhibition, and language dysfunction (not memory loss) due to frontal and temporal lobe atrophy, typically in patients under 65.
FTD and Alzheimer's both cause progressive cognitive decline, but they start in completely different domains because they affect different regions first. Alzheimer's attacks the hippocampus early, so memory loss is the presenting symptom. FTD attacks the frontal and temporal lobes, so the first symptoms are personality change, disinhibition, impulsive behavior, or language problems — memory is often relatively spared early on. The other key differentiator is age: FTD typically presents under 65, whereas late-onset Alzheimer's is more common after 65. A vignette describing a 58-year-old making socially inappropriate comments is FTD, not Alzheimer's.
Common mistake
Gap: Unable to differentiate PSP, CBD, and MSA based on their distinguishing clinical features
PSP is distinguished by vertical gaze palsy and axial rigidity; CBD by alien limb phenomenon and asymmetric apraxia; MSA by autonomic failure (orthostatic hypotension, urinary dysfunction) with parkinsonism and cerebellar signs.
These three atypical parkinsonian syndromes each have a 'signature' finding that separates them from each other and from Parkinson's disease. PSP's signature is vertical gaze palsy (especially downward) plus axial rigidity and early falls backward — think 'can't look down, falls back.' CBD's signature is the alien limb phenomenon (one limb moves involuntarily as if controlled by another force) plus asymmetric cortical signs like apraxia and cortical sensory loss. MSA's signature is prominent autonomic failure (orthostatic hypotension, bladder dysfunction) combined with either parkinsonism (MSA-P) or cerebellar ataxia (MSA-C). None of these respond well to levodopa, which is another exam hook.
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What the exam tests

  1. FTD: Given a patient under 65 with personality changes, disinhibition, or language dysfunction (not memory loss first), recognize FTD, identify the atrophy pattern (frontal and temporal lobes), and know it involves tau or TDP-43 proteinopathy with Pick bodies in some cases.
  2. DLB: Given a patient with fluctuating cognition, recurrent visual hallucinations, parkinsonism, and/or REM sleep behavior disorder, identify DLB and know that antipsychotics are contraindicated due to severe neuroleptic sensitivity reactions.
  3. CJD: Given a patient with rapidly progressive dementia, myoclonus, and death within months, identify CJD (prion disease), its EEG finding (periodic sharp-wave complexes), CSF marker (14-3-3 protein), and histology (spongiform encephalopathy).
  4. NPH: Given a patient with the classic triad, identify NPH, recognize that gait disturbance appears first (magnetic gait), know that imaging shows enlarged ventricles out of proportion to cortical atrophy, and that ventriculoperitoneal shunting is the treatment.
  5. PSP vs CBD vs MSA: Given a vignette with atypical parkinsonian features, differentiate PSP (vertical gaze palsy, axial rigidity, falls backward), CBD (alien limb phenomenon, asymmetric apraxia, cortical sensory loss), and MSA (autonomic failure — orthostatic hypotension and urinary dysfunction — plus parkinsonism and/or cerebellar signs).

Can you avoid these mistakes?

A 61-year-old man is brought in by his wife who reports he has been making crude jokes at work, spending recklessly, and recently exposed himself in public. His memory seems intact. MRI shows atrophy of the frontal and temporal lobes bilaterally. What is the diagnosis, and how does the initial presentation differ from Alzheimer's disease?
A 72-year-old woman with a 6-month history of progressive cognitive decline, involuntary jerking movements of her limbs, and a shuffling gait dies. EEG during her illness showed periodic sharp-wave complexes. What protein would you expect to find elevated in her CSF, what is the underlying pathological mechanism, and what is the expected time course from symptom onset to death?
A 68-year-old man presents with urinary incontinence and a 'magnetic' gait over the past few months. His wife notes he has become slightly forgetful only recently. Head CT shows enlarged ventricles disproportionate to any cortical atrophy. What is the diagnosis, what is the classic order of symptom onset, and what is the treatment?
An 70-year-old woman with parkinsonism is noted to have eyes that cannot move downward on command but can move reflexively. She frequently falls backward. A separate 65-year-old man with parkinsonism reports his left hand sometimes moves on its own without his intention, and he cannot perform learned motor tasks with that hand. What is each patient's diagnosis, and what single clinical feature distinguishes the two?

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