Carnitine uptake defect

Carnitine uptake defect is the fatty acid oxidation disorder where the enzyme machinery is intact but the cofactor cannot get into the cell. The treatment is the cofactor itself, taken orally for life. The screening pattern is distinctive (low free carnitine, the only such marker in the panel), the management is straightforward, and the outcomes when treatment is started early are excellent. The condition is one of the cleaner success stories of the expanded newborn screening era.

What CUD is

Carnitine uptake defect, CUD (also called primary systemic carnitine deficiency), is an autosomal recessive disorder caused by variants in SLC22A5 on chromosome 5q31. SLC22A5 encodes OCTN2, the high-affinity organic cation/carnitine transporter that imports carnitine into cells across the plasma membrane and reabsorbs filtered carnitine in the renal tubule. When OCTN2 activity is reduced or absent, intracellular carnitine cannot accumulate and renal reabsorption fails. Plasma carnitine drops dramatically. The clinical consequence is failure of long-chain fatty acid import into the mitochondrion, because carnitine is the obligate cofactor for the carnitine shuttle.

The clinical presentations span a wide spectrum. Some affected children present in the first year of life with hypoketotic hypoglycemia, hepatic encephalopathy, and acute liver failure. Others present in early childhood with cardiomyopathy, often dilated, that can progress to heart failure. Some present in late childhood or adulthood with skeletal muscle weakness and exercise intolerance. Some are entirely asymptomatic and are identified incidentally or through family screening after a sibling is diagnosed. The same SLC22A5 variant can produce different phenotypes in different individuals.

The maternal CUD phenomenon parallels maternal 3-MCC and maternal MCADD: an asymptomatic mother whose own carnitine deficiency lowers fetal and neonatal carnitine, producing an abnormal newborn screen that reflects the mother's biochemistry rather than the baby's. Confirmation testing distinguishes maternal carrier biochemistry from infant disease.

Reported live-birth incidence in newborn screening programs runs roughly 1 in 40,000 to 1 in 100,000 in unselected populations. Higher rates have been documented in the Faroe Islands, attributed to a founder variant.

Detection

Newborn screening uses tandem mass spectrometry on the dried blood spot to identify low free carnitine, C0. The marker is specific because plasma carnitine elevations are not flagged and the low-end pattern is distinctive. False positives can result from maternal CUD, maternal vegetarian diet, maternal valproate use during pregnancy, prematurity, and parenteral nutrition. Confirmation uses plasma carnitine measurement (low total and free carnitine), urine carnitine measurement (paradoxically normal or elevated relative to plasma due to failed reabsorption), and SLC22A5 sequencing.

What management looks like

Standard of care is lifelong oral L-carnitine supplementation, typically 100 to 400 mg per kg per day in divided doses. Plasma carnitine is monitored on therapy. Supplementation restores intracellular carnitine to levels that support fatty acid oxidation, and the clinical picture in screen-detected cases started on supplementation in the first weeks of life is generally normal development with no cardiomyopathy, no hepatic crises, and no muscle weakness.

Echocardiographic surveillance is part of routine follow-up because cardiomyopathy, when it has developed before treatment, may improve but does not always resolve completely on supplementation. Annual or biannual echocardiogram is standard in the first decade and at intervals appropriate to clinical trajectory thereafter.

Adherence to oral L-carnitine across decades is the central long-term management challenge. Discontinuation of carnitine in adolescents or adults has been associated with subsequent acute cardiac events in case reports. The supplement is inexpensive, but adherence to a daily medication for an asymptomatic condition is the same problem that affects every prophylactic medication.

What this looks like for a family

A baby is born and the heel-prick is sent. On day 4, the state lab reports a low free carnitine. On day 7, plasma carnitine confirms low total and free carnitine, urine carnitine analysis is consistent with renal carnitine wasting, and SLC22A5 sequencing returns biallelic pathogenic variants. The metabolic team starts oral L-carnitine within the first weeks of life. Plasma carnitine on therapy reaches the target range within weeks.

That child grows up taking carnitine three times a day, with periodic metabolic and cardiology surveillance. The condition that would have presented at 18 months with cardiomyopathy or at 4 years with hepatic encephalopathy in the unscreened pre-screening era never produces a symptomatic event.

That is what CUD care looks like in practice. The screen identifies the deficiency. The cofactor is the treatment. The lifelong adherence to a cheap oral supplement is the only difficult part of the management plan.