Fatty-acid oxidation disorders — branching viva

Opening framework

My framework has four layers. First, the recognition — a child with fasting- or illness-induced hypoketotic hypoglycaemia is a fatty-acid oxidation disorder until proven otherwise, and the bedside ketone measurement is the test that decides it. Second, the emergency protocol — IV 10 percent dextrose immediately to shut off lipolysis, collect the critical sample in parallel, and do not wait for the enzyme or molecular diagnosis. Third, the biochemistry — the carnitine shuttle and the beta-oxidation spiral, and each block produces a recognisable acylcarnitine signature. Fourth, the family — these are autosomal recessive, newborn screening detects most, and a written sick-day plan is what keeps the child safe. [1]

The hypoketotic hypoglycaemia signature

The single most important bedside observation is the relationship between glucose and ketones. In normal fasting, the glucose falls and ketones rise as the brain switches to fat-derived fuel. In a fatty-acid oxidation disorder the glucose falls but the ketones stay flat — the body cannot burn fat to compensate. A point-of-care beta-hydroxybutyrate that is low or undetectable alongside a glucose below 2.5 to 3.0 millimoles per litre should trigger the full metabolic work-up. [1] [5]

The acute protocol — F.A.S.T.

The protocol is F.A.S.T.: free glucose now (IV 10 percent dextrose, 2 millilitres per kilogram bolus then infusion at 6 to 8 milligrams per kilogram per minute to maintain glucose above five millimoles per litre), acylcarnitine profile and critical sample (insulin, ketones, free fatty acids, cortisol, growth hormone), support the organs (inotropes for cardiomyopathy in long-chain defects, hydration for rhabdomyolysis), and triage the long-term plan. Glucagon is ineffective because there is no insulin excess — the problem is inability to use fat fuel. [1] [2]

Branch 1: localising the defect with acylcarnitines

Each defect produces a chain-length-specific acylcarnitine signature. MCAD deficiency shows elevation of octanoylcarnitine (C8); VLCAD deficiency shows C14:1 elevation; the carnitine transporter defect shows profoundly low total carnitine; CPT I shows high free carnitine with low long-chain species. The critical sample taken at the time of hypoglycaemia separates the FAOD pattern (low insulin, low ketones, high free fatty acids) from hyperinsulinism (inappropriately high insulin) and endocrine failure. [1]

Branch 2: the carnitine transporter defect

The second case — a six-month-old with dilated cardiomyopathy and plasma carnitine of four micromoles per litre — is the primary carnitine transporter defect, OCTN2. Without the transporter, carnitine is lost in the urine and tissues are starved of the substrate needed to shuttle long-chain fats into the mitochondrion. The treatment is high-dose oral carnitine (100 to 400 milligrams per kilogram per day), which reverses the cardiomyopathy and is essentially curative. Missing this diagnosis by not checking a plasma carnitine in an unexplained cardiomyopathy is a classic and avoidable error. [1] [2]

Long-term management and prognosis

Once stabilised, management is built around three pillars: avoidance of fasting with age-appropriate safe intervals and cornstarch supplementation, a written emergency sick-day plan, and disease-specific therapy. For the long-chain defects, triheptanoin — an odd-chain triglyceride that bypasses the block and provides anaplerotic substrate — has demonstrated clinically meaningful reductions in hospitalisation and rhabdomyolysis in the Vockley extension study. For MCAD deficiency identified by newborn screening, the prognosis is excellent with near-normal survival and neurodevelopment — the Wilcken outcome study demonstrated this by comparing screened and unscreened cohorts. [9] [5] [6]