Carnitine is a naturally occurring hydrophilic amino acid derivative, produced endogenously in the kidneys and liver and derived from meat and dairy products in the diet. It plays an essential role in the transfer of long-chain fatty acids into the mitochondria for beta-oxidation. Carnitine binds acyl residues and helps in their elimination, decreasing the number of acyl residues conjugated with coenzyme A (CoA) and increasing the ratio between free and acylated CoA.
Carnitine deficiency is a metabolic state in which carnitine concentrations in plasma and tissues are less than the levels required for normal function of the organism. Biologic effects of low carnitine levels may not be clinically significant until they reach less than 10-20% of normal. Carnitine deficiency may be primary or secondary.
Primary carnitine deficiency is caused by a deficiency in the plasma membrane carnitine transporter, with urinary carnitine wasting causing systemic carnitine depletion.[1] Intracellular carnitine deficiency impairs the entry of long-chain fatty acids into the mitochondrial matrix. Consequently, long-chain fatty acids are not available for beta-oxidation and energy production, and the production of ketone bodies (which are used by the brain) is also impaired.
Regulation of the intramitochondrial free CoA also is affected, with accumulation of acyl-CoA esters in the mitochondria. This, in turn, affects the pathways of intermediary metabolism that require CoA (eg, Krebs cycle, pyruvate oxidation, amino acid metabolism, mitochondrial and peroxisomal beta oxidation).
SLC22A5 mutations can affect carnitine transport by impairing maturation of transporters to the plasma membrane.[2]
The 3 areas of involvement include (1) the cardiac muscle, which is affected by progressive cardiomyopathy (by far, the most common form of presentation), (2) the CNS, which is affected by encephalopathy caused by hypoketotic hypoglycemia, and (3) the skeletal muscle, which is affected by myopathy.
Muscle carnitine deficiency (restricted to muscle) is characterized by depletion of carnitine levels in muscle with normal serum concentrations. Evidence indicates that the causal factor is a defect in the muscle carnitine transporter.
In secondary carnitine deficiency, which is caused by other metabolic disorders (eg, fatty acid oxidation disorders, organic acidemias), carnitine depletion may be secondary to the formation of acylcarnitine adducts and the inhibition of carnitine transport in renal cells by acylcarnitines.
In disorders of fatty acid oxidation, excessive lipid accumulation occurs in muscle, heart, and liver, with cardiac and skeletal myopathy and hepatomegaly. Long-chain acylcarnitines are also toxic and may have an arrhythmogenic effect, causing sudden cardiac death.
Encephalopathy may be caused by the decreased availability of ketone bodies associated with hypoglycemia. Preterm newborns also may be at risk for developing carnitine deficiency because immature renal tubular function combined with impaired carnitine biosynthesis renders them strictly dependent on exogenous supplies to maintain normal plasma carnitine levels.
Valproic acid may cause an acquired type of secondary carnitine deficiency by directly impairing renal tubular reabsorption of carnitine. The effect on carnitine uptake and the existence of an underlying inborn error involving energy metabolism may be fatal; in other cases, it may primarily affect the muscle, causing weakness.
No studies have estimated the incidence of primary carnitine deficiency in the United States, however; it may be similar to the incidence in Japan from the cases already reported.
International
In a Japanese study, primary systemic carnitine deficiency was estimated to occur in 1 per 40,000 births.[3] In Australia, the incidence has been estimated to be between 1:37,000-1:100,000 newborns. The frequency of this condition in adults is not known. However, in the United Kingdom, a previous report identified 4 affected mothers in 62,004 infants screened, with a frequency of 1:15,500.
Mortality/Morbidity
In order to abate the mortality and morbidity of undiagnosed primary carnitine deficiency, this condition has been included in the expanded newborn screening program in several states within the United States.[4] Primary carnitine deficiency can be identified in infants by expanded newborn screening using tandem mass spectrometry.[5] Low levels of free carnitine (C0) are detected. However, low carnitine levels in newborns may also reflect maternal primary carnitine deficiency.
Sudden death: Unfortunately, the first clinical manifestation in asymptomatic individuals with primary carnitine deficiency may be sudden death. This also may occur in patients with secondary carnitine deficiency as a consequence of ventricular tachycardia or fibrillation.[6]
Heart failure: Patients with primary carnitine deficiency develop a progressive cardiomyopathy that usually presents at a later age. The cardiac function does not respond to inotropes or diuretics. If the condition is not correctly diagnosed and no carnitine is supplemented, progressive heart failure eventually leads to death. Heart failure caused by dilated cardiomyopathy may be the presenting syndrome in patients with secondary carnitine deficiency caused by defects in beta-oxidation, such as long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) and very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency.
