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Difficulty: MediumNeurocognitive Disorders

A 64-year-old man is brought to the neurologist by his wife due to a 3-month history of rapidly progressive cognitive decline and unusual involuntary movements. The wife notes that he initially had trouble concentrating at work and seemed unusually anxious. Over the past month, he has become profoundly disoriented, struggles to walk independently due to poor coordination, and frequently exhibits sudden, brief, shock-like muscle jerks, particularly when startled by a loud noise. He has no significant past medical history and takes no medications. Vital signs are normal. Neurological examination reveals marked gait ataxia, widespread myoclonus, and severe expressive aphasia. Magnetic resonance imaging (MRI) of the brain demonstrates hyperintensity in the basal ganglia and cerebral cortex on diffusion-weighted imaging. Which of the following best describes the underlying pathophysiology of this patient's condition?

  1. Conformational change of a normal cellular protein into a β\beta-pleated sheet isoformAnswer
  2. B
    Extracellular deposition of amyloid-β\beta peptides and intracellular neurofibrillary tangles
  3. C
    Hyperactivity of central serotonergic neurons leading to neuromuscular excitability
  4. D
    Severe deficiency of thiamine resulting in mammillary body necrosis
  5. E
    Progressive expansion of CAG trinucleotide repeats in an autosomal dominant gene

Answer

Conformational change of a normal cellular protein into a β\beta-pleated sheet isoform
The patient's rapidly progressive dementia, prominent startle myoclonus, cerebellar ataxia, and characteristic MRI findings (hyperintensity in the basal ganglia and cerebral cortex on diffusion-weighted imaging) strongly suggest a diagnosis of Creutzfeldt-Jakob disease (CJD). CJD is a fatal spongiform encephalopathy caused by prions. The underlying pathophysiology involves the conformational change of the normal cellular prion protein (PrPCPrP^C), which is rich in α\alpha-helices, into an abnormal, disease-causing isoform (PrPScPrP^{Sc}) that is predominantly composed of β\beta-pleated sheets. These abnormal proteins are resistant to protease degradation and accumulate in the brain, leading to widespread neurodegeneration, vacuolization, and rapidly progressive clinical decline.

Step-by-Step Solution

1
Identify the key clinical features and timeline in the vignette.
The patient has a rapidly progressive dementia (over 3 months), startle myoclonus, cerebellar ataxia, and aphasia.
Establishing the pace of cognitive decline is crucial for narrowing the differential diagnosis of neurocognitive disorders.
2
Correlate the clinical presentation with the neuroimaging results.
MRI shows diffusion restriction (hyperintensity) in the basal ganglia and cerebral cortex, known as cortical ribboning.
These specific MRI findings, combined with the rapid clinical triad of dementia, myoclonus, and ataxia, are pathognomonic for Creutzfeldt-Jakob disease (CJD).
3
Determine the underlying pathophysiology of the diagnosed condition.
CJD is caused by the transformation of a normal α\alpha-helical cellular prion protein (PrPCPrP^C) into an abnormal β\beta-pleated sheet isoform (PrPScPrP^{Sc}).
The question explicitly asks for the pathophysiological mechanism of the patient's condition.

Key Concept

Creutzfeldt-Jakob disease (CJD) is a rapidly progressive neurocognitive disorder characterized by dementia, startle myoclonus, and ataxia, caused by the conformational change of normal prion proteins into abnormal β\beta-pleated sheets.
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Topics

Neurocognitive Disorders

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