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Basic Neurosciences · Paper A

Acute intermittent porphyria: enzyme deficiency (HMBS/PBG deaminase)

The idea

A 28-year-old woman with severe abdominal pain, agitation and urine that darkens on standing is not a psychiatric admission wearing a physical mask — she is a single jammed enzyme wearing both. Acute intermittent porphyria comes from a partial deficiency of hydroxymethylbilane synthase (also called porphobilinogen deaminase, or PBG deaminase), the third enzyme in the pathway your body uses to build haem. The block itself does not make you ill; what makes you ill is everything that piles up behind it once something demands more haem than the pathway can supply, and that pile-up hits your nervous system hard enough to produce abdominal pain, peripheral neuropathy and genuine psychiatric symptoms together. Because it can look purely psychiatric at first presentation, this is exactly the kind of diagnosis the exam wants you to catch before a well-meaning but unsafe prescription makes things dramatically worse.

How it works

Haem is built in a stepwise pathway, and the whole story here follows from what happens when one step in that pathway is only partly working. Hydroxymethylbilane synthase, the enzyme deficient in acute intermittent porphyria, sits third in the sequence; a partial deficiency means the pathway can still limp along under normal demand, which is why most carriers of the gene fault never have an attack at all — this is an autosomal dominant condition with low penetrance, so inheriting the mutation is common among affected families but developing symptoms is not. Trouble starts when demand for haem rises, most often because a drug induces the liver's cytochrome P450 enzymes (which themselves need haem to function), or because fasting, infection, alcohol or hormonal change (the menstrual cycle, pregnancy, the contraceptive pill) increases the body's haem requirement in other ways. Haem itself normally sits on the FIRST enzyme in the pathway, ALA synthase, holding it back through negative feedback; when demand for haem rises, that feedback brake is released and ALA synthase output increases to compensate. In someone with a normal, fully working pathway, the extra material simply flows through to become more haem. In someone with a partly blocked third step, it cannot get through fast enough — so the two precursors made just before that blocked step, delta-aminolevulinic acid (ALA) and porphobilinogen (PBG), back up and accumulate to genuinely toxic levels. It is this build-up of ALA and PBG, not a shortage of haem itself, that is thought to drive the acute attack — abdominal pain from effects on the gut's autonomic nerve supply, motor-predominant peripheral neuropathy (which can progress to respiratory paralysis if unrecognised), and the psychiatric symptoms — anxiety, agitation, confusion, sometimes hallucinations — that so often bring these patients to a psychiatric team first. Followed step by step, the whole multisystem picture stops looking mysterious.

How to spot it

Severe abdominal pain in a young woman, with agitation or confusion and urine that darkens on standing, is an acute attack of acute intermittent porphyria until proven otherwise.
Progressive, motor-predominant limb weakness following an abdominal and psychiatric prodrome, with a normal cerebrospinal fluid protein, is porphyric neuropathy rather than Guillain-Barré syndrome.
An attack that begins after a new anticonvulsant or antibiotic, or after fasting or a hormonal change (menstruation, pregnancy, the contraceptive pill), is a precipitant acting through increased haem demand.
Skin blistering or photosensitivity added to the picture points to a different porphyria (variegate porphyria or hereditary coproporphyria if attacks also occur, porphyria cutanea tarda if they do not), not acute intermittent porphyria.

Tell it apart

Acute intermittent porphyria vs porphyria cutanea tarda — AIP causes acute neurovisceral attacks with no skin involvement, from a partial block earlier in the pathway (HMBS); porphyria cutanea tarda, the most common porphyria overall, causes only skin photosensitivity and blistering from a block further along the pathway (uroporphyrinogen decarboxylase), with no acute attacks.
Acute intermittent porphyria vs variegate porphyria/hereditary coproporphyria — these two cause BOTH acute neurovisceral attacks AND skin photosensitivity, because their enzyme blocks sit later in the pathway, downstream of the point where light-sensitising precursors accumulate; AIP's precursors (ALA, PBG) do not cause skin signs, which is the single feature that separates it from these two.
Acute intermittent porphyria vs lead poisoning — both raise ALA and can cause abdominal pain and neuropathy, because lead also inhibits an enzyme in the same haem pathway, but lead poisoning is confirmed or excluded with a blood lead level, not a porphyrin test.
Acute intermittent porphyria vs Guillain-Barré syndrome — porphyric neuropathy can mimic a progressive ascending motor weakness, but the preceding abdominal pain and psychiatric prodrome, plus a normal cerebrospinal fluid protein (unlike the raised protein typical of Guillain-Barré syndrome), point toward porphyria instead.
Acute intermittent porphyria vs a primary psychiatric presentation — the combination of severe abdominal pain, autonomic instability and urine darkening on standing should redirect you away from a purely psychiatric diagnosis and toward requesting a urinary porphobilinogen level before committing to a psychiatric formulation alone.