Hypoglycemic hypoketotic encephalopathy: Acute encephalopathy accompanied by hypoketotic hypoglycemic episodes usually presents in younger infants with primary carnitine deficiency. Periods of fasting in association with viral illness trigger these acute episodes. Some patients have developmental delay and CNS dysfunction associated with these episodes. If no carnitine replacement is given, recurrent episodes of encephalopathy may ensue.
A significant cohort of patients with primary carnitine deficiency do not present in infancy or early childhood as previously thought but remain asymptomatic into adulthood. These observations are derived from the experience of expanded newborn screening programs that identified maternal primary carnitine deficiency in mothers who were for the most part minimally symptomatic or asymptomatic. One mother with primary carnitine deficiency was reported to have a history of syncope that worsened during pregnancy, when plasma carnitine levels are physiologically lower.[7]
Race
Overall, this disorder is panethnic, and, in some families, consanguinity is present in cases of primary carnitine deficiency.
Sex
No sex predilection is observed in primary carnitine deficiency.
Age
The mean age at onset for primary carnitine deficiency not detected or ascertained by a newborn screening program is 2 years, with onset ranging from 1 month to 7 years. Infants typically present with hypoketotic hypoglycemia, whereas older children present with skeletal or heart myopathy. Symptoms of muscle carnitine deficiency may appear early yet generally occur later (ie, second or third decade of life).
In secondary carnitine deficiency caused by fatty acid oxidation disorders, the age of onset varies. Metabolic decompensation triggered by viral illness, associated with encephalopathy, and accompanied by liver involvement, hypotonia, or cardiomyopathy tends to occur in infancy. Cardiomyopathy or skeletal myopathy tends to present later. Carnitine deficiency also may occur in preterm newborns receiving total parenteral nutrition (TPN) with no carnitine supplementation.
One classic initial presentation of primary carnitine deficiency is hypoketotic hypoglycemic encephalopathy, accompanied by hepatomegaly, elevated liver transaminases, and hyperammonemia.
Cardiomyopathy is the other classic presentation (affecting older children); onset may occur with rapidly progressive heart failure. Cardiomyopathy can also be observed in older patients with a metabolic presentation, even if they are asymptomatic from a cardiac standpoint.
Pericardial effusion has also been observed in association with primary carnitine deficiency.[8]
Muscle weakness, the third manifestation of the disease, may accompany the heart failure or present by itself.
Carnitine deficiency may be a cause of GI dysmotility, with recurrent episodes of abdominal pain and diarrhea.
Hypochromic anemia and recurrent infections are other manifestations of the disease.
Few patients who were asymptomatic most of their lives have presented following the birth of a child.
Mild developmental delay can be the only manifestation in rare cases.
Muscle carnitine deficiency
Severe reduction in muscle carnitine levels and normal serum carnitine concentrations characterize muscle carnitine deficiency. This disorder is restricted to muscle, with no renal leak of carnitine or signs of liver involvement.
Symptoms of muscle carnitine deficiency can appear in the first years of life, but they may occur later during the second or third decade. Patients may experience proximal muscular weakness of varying degree, exercise intolerance, or myalgia.
Secondary carnitine deficiency
Breastfed infants may experience a catabolic state shortly after birth, when the production of milk is not adequate to meet nutritional requirements. Acute metabolic decompensation with hypoketotic or nonketotic hypoglycemia usually occurs in infancy, whereas cardiac and skeletal muscle disease manifest later. The episodes of metabolic decompensation, triggered by fasting or common viral illness, consist of altered consciousness that can be complicated by seizures, apnea, or cardiorespiratory arrest. Patients may have a history of failure to thrive, developmental delay, or nonspecific abdominal problems.
Patients with organic acidemias causing secondary carnitine deficiency may present with crises consisting of hypoglycemia, ketoacidosis, and hyperammonemia.
Patients with respiratory chain defects or mitochondrial disorders and secondary carnitine deficiency may present with abnormal fatigability and lactic acidosis associated with exertion. These children also may present with encephalopathy and/or lipid storage myopathy and carnitine depletion. Carnitine deficiency has been observed in children with urea cycle defects, and it may exacerbate episodes of hyperammonemia.
