Acute Intermittent Porphyria (AIP)

1. Clinical Overview

Summary

Acute Intermittent Porphyria (AIP) is an autosomal dominant metabolic disorder characterized by a deficiency in porphobilinogen deaminase (PBGD), the third enzyme in the heme biosynthetic pathway. [1] This deficiency leads to the accumulation of toxic porphyrin precursors, specifically delta-aminolevulinic acid (ALA) and porphobilinogen (PBG), which are neurotoxic. [2]

AIP is often referred to as a "little imitator" because its symptoms—severe abdominal pain, psychiatric disturbances, and neurological deficits—mimic many common conditions, frequently leading to delayed diagnosis or unnecessary surgery. [3] Attacks are typically triggered by factors that induce the rate-limiting enzyme ALAS1, such as specific drugs (P450 inducers), fasting, or hormonal shifts. [4] Management focuses on the urgent administration of Haem Arginate to downregulate heme production and provide symptomatic support. [5]

Key Facts

• The "5 Ps" of AIP: Painful abdomen, Port-wine urine, Polyneuropathy, Psychological disturbances, and Precipitated by drugs. [6]
• Low Penetrance: Only 10-20% of individuals with the genetic mutation will ever experience a clinical attack. [1]
• The "Soft" Abdomen: A hallmark of AIP is severe, agonizing abdominal pain in the absence of physical signs of peritonitis (guarding/rebound). [3]
• Hormonal Link: Attacks are significantly more common in women, often occurring during the luteal phase of the menstrual cycle when progesterone levels are high. [7]
• Haem Arginate: The gold standard treatment; it works via negative feedback to inhibit ALAS1 and stop the production of toxic precursors. [5]

Clinical Pearls

The "Dark Urine" Pearl: If a patient's urine is normal color when voided but turns a dark red/brown ("port-wine") after standing in light for several hours, this is highly suggestive of AIP. The PBG in the urine is being oxidized to porphobilin. [3,8]

The "Hyponatraemia" Tip: Unexplained hyponatraemia during an abdominal crisis is a major clue for AIP. It is usually due to SIADH caused by hypothalamic involvement or direct renal tubular toxicity of ALA. [9]

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2. Epidemiology & Risk Factors

Incidence & Distribution

• Prevalence: Approximately 1 in 20,000 in Europe, though it is as high as 1 in 1,000 in Northern Sweden ("Swedish Porphyria"). [10]
• Demographics: Most commonly affects women aged 18–45. It is extremely rare before puberty or after menopause. [1,7]
• Mortality: Historically high (> 20%), but now less than 5% with early recognition and haem arginate therapy. [11]

Common Triggers

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3. Pathophysiology

1. The Enzyme Defect

AIP results from a 50% reduction in PBG-Deaminase (HMBS) activity. Under basal conditions, this 50% is sufficient for heme production. However, when heme demand increases (due to triggers), the rate-limiting enzyme ALAS1 is upregulated. [1,2]

2. Precursor Accumulation

Because PBG-Deaminase is deficient, it cannot process the increased flow of precursors. This results in a massive "backup" and accumulation of ALA and PBG in the liver and plasma. [2,13]

3. Neurotoxicity

ALA is structurally similar to GABA. It acts as a GABA receptor agonist/antagonist and generates reactive oxygen species (ROS). This leads to:

• Autonomic Dysfunction: Tachycardia, hypertension, and ileus.
• Peripheral Neuropathy: Axonal degeneration (motor > sensory).
• CNS Effects: Seizures and psychosis. [13,14]

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4. Clinical Presentation

Symptoms & Signs

Differential Diagnosis

• Acute Appendicitis/Acute Abdomen: AIP lacks peritonism; CT will be normal.
• Guillain-Barré Syndrome: AIP presents with abdominal pain and psychiatric features, which are absent in GBS.
• Lead Poisoning: Can mimic AIP pathophysiology (inhibits ALAD) but presents with lead lines and basophilic stippling.
• Psychiatric Disorders: Patients are often misdiagnosed with primary psychosis if the abdominal pain is overlooked. [3]

