What is the differential diagnosis for a 21-year-old underweight female patient with a one-week history of headache, normal blood tests, and normal computed tomography (CT) head scan, who now presents with abdominal pain, inability to urinate, and a large volume of urine (1000 ml) collected after Foley catheter insertion?

Differential Diagnosis: Acute Urinary Retention with Neurologic and Systemic Features

The most likely diagnosis is acute intermittent porphyria (AIP), given the constellation of headache, abdominal pain, acute urinary retention with massive bladder distension (1000 mL), young underweight female, and normal initial workup. 1, 2

Primary Differential Considerations

1. Acute Intermittent Porphyria (Most Likely)

• Classic triad present: neuropsychiatric symptoms (headache), abdominal pain, and autonomic dysfunction (urinary retention) 2, 3
• Young female demographic is typical (women aged 20-40 years most commonly affected) 2
• Underweight appearance may reflect chronic malnutrition from recurrent attacks 3
• Normal CT head and routine blood tests are characteristic—diagnosis requires specific urine porphobilinogen testing 2
• Acute urinary retention occurs from autonomic neuropathy affecting bladder innervation, causing detrusor areflexia 4
• Immediate action required: Send urine for porphobilinogen and delta-aminolevulinic acid during acute attack (levels dramatically elevated during crisis) 2
• Avoid triggering medications (barbiturates, sulfonamides, estrogens) that can precipitate or worsen attacks 3

2. Neurogenic Bladder from Occult Spinal Pathology

• Acute transverse myelitis or spinal cord lesion can present with headache (if associated with increased intracranial pressure) and urinary retention 4
• Key distinguishing features to assess: lower extremity weakness, sensory level, absent bulbocavernosus reflex, perianal numbness 2, 3
• Normal CT head does not exclude spinal pathology—requires dedicated spinal MRI if neurologic deficits present 5
• Urodynamic finding would be detrusor areflexia from spinal shock 4

3. Meningitis-Retention Syndrome (MRS)

• Benign inflammatory condition causing acute urinary retention with or without meningeal signs 4
• Headache is prominent feature; abdominal pain less typical 4
• CSF analysis shows increased myelin basic protein in some cases 4
• Distinguishing feature: typically occurs following viral illness or vaccination 4
• Mechanism is acute spinal shock causing detrusor areflexia 4

4. Sacral Herpes (Elsberg Syndrome)

• Herpes simplex virus affecting sacral nerve roots causes acute urinary retention 4
• Look for: genital or perianal vesicular lesions, sacral paresthesias, constipation 4
• Headache can occur if associated with aseptic meningitis 4
• Laboratory confirmation: elevated herpes virus titers in serum/CSF 4
• Urodynamic abnormality is detrusor areflexia from direct pelvic nerve involvement 4

5. Fowler's Syndrome

• Primary disorder of young women causing painless urinary retention 2, 3
• Typical presentation: inability to void despite full bladder, minimal discomfort 3
• Associated with polycystic ovary syndrome in 40% of cases 2
• Does NOT explain headache or acute abdominal pain—less likely given symptom complex 2

6. Medication-Induced Retention

• Anticholinergics, alpha-adrenergic agonists, opioids commonly cause retention 2, 3
• Critical history: over-the-counter cold medications, antihistamines, herbal supplements 3
• Does not explain headache unless patient taking multiple medications 2

7. Functional/Psychogenic Retention

• Diagnosis of exclusion after organic causes ruled out 2, 6
• Underweight appearance raises concern for eating disorder with associated psychiatric comorbidity 3
• Does NOT explain abdominal pain and headache constellation 2

Immediate Diagnostic Workup Required

Laboratory Testing

• Urine porphobilinogen and delta-aminolevulinic acid (must be collected during acute attack—levels normalize between episodes) 2
• Complete metabolic panel including electrolytes (hyponatremia common in porphyria from SIADH) 3
• Serum sodium specifically (porphyria causes SIADH) 2
• Urine culture to exclude complicated UTI (though less likely given presentation) 5

Imaging

• MRI spine with and without contrast if any neurologic deficits detected on examination (weakness, sensory changes, reflex abnormalities) 5
• Renal ultrasound to assess for hydronephrosis from prolonged retention (1000 mL suggests chronic component) 5
• Post-void residual measurement after catheter removal trial 1, 7

Specialized Testing

• Neurologic examination documenting: lower extremity strength, sensation, reflexes, perianal sensation, bulbocavernosus reflex 2, 3
• Pelvic examination to exclude obstructive masses, prolapse (rare in 21-year-old but must exclude) 2, 8

Critical Management Principles

Bladder Management

• Foley catheter should remain in place minimum 7-10 days given 1000 mL retention (indicates significant detrusor dysfunction risk) 7
• Monitor for post-obstructive diuresis (can lose massive fluid/electrolytes after decompression of chronic retention) 8
• Implement bladder training protocol with intermittent catheterization every 4-6 hours once catheter removed 1, 7
• Do not attempt catheter removal until underlying cause identified and treated 1, 7

Monitoring for Complications

• Watch for hematuria post-catheterization (common after acute decompression) 8
• Serial creatinine to detect acute kidney injury from obstructive uropathy 5
• Strict intake/output monitoring for 24-48 hours post-catheterization 8

Common Pitfalls to Avoid

• Do not dismiss as simple UTI or functional disorder in young female with normal initial workup—porphyria is frequently missed for years 2
• Do not remove catheter prematurely before establishing diagnosis—1000 mL retention indicates serious underlying pathology requiring definitive diagnosis 7, 8
• Do not order routine urodynamics acutely—wait until acute episode resolves and diagnosis established 5, 2
• Do not assume psychiatric cause in underweight young female without excluding organic pathology first 2, 3

Specialist Consultation Triggers

• Immediate neurology consultation if any focal neurologic deficits, altered mental status, or concern for CNS pathology 2, 3
• Urology referral for urodynamic evaluation once acute episode resolves (assess detrusor function) 1, 7
• Genetics/hematology consultation if porphyria confirmed (requires family screening and long-term management) 2