Organic acidaemias — branching viva

Opening framework

My framework has four layers. First, the recognition — a well neonate deteriorating 24 to 72 hours into feeds with a high-anion-gap metabolic acidosis and ketosis is an organic acidaemia until proven otherwise, and the acid-base picture is the discriminator. Second, the emergency protocol — treat on suspicion: stop protein, calorie-load, give carnitine, correct the acidosis, and use haemofiltration if refractory. Third, the biochemistry — the organic acidaemias are branched-chain amino-acid catabolic blocks, and each produces a recognisable acylcarnitine and organic-acid pattern. Fourth, the cofactor responsiveness — methylmalonic acidaemia may be B12-responsive, and the trial defines the prognosis. [1]

The 'treat on suspicion' emergency protocol

The cardinal principle is that treatment begins before the enzyme or molecular diagnosis returns, because outcome tracks peak toxic-acid exposure multiplied by duration. The protocol is S.C.A.L.P.: stop protein and switch off catabolism (10% glucose + intralipid ± insulin to switch off breakdown, the single most important physiological move), carnitine (to conjugate and excrete the toxic acyl-CoA esters as acylcarnitines), acidosis correction with bicarbonate and alternative-pathway ammonia lowering (N-carbamylglutamate in propionic acidaemia), lower cofactor burden (trial B12 in MMA, biotin in multiple carboxylase deficiency), and purify with haemofiltration if the acidosis is refractory or the hyperammonaemia is severe and rising. [1]

The haemofiltration triggers are refractory metabolic acidosis, a very high or rising ammonia despite medical therapy, or deepening encephalopathy. Continuous kidney replacement therapy is preferred in the haemodynamically unstable neonate. The objective is to clear the toxic organic acids and ammonia within hours, because every hour of untreated metabolic acidosis adds to permanent basal-ganglia injury — so I do not wait for the ammonia to plateau or for the diagnosis. [1]

Localising the defect with the metabolic panel

The first-tier panel is interpreted together: blood gas (the acid-base discriminator), glucose, lactate, ketones, ammonia (free-flowing, on ice), acylcarnitines, urinary organic acids, plasma amino acids, and liver function and coagulation. The localising logic is the accumulating toxic acid. Propionic acidaemia shows raised propionylcarnitine (C3) with 3-hydroxypropionate and methylcitrate in urine; methylmalonic acidaemia shows raised C3 with methylmalonic acid; isovaleric acidaemia shows raised isovalerylcarnitine (C5) with isovalerylglycine and 3-hydroxyisovalerate; glutaric aciduria type 1 shows raised glutarylcarnitine (C5DC) with glutaric and 3-hydroxyglutaric acid. [1]

This pattern converts the acylcarnitine profile and urinary organic acids into a near-diagnostic first-tier screen. I confirm with molecular sequencing of the candidate gene — which also enables cascade carrier testing and prenatal or preimplantation diagnosis for the family, and for MMA distinguishes the B12-responsive cobalamin pathway defects from the non-responsive mut-type. [3]

The toxic-organic-acid mechanism of basal-ganglia injury

The injury is driven by the toxic organic acids themselves. They are acids, so they consume bicarbonate and produce the high-anion-gap metabolic acidosis; they inhibit the urea cycle to produce secondary hyperammonaemia; and they impair mitochondrial oxidative phosphorylation, depleting ATP and raising lactate. The basal ganglia — particularly the putamen and globus pallidus — are selectively vulnerable, which is why decompensation produces acute dystonia and choreoathetosis (a metabolic stroke) and why chronic disease accumulates intellectual disability. The Nizon cohort confirms that the number and severity of decompensations is the dominant predictor of long-term neurological outcome. [3]

Branch: the cobalamin responsiveness trial

For the methylmalonic acidaemia branch, I apply the cofactor responsiveness framework. MMA has a spectrum of causes: a defect in the methylmalonyl-CoA mutase enzyme itself (mut-type, typically B12-nonresponsive) or a defect in the cobalamin processing pathway (cblA–cblD, some B12-responsive). I perform a cobalamin trial at diagnosis — measuring urinary methylmalonic acid excretion before and after intramuscular hydroxocobalamin — because a substantial fall defines a responsive subtype that is milder and disease-modifying with lifelong pharmacological B12. A mut-type defect is non-responsive, defining a worse prognosis and the need to consider transplantation earlier. [6]

Closing: transplantation and the emergency sick-day plan

For severe disease, transplantation is the definitive therapy. Liver transplantation corrects the hepatic enzyme defect in propionic acidaemia and often resolves the cardiomyopathy; for methylmalonic acidaemia, kidney transplantation addresses the progressive renal failure and liver transplantation corrects the metabolic defect. The Sen ACMG points-to-consider statement guides the indications. Transplantation restores metabolic stability and liberates the child from strict dietary restriction, but it cannot reverse established neurological injury — which is why it is complementary to, not a substitute for, excellent emergency care. The closing point is that the emergency sick-day plan — stop protein, push calories, present early — is the single most powerful intervention for preventing recurrent decompensation and the cumulative basal-ganglia injury that dominates long-term morbidity. [8] [1]