3 Oct 2026 · Case #028
Acute intermittent porphyria
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Diagnosis
Acute intermittent porphyria
First-line treatment
Intravenous hemin with carbohydrate loading
How the diagnosis is reached
The pain is real and the abdomen is not its source. That is the paradox of an acute porphyric attack: severe colicky pain over a soft, non-tender abdomen with normal inflammatory markers and normal imaging, because the pain is neuropathic, arising from autonomic nerve dysfunction rather than from anything inside the abdomen. It is why last year's laparoscopy found nothing, and why a fourth negative scan would settle nothing either. Add a motor-predominant peripheral neuropathy, psychiatric disturbance, tachycardia and hypertension from autonomic instability, and the pattern becomes distinctive. The hyponatremia is genuine SIADH, confirmed by a low serum osmolality with inappropriately concentrated urine and high urine sodium; it is a common feature and can precipitate seizures. Urine darkening on standing in light reflects porphobilinogen oxidising to porphobilin, which is why samples must be protected from light and sent fresh. A markedly raised urine porphobilinogen with normal faecal porphyrins localises the defect to acute intermittent porphyria rather than variegate porphyria or hereditary coproporphyria. The recently started medication is almost certainly the precipitant: enzyme-inducing drugs increase haem demand and drive ALA synthase, and the classic culprits include many anticonvulsants, sulfonamides, rifampicin and the combined oral contraceptive. Treatment attacks the same pathway from the other end. Intravenous hemin suppresses ALA synthase by negative feedback and shuts off production of the toxic precursors, shortening the attack and preventing the neuropathy from progressing; high-dose carbohydrate does the same thing far less effectively and is started at once while the hemin is being sourced. Withdrawing every precipitant matters as much as either: the new drug, alcohol, and any period of fasting. The sodium of 119 mmol/L is the trap in the management rather than the diagnosis: it brings a real seizure risk, and most anticonvulsants are themselves porphyrinogenic, so the hyponatremia must be corrected rather than treated around.
Key clinical clues
- Severe abdominal pain with a soft, non-tender abdomen and normal CT
- Normal white cell count and CRP, no inflammatory process to explain the pain
- Motor-predominant neuropathy with psychiatric disturbance and autonomic instability
- SIADH-pattern hyponatremia: low serum osmolality, concentrated urine, high urine sodium
- Urine darkening on standing; raised porphobilinogen with normal faecal porphyrins
The case in full
Presenting complaint
Patient history
A 28-year-old woman presents with four days of severe, colicky, generalised abdominal pain and vomiting. This is her third such admission in two years; laparoscopy last year found nothing. She started a new medication three weeks ago.
Examination
Physical examination
She is agitated and tearful, and reports frightening visual disturbances overnight. The abdomen is soft and entirely non-tender despite the reported pain, with normal bowel sounds. There is symmetrical proximal weakness in all four limbs and reduced reflexes.
Observations
Vital signs
Observations are recorded.
| Parameter | Value | Reference |
|---|---|---|
| Heart rate | 128 bpmRef 60–100 | 60–100 |
| Blood pressure | 172/98 mmHg | – |
| Temperature | 36.8 °CRef 36.1–37.5 | 36.1–37.5 |
| Respiratory rate | 20 /minRef 12–20 | 12–20 |
Laboratory results
Laboratory results
Bloods are sent.
| Parameter | Value | Reference |
|---|---|---|
| Sodium | 119 mmol/LRef 135–145 | 135–145 |
| Serum osmolality | 252 mOsm/kgRef 275–295 | 275–295 |
| Urine osmolality | 486 mOsm/kgRef <100 when hyponatraemic | <100 when hyponatraemic |
| Urine sodium | 64 mmol/LRef <20 when hypovolaemic | <20 when hypovolaemic |
| White blood cells | 8.2 ×10⁹/LRef 4.5–11.0 | 4.5–11.0 |
| C-reactive protein | 4 mg/LRef <5 | <5 |
| Lipase | 38 U/LRef 13–60 | 13–60 |
Imaging and observation
Imaging
CT of the abdomen and pelvis with contrast is normal. A urine sample left on the windowsill during the ward round has turned a deep reddish-brown by the afternoon.
Specialist testing
Laboratory results
A fresh urine sample is sent to the regional laboratory, protected from light.
| Parameter | Value | Reference |
|---|---|---|
| Urine porphobilinogen | 148 µmol/LRef <8 | <8 |
| Urine delta-aminolaevulinic acid | 112 µmol/LRef <38 | <38 |
| Faecal porphyrins | NormalRef Normal | Normal |
| Erythrocyte PBG deaminase | ReducedRef Normal activity | Normal activity |