Signs and symptoms related to carnitine deficiency are not completely defined in the newborn. Apnea, cardiac death, and sudden death have been found in infants with carnitine depletion.
Carnitine deficiency can develop in children with renal Fanconi tubulopathy; it may be idiopathic and present with renal tubular acidosis or secondary to acquired or inherited conditions.
Carnitine deficiency may present in children being treated with valproic acid and may be associated with fulminant liver failure and presentation similar to that in Reye syndrome. It also may present with a myopathy and increased lipid storage in patients with AIDS who are being treated with zidovudine.
In primary carnitine deficiency, physical findings may vary depending on the form of presentation.
CNS: If the presentation is encephalopathy caused by hypoketotic hypoglycemia, the patient may present limp, unresponsive, and comatose after a prolonged fast. Pyramidal movements or minimal athetoid movements can persist after this type of presentation. Modest hepatomegaly also can be appreciated.
Skeletal muscle: In the myopathic presentation, patients may have mild motor delays, hypotonia, or progressive proximal weakness.
Cardiac muscle: Patients with primary carnitine deficiency may present with cardiomyopathy. Onset may occur with rapidly progressive heart failure or murmur. Cardiomegaly may be found on the physical examination, associated with the presence of a heart murmur. A gallop rhythm can be found, associated with a dilated cardiomyopathy.[9]
Respiratory symptoms are associated with heart failure.
Muscle carnitine deficiency
Muscle carnitine deficiency findings are limited to muscle and can be associated with proximal weakness and signs of exercise intolerance and cardiomyopathy.
Secondary carnitine deficiency
Secondary carnitine deficiency presents with clinical manifestations of fatty acid oxidation disorders.
Episodes of metabolic decompensation triggered by infection or fasting may present with lethargy that may be accompanied by seizures or apnea.
This encephalopathy may also present with hypotonia and hepatomegaly.
Signs of cardiac hypertrophy may be evident, with gallop or heart murmur on the cardiac examination.[10]
Less frequently, these patients may have other findings, such as pigmentary retinopathy, peripheral neuropathy, cardiac arrhythmias, or myoglobinuria.
Disorders such as glutaric aciduria type II or carnitine palmitoyltransferase II (CPT-II) deficiency can present with dysmorphic features, such as mid-facial hypoplasia and frontal bossing (Zellwegerlike phenotype) and congenital abnormalities of the abdominal wall.
Primary carnitine deficiency is caused by a defect in the plasma membrane carnitine transporter in kidney and muscle. The lack of the plasma membrane carnitine transporter OCTN2 results in urinary carnitine wasting and in decreased intracellular carnitine accumulation. Causative mutations in a gene called SLC22A5 are responsible for this condition.
Muscle carnitine deficiency
Carnitine deficiency limited to the muscle is observed in myopathic carnitine deficiency with severe reduction in muscle carnitine levels. The basic biochemical defect has not been identified.
Secondary carnitine deficiency
Secondary carnitine deficiency, which manifests with a decrease of carnitine levels in plasma or tissues, may be associated with genetically determined metabolic conditions, acquired medical conditions, or iatrogenic states.
Disorders of the carnitine cycle or disorders of fatty acid beta-oxidation can cause secondary carnitine deficiency via several mechanisms. Block in fatty acid oxidation contributes to the accumulation of acyl-CoA intermediates. Transesterification with carnitine leads to the formation of acylcarnitine and the release of free CoA. These acylcarnitines are excreted readily in the urine. They inhibit carnitine uptake at the level of the carnitine transporter in renal cells, causing increased carnitine losses in the urine and systemic secondary depletion of carnitine.
Other genetic conditions that are associated with Fanconi syndrome (eg, Lowe syndrome, cystinosis) may present with secondary carnitine deficiency because of increased renal losses of carnitine. Lysinuric protein intolerance is associated with an increased excretion of lysine in the urine, and the biosynthesis of carnitine needs lysine. Other metabolic disorders (eg, propionic acidemia, methylmalonic acidemia) may also present with secondary carnitine deficiency. Secondary carnitine deficiency may also be observed in respiratory chain defects.
