17839579395967034185712000820637.jpg
Give answer
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Finding Sources
Reading File
Finding Sources
I now have comprehensive data to answer all parts of this question. Let me compile the full answer.
Exam Answer: Porphyria (Clinical Case)
Case Diagnosis
This 24-year-old woman presents with the classic triad of Acute Intermittent Porphyria (AIP):
- Abdominal pain, vomiting, anxiety (neurovisceral symptoms)
- Reddish/dark urine (porphyrin metabolites)
- Neuropsychiatric symptoms + seizures
- Tachycardia, hypertension (sympathetic overactivity)
- Precipitated by an antiepileptic drug (many AEDs are P450 inducers that trigger attacks)
- Lab: elevated PBG, normal LFTs, no hemolysis, hyponatremia (SIADH)
Part i. Most Likely Type of Porphyria (1 mark)
Acute Intermittent Porphyria (AIP)
This is the most common acute hepatic porphyria. It is caused by a deficiency of hydroxymethylbilane synthase (HMBS), also called PBG deaminase (porphobilinogen deaminase). It is inherited in an autosomal dominant fashion. Key distinguishing feature: no photosensitive rash (unlike variegate porphyria or hereditary coproporphyria, which do cause skin manifestations).
- Harrison's Principles of Internal Medicine 22E, p. 3646
Part ii. Biochemical Defect in This Patient (1 mark)
Deficiency of PBG deaminase (Hydroxymethylbilane synthase / HMBS)
- This enzyme normally catalyzes the condensation of 4 molecules of porphobilinogen (PBG) into the linear tetrapyrrole hydroxymethylbilane (HMB)
- When HMBS is deficient, PBG and its precursor δ-aminolevulinic acid (ALA) accumulate in the liver and are excreted in excess in the urine
- The accumulating ALA and PBG are directly neurotoxic, producing the neurovisceral symptoms
- The antiepileptic drug she was prescribed (likely a barbiturate or carbamazepine) induces hepatic ALAS1 (ALA synthase), driving more flux through the pathway and worsening accumulation
Triggering mechanism: Drugs metabolized by P450 deplete hepatic heme, inducing ALAS1 -> massive ALA and PBG production -> overwhelms the deficient HMBS step -> symptoms appear.
- Harrison's Principles of Internal Medicine 22E, p. 3646
- Fitzpatrick's Dermatology, Table 124-1
Part iii. Steps of Heme Synthesis (3 marks)
Heme is synthesized in 8 steps involving both mitochondria and cytosol:
| Step | Location | Enzyme | Substrate -> Product |
|---|---|---|---|
| 1 | Mitochondria | ALA synthase (ALAS1/ALAS2) | Glycine + Succinyl-CoA -> δ-Aminolevulinic acid (ALA) |
| 2 | Cytosol | ALA dehydratase (ALAD / PBG synthase) | 2 ALA -> Porphobilinogen (PBG) |
| 3 | Cytosol | HMB synthase (PBG deaminase) [DEFICIENT IN AIP] | 4 PBG -> Hydroxymethylbilane (HMB) |
| 4 | Cytosol | Uroporphyrinogen III synthase (UROS) | HMB -> Uroporphyrinogen III |
| 5 | Cytosol | Uroporphyrinogen decarboxylase (UROD) | Uroporphyrinogen III -> Coproporphyrinogen III |
| 6 | Mitochondria | Coproporphyrinogen oxidase (CPOX) | Coproporphyrinogen III -> Protoporphyrinogen IX |
| 7 | Mitochondria | Protoporphyrinogen oxidase (PPOX) | Protoporphyrinogen IX -> Protoporphyrin IX |
| 8 | Mitochondria | Ferrochelatase (FECH) | Protoporphyrin IX + Fe²⁺ -> Heme |
Regulation: The first enzyme ALAS1 is the rate-limiting step. Heme feedback inhibits ALAS1. Drugs (P450 inducers), fasting, and hormones induce ALAS1, precipitating acute attacks.
