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Difficulty: Very hardOrdering Laboratory Tests

Match each highly specific neurologic clinical vignette to the most appropriate initial or confirmatory diagnostic laboratory test.

  • A 38-year-old woman presenting with severe, bilateral optic neuritis, sequential episodes of paraparesis, and intractable vomiting.Serum aquaporin-4 (AQP4) immunoglobulin G testing
  • A 24-year-old woman with new-onset severe psychiatric symptoms, autonomic instability, orofacial dyskinesias, and an adnexal mass.Cerebrospinal fluid assay for anti-N-methyl-D-aspartate (NMDA) receptor antibodies
  • A 55-year-old man with asymmetric, slowly progressive distal upper extremity weakness without sensory loss, and absent tendon reflexes in the affected limbs.Serum anti-GM1 ganglioside antibody testing
  • A 35-year-old woman with a subacute onset of gait ataxia, ophthalmoplegia, and diffuse areflexia following a recent diarrheal illness.Serum anti-GQ1b ganglioside antibody testing
  • A 65-year-old man exhibiting rapidly progressive cognitive decline over two months, accompanied by startle myoclonus and akinetic mutism.Cerebrospinal fluid real-time quaking-induced conversion (RT-QuIC) assay

Answer

Each distinct neurologic presentation matches a highly specific confirmatory laboratory test: NMOSD to AQP4 IgG, anti-NMDA receptor encephalitis to CSF NMDA receptor antibodies, multifocal motor neuropathy to anti-GM1 antibodies, Miller Fisher syndrome to anti-GQ1b antibodies, and Creutzfeldt-Jakob disease to the CSF RT-QuIC assay.
Correctly identifying these diseases requires recognizing highly specific clinical clues (e.g., an adnexal mass pointing to an ovarian teratoma in anti-NMDA receptor encephalitis, or intractable vomiting pointing to area postrema involvement in NMOSD) and knowing their paired autoantibodies or protein assays.

Step-by-Step Solution

1
Evaluate the patient presenting with optic neuritis, paraparesis, and intractable vomiting.
Identify the presentation as neuromyelitis optica spectrum disorder (NMOSD) featuring area postrema syndrome.
Intractable vomiting originates from area postrema involvement, which is highly specific for NMOSD. The diagnostic test is serum AQP4 IgG.
2
Analyze the young female patient with acute psychiatric symptoms, autonomic instability, and an adnexal mass.
Diagnose suspected paraneoplastic anti-NMDA receptor encephalitis secondary to an ovarian teratoma.
This classic paraneoplastic syndrome requires diagnostic confirmation via a CSF assay for NMDA receptor antibodies.
3
Assess the patient with asymmetric, progressive distal upper extremity weakness, absent reflexes, and intact sensation.
Identify multifocal motor neuropathy (MMN).
MMN mimics amyotrophic lateral sclerosis (ALS) but uniquely lacks upper motor neuron signs and sensory deficits, and is diagnostically associated with anti-GM1 antibodies.
4
Evaluate the patient with the triad of gait ataxia, ophthalmoplegia, and areflexia following a diarrheal illness.
Diagnose Miller Fisher syndrome, a specific variant of Guillain-Barré syndrome.
This specific triad is strongly and uniquely associated with anti-GQ1b antibodies.
5
Assess the older patient with rapidly progressive cognitive decline, startle myoclonus, and akinetic mutism.
Identify suspected Creutzfeldt-Jakob disease (CJD).
The RT-QuIC assay of the CSF is currently the most sensitive and specific laboratory test for confirming the presence of misfolded prion proteins.

Key Concept

Targeted laboratory evaluation for specific neuro-immunologic and prion diseases
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Topics

Ordering Laboratory Tests

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