Aminoacidopathies (eg, isovaleric acidemia, propionic acidemia, methylmalonic acidemia, glutaric acidemia type I, 3-hydroxymethylglutaryl-CoA lyase deficiency) also contribute to the accumulation of acyl-CoA intermediates at the site of the metabolic block. This occurs with the formation of acylcarnitine esters, which are transported out of the cell and excreted in the urine. The decreased threshold for carnitine excretion causes low total carnitine levels in plasma and tissue.
Carnitine deficiency has been observed in children with urea cycle defects (eg, ornithine transcarbamylase deficiency, carbamoyl phosphate synthetase deficiency). Whether carnitine deficiency is related to the primary metabolic defect, to the concomitant liver disease observed in the initial presentation, or to benzoate therapy is unclear.
Carnitine deficiency is observed in disorders of the mitochondrial respiratory chain, such as cytochrome c oxidase deficiency, in which the ATP depletion may compromise the energy-dependent carnitine uptake. An interference with carnitine transport occurs in tissues, including renal reabsorption, which explains the low plasma and tissue levels in these patients.
Other inborn errors of metabolism or genetic disorders may cause secondary carnitine deficiency because of impairment of carnitine biosynthesis secondary to increased urinary losses of lysine, which occurs in lysinuric protein intolerance. Increased urinary loss of carnitine associated with Fanconi syndrome may be observed in syndromes such as cystinosis or Lowe syndrome (ie, X-linked oculocerebrorenal syndrome).
Acquired medical conditions may affect carnitine homeostasis. Cirrhosis or chronic renal failure may impair the biosynthesis of carnitine. Diets with low carnitine content (eg, lacto-ovo–vegetarian diet) or malabsorption syndromes may cause secondary carnitine deficiency. It may also be observed in conditions of increased catabolism present in patients with critical illness. Increased losses of carnitine in the urine, which occur in renal tubular acidosis or Fanconi syndrome, may cause secondary carnitine deficiency. Preterm neonates are at risk for developing carnitine deficiency because they have impaired reabsorption of carnitine at the level of the proximal renal tubule and immature carnitine biosynthesis.
In cases of maternal primary carnitine deficiency, few infants were found to have dramatically reduced levels of carnitine in newborn screening. However, these levels rapidly normalized with supplementation. The diagnostic work-up revealed that their mothers had primary carnitine deficiency and were asymptomatic all of their lives, with the mother's disorder being unmasked by low carnitine levels in their infants.
Iatrogenic causes of secondary carnitine deficiency include several drugs associated with secondary carnitine deficiency (eg, valproate, pivampicillin, emetine, zidovudine).
Valproate: Numerous mechanisms have been cited, such as sequestration of CoA by valproic acid and metabolites (causing a secondary disturbance of intermediary metabolism) and direct inhibition of fatty acid oxidation enzymes by valproic acid metabolites. In cultured fibroblasts, valproic acid impairs the plasma membrane carnitine uptake in vitro. This impairment of carnitine uptake may explain serum depletion caused by decreased renal tubular reabsorption of carnitine and muscle depletion caused by decreased muscle uptake.
Zidovudine: Muscle mitochondrial impairment caused by zidovudine in patients with AIDS results in decreased content of muscle carnitine levels caused by decreased carnitine uptake in muscle.
If the patient is suspected of having primary carnitine deficiency or other metabolic disorders associated with secondary carnitine deficiency and is presenting with a metabolic emergency, the following studies are indicated:
Immediate assessment: Immediately check blood glucose and urine ketones if a child presents to the emergency room with lethargy, seizures, apnea, or any episode of decreased consciousness. The absence or low amounts of ketones in the urine, combined with the episode of hypoglycemia in primary carnitine deficiency (as well as in other defects in the carnitine cycle or fatty acid oxidation), causes secondary carnitine deficiency.
Ammonia levels can be moderately elevated, especially in primary carnitine deficiency and particularly if the child has a presentation similar to that of Reye syndrome.
Transaminases are usually moderately elevated in primary carnitine deficiency.
In some defects of the carnitine cycle that cause secondary carnitine deficiency (eg, CPT-II deficiency), a hepatocardiomuscular form can present with liver involvement. Other fatty acid oxidation disorders, such as long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency, can present with liver involvement.
A chemistry panel may show evidence of metabolic acidosis.
Hyperuricemia may be present in carnitine deficiency because carnitine competes for renal tubular excretion.
Elevated serum CK levels may be observed in primary carnitine deficiency and in fatty acid oxidation disorders.
Elevated lactate can be observed in respiratory chain defects or in LCHAD deficiency.
