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Difficulty: MediumFormulating a Differential Diagnosis

A 28-year-old woman is brought to the emergency department by her husband due to severe, diffuse abdominal pain that began 2 days ago. She also reports generalized weakness and a tingling sensation in her hands and feet. Her husband notes that she has been unusually anxious, restless, and confused since yesterday. Her medical history is significant for an uncomplicated urinary tract infection, for which she began taking trimethoprim-sulfamethoxazole 3 days ago. Her temperature is 37.1°C (98.8°F), blood pressure is 158/98 mm Hg, and pulse is 112/min. Physical examination shows a soft, non-distended abdomen with diffuse tenderness to palpation but no guarding or rebound. Bowel sounds are hypoactive. Neurologic examination reveals decreased sensation to pinprick in the bilateral hands and feet, as well as diminished deep tendon reflexes. Complete blood count, comprehensive metabolic panel, and serum lipase are within normal limits. A urine pregnancy test is negative. Which of the following is the most likely diagnosis?

  1. A
    Brief psychotic disorder
  2. Acute intermittent porphyriaAnswer
  3. C
    Lead poisoning
  4. D
    Drug-induced lupus erythematosus
  5. E
    Guillain-Barré syndrome

Answer

Acute intermittent porphyria
The correct answer is acute intermittent porphyria (AIP). This patient presents with the classic triad of unexplained abdominal pain without peritoneal signs, peripheral neuropathy (sensory loss, diminished reflexes), and psychiatric symptoms (anxiety, confusion). Her autonomic instability (tachycardia, hypertension) is also highly characteristic. AIP is an autosomal dominant disorder of heme synthesis caused by a deficiency in porphobilinogen deaminase. Symptoms are typically episodic and triggered by factors that increase heme demand and induce hepatic cytochrome P450 enzymes, such as fasting, alcohol, and certain medications. Sulfonamides (e.g., trimethoprim-sulfamethoxazole) are notorious porphyrinogenic triggers. The diagnosis is confirmed by elevated urinary porphobilinogen during an acute attack.

Step-by-Step Solution

1
Analyze the patient's presenting symptoms.
The patient exhibits a triad of severe abdominal pain, peripheral neuropathy (tingling, diminished reflexes), and neuropsychiatric disturbances (anxiety, confusion).
Identifying the multi-system nature of the presentation is the first step in formulating a differential diagnosis.
2
Evaluate the objective physical examination findings.
Autonomic hyperactivity (tachycardia, hypertension) is present alongside a lack of peritoneal signs despite severe abdominal pain.
These findings point away from a primary intra-abdominal surgical emergency and toward a systemic or metabolic etiology.
3
Correlate symptom onset with recent medical history.
The symptoms began shortly after the initiation of trimethoprim-sulfamethoxazole.
Identifying a medication trigger helps narrow down specific drug-induced or drug-precipitated metabolic conditions.
4
Synthesize findings to confirm the diagnosis.
The clinical picture matches acute intermittent porphyria triggered by a sulfonamide.
Sulfonamides are known cytochrome P450 inducers that can precipitate acute attacks of porphyria in genetically susceptible individuals.

Key Concept

Acute intermittent porphyria presents with a triad of abdominal pain, neurological deficits, and psychiatric symptoms, and is commonly precipitated by cytochrome P450 inducers such as sulfonamides.
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Topics

Formulating a Differential Diagnosis

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