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Difficulty: HardMedical History Evaluation

A 21-year-old male college student is evaluated in the emergency department for a three-week history of progressive unsteadiness and frequent falls. He also reports a constant "pins and needles" sensation in both of his hands and feet. He has no significant past medical history and takes no medications. He consumes a regular diet that includes meat, dairy, and fresh vegetables. He drinks alcohol occasionally but denies intravenous drug use. Vital signs are within normal limits. Neurologic examination reveals a spastic, broad-based gait. Vibratory and position senses are profoundly decreased in the bilateral lower and upper extremities. Deep tendon reflexes are 3+ in the patellar and Achilles tendons bilaterally, and bilateral upgoing plantar reflexes are present. Complete blood count reveals a hemoglobin of 11.0 g/dL and a mean corpuscular volume of 108 fL. Which of the following historical elements is most likely to be elicited upon further directed interviewing of this patient?

  1. Frequent inhalation of gas from aerosol whipped cream dispensersAnswer
  2. B
    Strict adherence to a vegan diet over the preceding three months
  3. C
    History of a painless genital ulcer that resolved without medical intervention
  4. D
    Previous completion of a six-month course of isoniazid therapy
  5. E
    Daily use of over-the-counter naproxen for recurrent headaches

Answer

The correct historical element is the frequent inhalation of gas from aerosol whipped cream dispensers, indicating nitrous oxide abuse.
The correct answer identifies recreational nitrous oxide abuse. Nitrous oxide, often inhaled from whipped cream dispensers ('whippets'), irreversibly oxidizes the cobalt atom of vitamin B12 from the 1+ to the 3+ valence state. This inactivates the vitamin and completely halts B12-dependent enzymes (methionine synthase and methylmalonyl-CoA mutase), rapidly producing a functional B12 deficiency. This leads to the classic presentation of subacute combined degeneration of the spinal cord (loss of proprioception/vibration due to dorsal column damage, and spasticity/hyperreflexia/Babinski signs due to lateral corticospinal tract damage) alongside macrocytic anemia. Eliciting this history is critical because cessation of use and immediate B12 supplementation are required to prevent irreversible neurologic damage.

Step-by-Step Solution

1
Identify the primary clinical syndrome from the examination and laboratory findings.
The patient presents with subacute combined degeneration of the spinal cord (dorsal column loss and upper motor neuron signs) accompanied by a macrocytic anemia.
This classic triad is highly specific for a severe vitamin B12 (cobalamin) deficiency.
2
Evaluate the temporal kinetics and dietary context provided in the stem.
The patient eats a regular diet and has only had symptoms for three weeks, making a purely dietary etiology highly improbable.
Dietary deficiency of vitamin B12 takes 3 to 5 years to manifest due to massive hepatic stores. The rapid onset implies acute functional depletion.
3
Synthesize demographic clues with potential toxicologic exposures that mimic this syndrome.
The patient is a college student, pointing toward recreational nitrous oxide use.
Nitrous oxide irreversibly oxidizes the cobalt core of vitamin B12, rapidly inactivating it and producing a profound functional deficiency that causes immediate neurologic damage.

Key Concept

Nitrous oxide toxicity rapidly inactivates vitamin B12, causing functional deficiency and subacute combined degeneration of the spinal cord despite adequate prior hepatic stores.
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Topics

Medical History Evaluation

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