Altered coagulation with prolonged prothrombin time may be found.
Plasma carnitine level: In primary carnitine deficiency, the carnitine level in plasma is usually less than 5% of normal, with acylcarnitines proportionately reduced. The ratio between acylcarnitine and free carnitine is normal. A feature of most fatty acid oxidation disorders is that they are associated with decreased plasma carnitine concentrations. This feature is also observed in other inborn errors of metabolism that cause secondary carnitine deficiency, such as organic acidemias caused by the formation of carnitine esters.
Urine carnitine level: This is only useful in primary carnitine deficiency in which the transporter in kidney cells has decreased capacity for reabsorption, causing increased carnitine excretion.
Newborn screen (see Other Tests): Recently, several patients with primary carnitine deficiency have been ascertained through newborn screening programs. In these cases, the acylcarnitine profile reveals a low level of free carnitine and all acylcarnitine species. However, plasma carnitine levels can be within the reference range if obtained too early, due to the transfer of carnitine through the placenta to the fetus.
Urine organic acid levels: In primary carnitine deficiency, the urine organic acid analysis usually is normal. In cases of fatty acid oxidation disorders that cause secondary carnitine deficiency, inappropriate dicarboxylic aciduria occurs during periods of illness. Urinary organic acid profile usually is normal in these patients when they are well, except in cases of medium-chain 3-hydroxyacyl-CoA dehydrogenase (MCAD) deficiency. In some disorders (eg, MCAD, LCHAD, short-chain acyl-CoA dehydrogenase [SCAD] deficiency) specific patterns can be seen. Collecting this specimen during illness is important.
Urine acylglycine level: In MCAD deficiency, the urine contains increased amounts of glycine conjugates. The test may also be used in individuals with suspected glutaric aciduria type II or SCAD deficiency.
Acylcarnitine profile and free fatty acid levels: Tandem mass spectrometry analyses of acylcarnitine profile and free fatty acids may be used to detect metabolic defects that cause secondary carnitine deficiency (eg, fatty acid oxidation disorders, organic acidemias) because acyl-CoA intermediates proximal to the block in fatty acid or amino acid oxidative pathway may be transesterified to carnitine. Modest amounts of long-chain 3-hydroxy fatty acids consistently are found in the plasma of patients with LCHAD deficiency, even if these patients are asymptomatic.
Fasting test: In a fasting test, patients undergo a controlled and prolonged fast under strict medical supervision. Take blood samples at regular intervals to measure glucose, ketone bodies, and free fatty acids. Acylcarnitine profile may be obtained at the same time. Fasting may be continued in children for up to 24 hours, unless blood glucose drops to less than 3 mmol/L. An inadequate production of ketones with a high free fatty acid–to–ketone bodies ratio suggests a defect in long-chain fatty acid oxidation.
Fatty acid oxidation study: This is used if a fatty acid oxidation defect is suspected. The most appropriate first line of investigation in these patients is to study the entire fatty acid oxidation pathway. Methods involve (1) monitoring the rate of production of radioactive end products of fatty acid oxidation disorders for radiolabeled precursor fatty acids or (2) measuring by tandem mass spectrometry the disease-specific acylcarnitines produced when stable isotope fatty acid precursors are incubated with cells in the presence of excess L-carnitine.[11]
Enzyme assay: This criterion standard for demonstrating an enzyme defect measures the activity in cultured fibroblasts or in some other tissue, such as muscle or liver. To account for the frequent finding of overlapping chain-length specificities, complex analysis using a mixture of different chain-length substrates and immunoprecipitation with antibodies to different enzymes is required.
Carnitine transport assay: Carnitine transport assay in cultured fibroblasts specifically demonstrates the absence of active carnitine transport in cultured fibroblasts. This finding is specific for primary carnitine deficiency.
Molecular diagnosis: Molecular diagnosis provides information on the gene for the carnitine transporter defective in primary carnitine deficiency, which has been cloned (SLC22A5) and can be screened for mutations. Most patients have private mutations. However, the R245X mutation has been found in Taiwanese, Saudi, and Lebanese kindreds.[12] The R245X and V295X mutations are associated with cardiomyopathy as the only clinical phenotype. Phenotypic variability has been observed among patients harboring the same mutations raising the possibility of modifier genes or epigenetic factors as responsible for these differences.