Watch for

⚠️ Prescribing a porphyrinogenic drug (classically a barbiturate, but also several anticonvulsants used to treat the seizures this condition itself can cause) to a patient during an undiagnosed attack — this can precipitate a life-threatening deterioration, which is exactly why checking the NAPOS porphyria drug database (drugs-porphyria.org, run by the Norwegian Porphyria Centre) or the British and Irish Porphyria Network before prescribing anything new is a real safety step, not textbook decoration.
Assuming every carrier of the HMBS mutation will become symptomatic — this is an autosomal dominant condition with LOW penetrance, so most gene carriers never have a clinical attack.
Missing the diagnosis because the presentation looks purely psychiatric — agitation, anxiety and confusion are genuine, common presenting features, not a coincidental comorbidity layered on top of the abdominal pain.
Forgetting that the urine darkening on standing is a bedside clue, not the diagnostic test itself — the actual diagnosis rests on a raised urinary porphobilinogen level during an attack, confirmed by enzyme or genetic testing.
Confusing AIP with the porphyrias that cause skin photosensitivity — AIP has no skin signs, and a stem describing blistering or photosensitivity is describing a different porphyria entirely.

Use it again

First check whether the vignette combines abdominal pain, a neuropsychiatric feature and a precipitating drug or physiological stressor — that combination should trigger porphyria as a differential before any purely psychiatric label is applied.
If skin signs (blistering, photosensitivity) are present, redirect away from acute intermittent porphyria toward variegate porphyria, hereditary coproporphyria, or porphyria cutanea tarda depending on whether acute attacks also occur.
If a new drug has just been started, check it against the NAPOS porphyria drug database — classically a barbiturate, but several anticonvulsants qualify too — before assuming the psychiatric symptoms are unrelated to the prescription.
If neuropathy is present, compare the cerebrospinal fluid protein and the preceding history against Guillain-Barré syndrome before settling on either diagnosis.
A normal-looking mental state between attacks is not evidence against the diagnosis: most gene carriers are asymptomatic between attacks, or for life, given low penetrance, so a quiet interval does not rule porphyria out once an attack has occurred.

In detail

Hook — the abdominal pain that turns out to be biochemistry

A young woman is admitted with severe abdominal pain so far unexplained by any scan, and over the next two days she becomes agitated, confused, and starts to lose strength in her legs; someone notices her urine has gone from pale to almost port-wine dark sitting in the sample pot. Every individual finding here could belong to a dozen different diagnoses. Put together, with the right precipitant in the history, they belong to one: acute intermittent porphyria, a disease of a single overworked enzyme rather than any structural damage you could see on a scan. Everything below unpacks that one enzyme, from the biochemistry outward.

Plain definition — one faulty copy of one enzyme's gene

Your body makes haem (the iron-containing molecule at the centre of haemoglobin and several other essential proteins) through a chain of enzyme-driven steps, one after another. Acute intermittent porphyria happens when you inherit one faulty copy of the gene for the third enzyme in that chain, hydroxymethylbilane synthase — also called porphobilinogen deaminase, or PBG deaminase for short. You are left with roughly half the normal amount of a working enzyme at that one step, which is usually just enough to get by on.

The critical caveat — carrying the gene fault is not the same as being ill

Inheriting the gene fault and having attacks are two different things, and running them together is the commonest error on this topic. Most carriers never have an attack. This is an autosomal dominant condition (one faulty copy from one parent is enough to inherit it) with LOW penetrance, meaning only a minority of people who carry the mutation ever go on to have a clinical attack in their lifetime. Carrying the gene is necessary for the disease but nowhere near sufficient — something else has to tip the partly-working pathway over the edge, and you should never assume a family history alone means a patient will become symptomatic.

First principles — why a partial block causes a build-up, not just a shortage

Work through what haem does to its own production line. Haem itself acts as a brake on the FIRST enzyme in the chain, ALA synthase — plenty of haem around, and that first station is told to slow down. When your body suddenly needs more haem (because a drug has switched on liver enzymes that consume it, or because fasting, infection, alcohol or a hormonal shift increases demand another way), that brake is released and the first station speeds up to compensate.