- Fitzpatrick's Dermatology, p. 2264 (Heme Synthesis and Functions)
- Harrison's Principles of Internal Medicine 22E
Part iv. Types of Porphyrias (3 marks)
Porphyrias are classified by site of enzyme deficiency (hepatic vs. erythropoietic) and by clinical presentation (acute neurovisceral vs. cutaneous):
A. Hepatic Porphyrias (Acute)
| Type | Enzyme Defect | Inheritance | Features |
|---|---|---|---|
| ALA dehydratase porphyria (ADP) | ALA dehydratase | AR | Neurovisceral (rare) |
| Acute Intermittent Porphyria (AIP) | PBG deaminase (HMBS) | AD | Neurovisceral, NO skin |
| Hereditary Coproporphyria (HCP) | Coproporphyrinogen oxidase | AD | Neurovisceral + skin |
| Variegate Porphyria (VP) | Protoporphyrinogen oxidase | AD | Neurovisceral + skin |
B. Hepatic Porphyrias (Non-Acute)
| Type | Enzyme Defect | Features |
|---|---|---|
| Porphyria Cutanea Tarda (PCT) | Uroporphyrinogen decarboxylase | Blistering photosensitivity, NO acute attacks |
C. Erythropoietic Porphyrias
| Type | Enzyme Defect | Features |
|---|---|---|
| Congenital Erythropoietic Porphyria (CEP) | Uroporphyrinogen III synthase | Severe photosensitivity, hemolytic anemia |
| Erythropoietic Protoporphyria (EPP) | Ferrochelatase | Non-blistering photosensitivity |
| X-linked Protoporphyria (XLP) | ALAS2 (gain of function) | Non-blistering photosensitivity |
- Fitzpatrick's Dermatology, Table 124-1; Harrison's Principles of Internal Medicine 22E
Part v. Cause of Abdominal Pain, Neuropsychiatric Symptoms, and Reddish Urine (2 marks)
Abdominal Pain
- Accumulation of ALA and PBG causes autonomic neuropathy of the gut, leading to visceral smooth muscle dysfunction
- Results in colicky abdominal pain, nausea, vomiting, and constipation
- Sympathetic overactivation (tachycardia, hypertension) is a hallmark
Neuropsychiatric Symptoms (anxiety, hallucinations, seizures)
- ALA and PBG are directly neurotoxic
- ALA structurally resembles GABA and may act as a false neurotransmitter
- CNS accumulation causes anxiety, psychosis, hallucinations, and seizures
- Hyponatremia (from SIADH - inappropriate ADH secretion) worsens CNS symptoms and can independently cause seizures
- Peripheral neuropathy (axonal degeneration) explains weakness and pain
Reddish/Dark Urine
-
Excess uroporphyrins and coproporphyrins (oxidized porphyrin intermediates) are excreted in the urine
-
Porphyrins are photoactive - urine darkens and turns reddish/port-wine colored on exposure to light (oxidation of colorless porphyrinogens to colored porphyrins)
-
Urinary PBG is classically positive; urine also shows excess ALA
-
This feature is pathognomonic - fresh urine is often normal colored, but darkens on standing (characteristic finding in this case: "reddish on standing")
-
Harrison's Principles of Internal Medicine 22E, p. 3646
-
Goldman-Cecil Medicine, The Acute Attack section
Summary Box:
| Feature in Case | Explanation |
|---|---|
| Reddish urine on standing | Porphyrins/porphyrinogens oxidize in urine |
| Abdominal pain | Autonomic neuropathy + GI dysmotility |
| Tachycardia, hypertension | Sympathetic overactivation |
| Seizures | ALA neurotoxicity + hyponatremia |
| Normal LFTs, no hemolysis | Hepatic porphyria, not hemolytic |
| Elevated PBG | Hallmark of AIP - diagnostic |
| Precipitated by AED | Drug-induced P450 induction -> ALAS1 upregulation |
This is a shared conversation. Sign in to Orris to start your own chat.