Mutation analysis: The genes for most of the enzymes of fatty acid oxidation that are defective in fatty acid oxidation disorders and may cause secondary carnitine deficiency have been identified, and mutation analysis is available for numerous genes (eg, CPT1, CPT2, VLCAD, TFP, MCAD). Prevalent mutations have been identified in patients with MCAD deficiency (A985G) and LCHAD deficiency (G1528C). In the adult form of CPT-II deficiency, a C439T mutation accounts for 60% of mutations in patients with adult onset.
Roentgenograms reveal cardiac enlargement in primary carnitine deficiency and fatty acid oxidation disorders, such as LCHAD or very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, which may cause secondary carnitine deficiency.
In primary carnitine deficiency (as well as in fatty acid oxidation disorders, which also may present with cardiomyopathy), the echocardiogram may reveal cardiac enlargement and increased thickness of the left ventricular wall.
Brain imaging studies (eg, cranial ultrasound, brain MRI) may show cystic lesions in glutaric aciduria type II or basal ganglia involvement in mitochondrial disorders that may be associated with secondary carnitine deficiency.
ECG: The ECG reveals left ventricular hypertrophy and peaked T waves in primary carnitine deficiency. Cardiac arrhythmias can be observed in translocase deficiency and in the lethal neonatal form of carnitine palmitoyltransferase II (CPT-II) deficiency.
Newborn screen
Primary carnitine deficiency can be identified in infants by expanded newborn screening using tandem mass spectrometry by detection of low levels of free carnitine (C0). In addition, newborns’ low carnitine levels may result from primary carnitine deficiency in their affected mothers.
Pediatrician needs to contact the family to inform them of the newborn screening result and ascertain clinical status and whether the newborn presents with poor feeding, lethargy, or tachypnea.
Consultation with a pediatric metabolic specialist has to be immediately activated and the newborn should be evaluated for tachycardia, hepatomegaly, or reduced muscle tone. After obtaining confirmatory testing with total and free plasma carnitine levels in the newborn and mother, carnitine supplementation with 100 mg/kg/d by mouth in 3 divided doses should be initiated by the metabolic specialist.
Confirmatory and diagnostic testing can be performed with carnitine uptake assay in cultured fibroblasts and SLC22A5 gene sequencing.
Clinical availability of SLC22A5 gene sequencing may preclude the need of a skin biopsy and carnitine uptake assay on cultured fibroblasts.
The family has to be educated about signs, symptoms and need for urgent treatment if infant becomes ill.
Skin biopsy can be performed to confirm diagnosis of primary carnitine deficiency by demonstrating reduced carnitine transport in fibroblasts that express the transporter. Fibroblasts may be used for fatty acid oxidation studies or enzyme assay. However, with the clinical availability of SLC22A5 sequencing, it may not be strictly necessary to perform a skin biopsy for the transport assay on cultured fibroblasts.
Muscle biopsy may be necessary to confirm the diagnosis of some conditions that may cause secondary carnitine deficiency (eg, respiratory chain defect) or to measure the carnitine concentration in muscle in cases of myopathic carnitine deficiency.
Biopsy of the liver may show microvesicular lipid steatosis that, along with the rest of the clinical picture, may lead to a diagnosis of Reye syndrome. If muscle biopsy is performed, very low fatty infiltration may be seen.
In infants with carnitine deficiency ascertained via newborn screen program, oral carnitine supplementation is followed by a slow increase of plasma carnitine levels. If the infants’ levels reflect maternal primary carnitine deficiency, the rise in plasma levels is fast and this should prompt the work-up towards the diagnosis of maternal primary carnitine deficiency. Guidelines for the management of carnitine deficiency and other fatty acid mitochondrial disorders have been established.[13]
Evaluation for carnitine deficiency may be performed on an outpatient basis. In cases of acute decompensation, inpatient studies may be necessary in the acute phase and following stabilization of the patient.
In acute situations, if the patient presents with hypoketotic hypoglycemic encephalopathy, insure stabilization with 10% dextrose in water at rates of 10 mg/kg/min intravenous (IV) initially; adjust infusion rate according to blood glucose concentrations.
IV carnitine restores tissue carnitine concentrations for the transport of fatty acids in the mitochondria. This treatment removes toxic metabolites in the form of carnitine esters that are readily excreted in the urine. The use of IV carnitine should be considered only when the diagnosis of primary carnitine deficiency is entertained or confirmed. The use of IV carnitine in disorders of fatty acid oxidation in which long-chain acylcarnitines accumulate and have the potential of being arrhythmogenic is controversial. IV carnitine may be considered in cases of organic acidemias (eg, isovaleric acidemia, propionic acidemia, methylmalonic acidemia) when oral intake is not feasible.