In someone with a fully working line, everything simply flows through faster and more haem gets made. In someone with a narrowed third station, the extra material entering at the start cannot get past that narrow point fast enough — so the two substances made just before it, delta-aminolevulinic acid (ALA) and porphobilinogen (PBG), pile up to genuinely toxic levels.

Notice which way round that runs. It is the pile-up, not a shortage of haem itself, that is thought to poison the gut's nerve supply (abdominal pain), peripheral nerves (weakness, which can progress to breathing difficulty if severe) and the brain (the psychiatric symptoms). You are looking for what accumulates, not for what runs out.

Structured walk-through — from trigger to attack, in order

For example, take a patient who starts a new anticonvulsant that induces liver enzymes. Those enzymes use up haem doing their job, so the liver's internal haem level drops. That drop removes haem's brake on ALA synthase, which ramps up.

Now the line is being pushed harder than its narrowed third station can clear, and ALA and PBG accumulate behind that station. Specialist laboratories quantify exactly this as urinary porphobilinogen, which rises many times above the upper limit of normal during a genuine attack. The abdominal pain usually arrives first, with the neuropsychiatric features following as the toxic effect builds.

Removing the precipitant and giving intravenous haem arginate restores the missing brake. Recovery is gradual rather than immediate, so you should not read a slow first day as treatment failure.

Look-alikes & differentials — four conditions that borrow this picture

Test your own first impression against this table before you settle on an answer.

Looks like acute intermittent porphyria	How to tell it apart
Porphyria cutanea tarda	Only skin photosensitivity and blistering, no acute attacks, from a different enzyme block further along the pathway
Variegate porphyria / hereditary coproporphyria	BOTH acute attacks and skin photosensitivity, unlike AIP's attacks-only picture
Lead poisoning	Also raises ALA and causes abdominal pain/neuropathy, but confirmed by a blood lead level, not a porphyrin test
Guillain-Barré syndrome	Progressive ascending weakness looks similar, but porphyria's preceding abdominal/psychiatric prodrome and normal protein point away from it

Diagnosis — one urine test does most of the work

During a genuine attack, urinary porphobilinogen is markedly raised, and this single test is the mainstay of diagnosis at the bedside — the urine-darkening-on-standing sign is a helpful clue that should prompt you to send it, not a substitute for actually sending it. Confirmation of the underlying condition (as opposed to confirming an attack is happening right now) comes from measuring red blood cell hydroxymethylbilane synthase activity or from genetic testing for the specific HMBS mutation, which also allows screening of at-risk relatives.

Management — replace haem, remove the trigger, and check every drug

There is no NICE guideline for this rare inherited condition; management runs through specialist national porphyria services. The definitive treatment for an acute attack is intravenous haem arginate, which restores haem's negative feedback on ALA synthase directly, alongside stopping any precipitating drug and correcting dehydration, pain and electrolyte disturbance (hyponatraemia is common). High carbohydrate intake gives a smaller version of the same feedback effect and is sometimes tried first for a milder attack.

Every drug prescribed to a patient with known or suspected porphyria — including any anticonvulsant needed for a seizure the attack itself may cause — has to be checked against the NAPOS porphyria drug database (drugs-porphyria.org), or with the British and Irish Porphyria Network, before use, because a genuinely porphyrinogenic drug can turn a manageable attack into a life-threatening one. Make this database check part of your own routine whenever porphyria is even a possibility, not just once the diagnosis is confirmed.

Exam pearl — never let the psychiatry stand alone

The single highest-yield habit for this topic is refusing to let a purely psychiatric presentation stand alone: whenever agitation or confusion arrives together with severe abdominal pain, ask about recent new drugs, fasting, alcohol and hormonal changes, and send a urinary porphobilinogen level before committing to a psychiatric diagnosis in isolation. Set yourself this test before moving on: why does the enzyme block itself not make someone ill most of the time, and what actually has to happen for it to?

In one line — a narrowed step meets a rush order

Remember it this way: you are looking for a partial hydroxymethylbilane synthase deficiency that, once a precipitant raises demand on the haem pathway, lets toxic ALA and PBG pile up behind the block and produce abdominal pain, peripheral neuropathy and psychiatric disturbance together — treated by removing the trigger and giving intravenous haem arginate.