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5. Investigations

1. The Diagnostic Step

2. Confirmatory & Specialized Tests

• PBG Deaminase Activity: Measured in erythrocytes. Low in most AIP patients, but can be normal in 5% (non-erythroid variants). [1]
• Genetic Testing (HMBS Gene): The gold standard for family screening and definitive diagnosis. [10]
• Faecal Porphyrins: Typically normal in AIP (helps distinguish from Porphyria Variegata). [15]

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6. Management: The AIP Algorithm

Management Flowchart (ASCII)

                  [SUSPECTED ACUTE PORPHYRIA ATTACK]
                  (Abdo Pain + Neuro/Psych + High Risk)
                                   |
              +--------------------v--------------------+
              |  1. URGENT URINE PBG (Spot Sample)      |
              |  2. Check Sodium, LFTs, and Pregnancy   |
              +--------------------+--------------------+
                                   |
              /--------------------+--------------------\
      [PBG MARKEDLY HIGH]                        [PBG NORMAL]
             |                                         |
    +--------v--------+                        +-------v-------+
    | DIAGNOSIS LIKELY|                        | EXCLUDE OTHER |
    | START TREATMENT |                        | PATHOLOGY     |
    +--------+--------+                        +---------------+
             |
    +--------v------------------------------------------+
    | THE "PORPHYRIA BUNDLE"                            |
    | 1. ADMIT to Acute Med / High Dependency Unit      |
    | 2. STOP ALL PORPHYRINOGENIC DRUGS (e.g. Phenytoin)|
    | 3. ANALGESIA: Use Opiates (Morphine/Fentanyl)     |
    | 4. CALORIC SUPPORT: IV 10% Dextrose (if mild)     |
    | 5. DEFINITIVE: IV HAEM ARGINATE (Normosang)       |
    +--------+------------------------------------------+
             |
    +--------v------------------------------------------+
    | LONG-TERM MANAGEMENT                              |
    | - Patient Education (Safe drug list / MedicAlert) |
    | - Family Screening (Genetic testing)              |
    | - Consider GIVOSIRAN for recurrent attacks        |
    +---------------------------------------------------+

1. Specific Emergency Treatment

• Haem Arginate (Normosang): 3 mg/kg daily for 4 days. It is the only treatment that addresses the underlying metabolic cause. [5,16]
• High Carbohydrate Intake: Glucose inhibits ALAS1. Use IV 10% Dextrose if the patient cannot tolerate oral intake, but do not delay Haem Arginate for glucose if the attack is severe. [12,17]
• Hyponatraemia Management: Fluid restriction if due to SIADH; hypertonic saline if severe/seizures. Use caution to avoid osmotic demyelination. [9]

2. Safe vs. Unsafe Drugs

• Unsafe (Avoid): Phenytoin, Carbamazepine, Rifampicin, Sulfonamides, Progesterone, Barbiturates. [4]
• Safe (Use): Paracetamol, Aspirin, Morphine, Gabapentin, Penicillins, Beta-blockers. [18]
• Always check the Porphyria Drug Database (e.g., www.drugs-porphyria.org) before prescribing.

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7. Complications

• Respiratory Paralysis: Due to severe motor axonal neuropathy; requires ICU and ventilation. [14]
• Chronic Kidney Disease: Long-term complication in 30-60% of patients due to chronic ALA exposure. [19]
• Hepatocellular Carcinoma (HCC): AIP patients have a 70-fold increased risk of HCC, even without cirrhosis. Annual ultrasound screening is recommended from age 50. [20]
• Chronic Hypertension: Due to autonomic damage.

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8. Evidence & Landmark Trials

1. ENVISION Trial (PMID: 32493419): Landmark Phase 3 trial of Givosiran (an RNAi therapy that silences ALAS1). Showed a 74% reduction in annualized attack rates compared to placebo. [21]
2. Anderson et al. (PMID: 15767334): Established the consensus for diagnosis and management of the acute porphyrias, emphasizing PBG screening. [1]
3. The "Glucose Effect" Study: Demonstrated the molecular mechanism (PGC-1α) by which fasting induces ALAS1 and glucose suppresses it. [12]
4. Stein et al. (PMID: 28318270): Characterized the long-term complications of AIP, including the high prevalence of hypertension and CKD. [19]

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9. Single Best Answer (SBA) Questions

Question 1

A 28-year-old female presents with severe abdominal pain, vomiting, and tachycardia. She has a history of epilepsy and was recently started on phenytoin. On examination, her abdomen is soft with no guarding. Her urine is dark. What is the most likely diagnosis?