Consider pharmacological support for cardiomyopathy.
Medical therapy with oral carnitine in primary carnitine deficiency improves fasting ketogenesis, cardiac function, growth, and cognitive performance.
Direct the therapy in secondary carnitine deficiency to replenish carnitine and treat the primary metabolic defect with specific diet and other supplements, such as riboflavin, glycine, or biotin.
Actively avoid periods of fasting in these patients.
Patients with primary carnitine deficiency requires no special diet as long as they are taking carnitine supplementation and are not faced with situations of stress and starvation. Actively avoid periods of fasting in these patients.
Patients with fatty acid oxidation disorders require a high-carbohydrate fat-restricted diet (30% calories from fat) and must eat frequently.
Prescribe medium-chain triglyceride supplementation in patients with long-chain fatty acid disorders.
Advise use of uncooked cornstarch at bedtime to prevent early morning hypoglycemia after the overnight fast.
Supplementation of essential fatty acids (ie, linoleic acids, linolenic acids) prevents the growth restriction and dermatitis that are associated with fatty acid deficiency.
Consider specific protein-restricted diets in patients with aminoacidopathies and organic acidemias associated with secondary carnitine deficiency.
Once carnitine supplementation has been instituted for primary carnitine deficiency, cardiac function, strength, and growth improve significantly. No specific recommendations to limit physical activity are indicated if the cardiomyopathy has reverted.
Secondary carnitine deficiency caused by fatty acid oxidation disorders may require tempered or restricted activity in certain cases, including the following:
Conditions associated with increased risk for rhabdomyolysis and myoglobinuria (eg, carnitine palmitoyltransferase II [CPT-II] deficiency, very long-chain acyl-CoA dehydrogenase [VLCAD] deficiency)
Conditions in which a cardiomyopathy is present (eg, long-chain 3-hydroxyacyl-CoA dehydrogenase [LCHAD] deficiency, VLCAD deficiency)
Strenuous exercise or activity should be avoided, and frequent snacks and good hydration should be procured with physical activity.
Use of L-carnitine in primary carnitine deficiency restores plasma carnitine levels to nearly normal, but muscle carnitine levels rise slightly. Muscle function can be normalized in patients with carnitine deficiency when muscle carnitine levels remain less than 10% of controls. Cardiomyopathy often responds well to carnitine supplementation. Carnitine supplementation in fatty acid oxidation disorders and other organic acidurias is to correct carnitine deficiency and to allow removal of toxic intermediates. The other goal of therapy is to restore CoA levels. Carnitine therapy for long-chain fatty acid oxidation defects has become questionable because it promotes formation of long-chain acylcarnitines that may cause arrhythmogenesis and membrane dysfunction. Carnitine supplementation in total parenteral nutrition (TPN) prevents secondary carnitine deficiency in preterm newborns.
Clinical Context:
An amino acid derivative synthesized from methionine and lysine, required in energy metabolism. Can promote excretion of excess fatty acids in patients with defects in fatty acid metabolism or specific organic acidopathies, which bioaccumulate acyl CoA esters. Normal levels occur in liver, and mild level increases occur in skeletal muscle. May cause reversal of skeletal and heart muscle abnormalities.
Clinical Context:
Monosaccharide absorbed from intestines and distributed, stored, and used by tissues.
Parenterally injected dextrose is used in patients unable to sustain adequate PO intake. Direct PO absorption results in a rapid increase in blood glucose concentrations. Dextrose is effective in small doses. Concentrated dextrose infusions provide higher amounts of glucose and increased caloric intake in a small volume of fluid.
Clinical Context:
Essential in activation of pyridoxine and conversion of tryptophan to niacin; component of flavoprotein enzymes, which are necessary for tissue respiration. Riboflavin functions as a cofactor for electron transport in complex I, complex II, and in the electron transfer of flavoprotein. It has proven useful for the treatment of some patients with SCAD deficiency, riboflavin-responsive multiple acyl-CoA dehydrogenase deficiency, and milder forms of glutaric aciduria type II.
