What are the possible diagnoses for an 81‑year‑old woman presenting with unilateral hemichorea, urinary incontinence, magnetic gait, and apathy, with a non‑contrast head CT showing no acute lesion?

Differential Diagnosis for Hemichorea with Urinary Incontinence, Magnetic Gait, and Apathy

The clinical triad of magnetic gait, urinary incontinence, and apathy (frontal cognitive impairment) in an 81-year-old woman with normal CT strongly suggests Normal Pressure Hydrocephalus (NPH) as the primary diagnosis, with hemichorea representing either a concurrent vascular event or a manifestation of NPH itself. 1, 2

Primary Consideration: Normal Pressure Hydrocephalus

NPH is one of the few reversible causes of dementia and must be prioritized in this clinical presentation. 1

Classic NPH Features Present:

• Magnetic gait (feet appearing "glued to the floor") is the cardinal and earliest sign of NPH, occurring first in approximately 70% of patients 1, 3
• Urinary incontinence develops as part of the classic triad 1, 4
• Apathy and frontal cognitive impairment (psychomotor slowing, deficits in attention, working memory, executive function) represent the cognitive component 1

Critical Diagnostic Limitation:

• CT has limited sensitivity for NPH diagnosis - the non-contrast CT showing "no significant findings" does not exclude NPH, as CT is inferior to MRI for detecting the characteristic ventriculomegaly and periventricular changes 1, 2
• MRI brain without IV contrast is the preferred imaging modality and should be obtained urgently to evaluate for ventriculomegaly, narrowed posterior callosal angle, effaced sulci, widened sylvian fissures, and periventricular white matter changes 1, 2

Hemichorea Etiology in This Context

Most Likely Vascular Causes:

• Acute ischemic stroke involving the contralateral basal ganglia (particularly striatum) is the most common cause of hemichorea 5
• Small lacunar infarcts can present without obvious focal deficits on routine CT and may require MRI with diffusion-weighted imaging for detection 6
• Hyperglycemic hemichorea should be evaluated with point-of-care glucose testing and hemoglobin A1c 7, 5

Alternative Considerations:

• Subdural hematoma from unrecognized trauma, especially given age and potential falls from gait disturbance - though typically visible on CT 7
• Metabolic derangements (hyponatremia, hypernatremia, hypercalcemia, thyroid disorders) can cause movement disorders 7

Immediate Diagnostic Algorithm

Step 1: Urgent Laboratory Evaluation

• Point-of-care glucose and comprehensive metabolic panel (evaluate for hyperglycemia, electrolyte abnormalities) 7
• Complete blood count, thyroid-stimulating hormone, liver function tests 1, 7
• Hemoglobin A1c if glucose elevated 5

Step 2: Advanced Neuroimaging

MRI brain without IV contrast should be obtained as the next imaging study to simultaneously evaluate for:

• NPH features (ventriculomegaly, callosal angle narrowing, periventricular changes) 1, 2
• Acute/subacute ischemic stroke (diffusion-weighted imaging) 6
• Basal ganglia lesions causing hemichorea 6, 5
• Subdural collections or other structural abnormalities 6

Step 3: CSF Evaluation if NPH Confirmed

• CSF tap test (removal of 30-50 mL) to assess for transient gait improvement, which predicts shunt responsiveness 4, 3
• If single tap test negative but clinical suspicion high, proceed to repeated CSF tap test or continuous lumbar external drainage 4

Critical Prognostic Considerations

NPH Treatment Implications:

• Properly selected NPH patients have 80-90% chance of responding to ventriculoperitoneal shunt surgery 1
• Comorbid Alzheimer's disease (present in 20-57% of NPH patients) significantly reduces shunt responsiveness 1, 2
• Early diagnosis and treatment are essential, as this represents a reversible cause of dementia 1, 3

Common Pitfalls to Avoid:

• Do not dismiss NPH based on normal CT alone - CT has poor sensitivity for NPH features compared to MRI 1
• Do not attribute all symptoms to a single diagnosis - this patient may have concurrent NPH and a separate vascular event causing hemichorea 5
• Do not delay MRI - the combination of potentially reversible dementia (NPH) and acute stroke requires urgent advanced imaging 6, 1

Secondary Differential Considerations

If MRI Rules Out NPH and Stroke:

• Huntington's disease - though age 81 is atypical for new-onset HD; genetic testing would be diagnostic 6
• Neurodegeneration with brain iron accumulation (NBIA) - MRI with susceptibility-weighted sequences would show "eye-of-the-tiger sign" in globus pallidus 6
• Lewy body dementia - characterized by visual hallucinations and fluctuating cognition, less consistent with this presentation 1
• Vascular parkinsonism - stepwise progression with multiple lacunar infarcts 6