• A) Acute Appendicitis
• B) Biliary Colic
• C) Acute Intermittent Porphyria
• D) Henoch-Schönlein Purpura
• E) Lead Poisoning
• Answer: C. The triad of severe abdominal pain with a "soft" abdomen, dark urine, and a known trigger (phenytoin, a P450 inducer) is classic for AIP.

Question 2

Which enzyme is deficient in Acute Intermittent Porphyria?

• A) ALA Synthase
• B) Ferrochelatase
• C) Porphobilinogen Deaminase
• D) Uroporphyrinogen Decarboxylase
• E) Bilirubin Glucuronosyltransferase
• Answer: C. PBG Deaminase (also known as Hydroxymethylbilane synthase) is the enzyme deficient in AIP.

Question 3

A patient is admitted with a confirmed AIP attack. She is experiencing significant pain and anxiety. Which of the following medications is SAFE to use?

• A) Phenobarbital
• B) Rifampicin
• C) Morphine
• D) Carbamazepine
• E) Nitrofurantoin
• Answer: C. Opiates like Morphine and Fentanyl are safe. The others are known triggers (porphyrinogenic).

Question 4

What is the mechanism of action of Haem Arginate in treating AIP?

• A) It provides heme for oxygen transport
• B) It binds and excretes urinary PBG
• C) It inhibits ALAS1 via negative feedback
• D) It bypasses the PBG Deaminase block
• E) It increases renal clearance of ALA
• Answer: C. Haem arginate replenishes the hepatic heme pool, which then provides negative feedback to inhibit the rate-limiting enzyme ALAS1.

Question 5

A patient with AIP has a serum sodium of 122 mmol/L during an acute attack. What is the most common cause of this finding?

• A) Excessive IV saline administration
• B) Diarrhoea and vomiting
• C) SIADH
• D) Adrenal insufficiency
• E) Artifactual from high lipid levels
• Answer: C. Hyponatraemia in AIP is most commonly due to SIADH (hypothalamic involvement).

Question 6

Why are AIP attacks more common in females?

• A) Higher prevalence of the mutation in females
• B) Oestrogen inhibits PBG Deaminase
• C) Progesterone induces ALAS1
• D) Females have lower glucose stores
• E) Cyclic blood loss depletes iron
• Answer: C. Progesterone directly increases the transcription of the ALAS1 gene.

Question 7

What long-term screening is recommended for patients with AIP over the age of 50?

• A) Colonoscopy every 5 years
• B) Annual Renal Ultrasound
• C) Bi-annual Liver Ultrasound (for HCC)
• D) Annual DEXA scan
• E) Monthly serum urate levels
• Answer: C. Due to the high risk of HCC even in the absence of cirrhosis, annual or bi-annual liver ultrasound is recommended.

Question 8

Which laboratory finding is required to confirm a diagnosis of an acute attack of porphyria?

• A) Low Haemoglobin
• B) Elevated Serum Bilirubin
• C) Markedly elevated urinary PBG
• D) High Serum Iron
• E) Low WBC count
• Answer: C. A markedly elevated urinary PBG (usually 10-100x normal) is the diagnostic hallmark of an acute attack.

Question 9

A patient with recurrent AIP attacks is being considered for Givosiran therapy. What is the target of Givosiran?

• A) Heme protein
• B) PBG Deaminase enzyme
• C) ALAS1 mRNA
• D) GABA receptors
• E) Cytochrome P450
• Answer: C. Givosiran is an siRNA that targets and degrades ALAS1 mRNA in the liver.

Question 10

In the heme pathway, which precursor is primarily associated with the neurological symptoms of AIP?

• A) Succinyl-CoA
• B) Delta-Aminolevulinic Acid (ALA)
• C) Glycine
• D) Protoporphyrin IX
• E) Biliverdin
• Answer: B. ALA is the primary neurotoxin in AIP, likely due to its structural similarity to GABA.

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10. Patient Explanation

"Acute Intermittent Porphyria (AIP) is a rare genetic condition where your body has trouble making 'heme,' which is a vital part of your blood. Because of a missing enzyme, your body starts to build up certain chemicals (called ALA and PBG) that can be toxic to your nerves.

An 'attack' usually causes severe stomach pain, even though there is nothing physically wrong with your stomach itself. It can also cause confusion, a fast heartbeat, or weakness in your arms and legs. These attacks are often set off by triggers like certain medications, skipping meals, or changes in hormones.

When you have an attack, we treat you in the hospital with a medication called Haem Arginate, which tells your body to stop making those toxic chemicals. The most important part of staying well is knowing which medications are 'safe' and which are 'unsafe,' and making sure you eat regular meals. We will give you a card to carry so that other doctors know about your condition."

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11. References

1. Anderson KE, et al. Recommendations for the diagnosis and treatment of the acute porphyrias. Ann Intern Med. 2005;142(6):439-50. [PMID: 15767334]
2. Puy H, et al. Porphyrias. Lancet. 2010;375(9718):924-37. [PMID: 20226990]
3. Whatley SD, et al. Acute intermittent porphyria: an update. Ann Clin Biochem. 2005;42(Pt 2):121-41. [PMID: 15842795]
4. Hift RJ, et al. Drugs in porphyria: From observation to a solid evidence base. Pharmacology & Therapeutics. 2011.
5. Mustajoki P, Nordmann Y. Early administration of heme arginate for acute porphyric attacks. Arch Intern Med. 1993.
6. Bonkovsky HL, et al. Porphyrias: What Is New? Current Rheumatology Reports. 2019.
7. Marsden JT, Rees DC. Urinary excretion of porphyrins and precursors during the menstrual cycle in healthy women. Ann Clin Biochem. 2014. [PMID: 24334346]
8. Kauppinen R. Porphyrias. Lancet. 2005;365(9457):419-31. [PMID: 15680459]
9. L’Aubriet AC, et al. Hyponatremia in acute intermittent porphyria. Journal of Nephrology. 2022.
10. Elder G, et al. The incidence of inherited porphyrias in the UK. J Inherit Metab Dis. 1990. [PMID: 2121980]
11. Kauppinen R, Mustajoki P. Prognosis of acute porphyria: occurrence of acute attacks, clinical findings, and elimination of triggers. Arch Intern Med. 1992.
12. Handschin C, et al. Nutritional regulation of hepatic heme biosynthesis and porphyria through PGC-1alpha. Cell. 2005. [PMID: 16122421]
13. Lin CS, et al. The acute porphyrias: clinical and pathophysiological aspects. Clin Neurophysiol. 2011;122(12):2336-44. [PMID: 21742548]
14. Albers JW, Fink JK. Porphyric neuropathy. Muscle Nerve. 2004;30(5):556-66. [PMID: 15455442]
15. Marsden JT, et al. A Practical Guide to the Diagnostic Center for Porphyria. Clin Chim Acta. 2015.
16. Normosang (Haem Arginate) Summary of Product Characteristics. [Online]
17. Bonkovsky HL, et al. The Porphyrias. Clin Liver Dis. 2017.
18. The Porphyria Drug Database. www.drugs-porphyria.org
19. Stein PE, et al. Chronic Kidney Disease in the Acute Hepatic Porphyrias. Medicine (Baltimore). 2017. [PMID: 28318270]
20. Sardh E, et al. Hepatocellular Carcinoma in Acute Intermittent Porphyria. Clinical Gastroenterology and Hepatology. 2013.
21. Balwani M, et al. Phase 3 Trial of Givosiran for Acute Intermittent Porphyria. N Engl J Med. 2020;382(24):2289-2301. [PMID: 32493419]

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Last Updated: 2026-01-04 | MedVellum Editorial Team | Status: Gold Standard (V4)