Clinical Context:
Methyl group donor used in the treatment of homocystinuria. Decreases elevated homocysteine blood levels. Used for conditions that can cause hyperhomocysteinemia and secondary carnitine deficiency (ie, cobalamin C deficiency).
Clinical Context:
Deoxyadenosylcobalamin and hydroxocobalamin are active forms of vitamin B-12 in humans. Vitamin B-12 synthesized by microbes but not humans or plants. Vitamin B-12 deficiency may result from intrinsic factor deficiency (pernicious anemia), partial or total gastrectomy, or diseases of the distal ileum. Used to treat conditions caused by altered cobalamin metabolism that may cause secondary carnitine deficiency (ie, cobalamin C deficiency).
Clinical Context:
Coenzyme involved in mitochondrial energy production. Controls flow of oxygen within individual cells. Has essential antioxidant and membrane-stabilizing properties.
Clinical Context:
The simplest amino acid that helps improve glycogen storage is used in the synthesis of hemoglobin, collagen, and glutathione, and it facilitates the amelioration of high blood fat and uric acid levels. Glycine is primarily used for the treatment of isovaleric acidemia, which is an organic acidemia that causes secondary carnitine depletion.
Clinical Context:
Water-soluble vitamin, generally classified as a B-complex vitamin. An essential coenzyme in fat metabolism and in other carboxylation reactions. Used for the treatment of biotin responsive propionic acidemia, which can lead to secondary carnitine deficiency.
At high doses, L-carnitine corrects severe carnitine depletion and associated metabolic abnormalities observed in primary carnitine deficiency and enables the production of ketone bodies during fasting. In secondary carnitine deficiency, carnitine enhances excretion of toxic metabolites and generation of free CoA.
Carefully monitor adequate carnitine dose in primary and secondary carnitine deficiencies by evaluating plasma carnitine levels during follow-up visits. Carefully review diet compliance in secondary carnitine deficiency, considering avoidance of fasting, intake of fat-restricted, high-carbohydrate diet, and other dietary supplements that may be needed, such as riboflavin or glycine. Treat infections aggressively.
Admit patients with carnitine deficiency for medical management of acute metabolic decompensation.
Prescribe 10% dextrose in water at rates of 10 mg/kg/min or higher to achieve normal glucose concentrations. If the rate of glucose infusion is based on blood glucose level alone, it may underestimate carbohydrate demand because tissues are depleted of glycogen stores.
Provide intravenous (IV) carnitine if the patient is known to have carnitine deficiency and a defect affecting the oxidation of long chain fatty acids has been excluded.
Medications include carnitine for primary and secondary carnitine deficiency, as well as other cofactors that may be needed for different conditions associated with secondary carnitine deficiency (eg, riboflavin, coenzyme Q, biotin, hydroxocobalamin, betaine, glycine). If a seizure disorder has developed secondary to a past episode of hypoglycemia, valproic acid should not be used as an anticonvulsant.
Patients may require transfer to a tertiary care center in which a more specialized metabolic workup for further diagnostic evaluation can be performed.
Prevent fasting with frequent feedings to avoid triggering episodes of hypoglycemia.
Treat infections aggressively to prevent a catabolic state.
Snacks and liquids should be consumed before exercise.
Avoid exercise and dehydration with warm temperatures because attacks of rhabdomyolysis may occur with certain conditions that cause secondary carnitine deficiency.
For fatty acid oxidation disorders, follow a fat-restricted diet with high carbohydrate content.
Patients with primary carnitine deficiency have excellent prognosis with oral carnitine supplementation.
If the disorder is unrecognized, mortality may occur from cardiac failure, arrhythmias, or sudden death.
Lifelong treatment with L-carnitine and avoidance of fasting are required. Hypoglycemia or sudden deaths from arrhythmias (even without cardiomyopathy) have been reported in patients who stop their carnitine supplementation against medical advice.
Secondary carnitine deficiency
Prognosis of secondary carnitine deficiency depends on the nature of the disorder.
Translocase deficiency and the infantile form of carnitine palmitoyltransferase II (CPT-II) deficiency have very poor prognosis regardless of treatment.[14]
In general, disorders of fatty acid oxidation require lifelong prevention of fasting and diet modification.
Other metabolic disorders that cause secondary carnitine deficiency, such as organic acidemias, require lifelong diet modification and nutritional supplements.
Family members should receive education once the work-up initiated after newborn screening results suggests primary carnitine deficiency in the newborn or in the mother.
Family members should receive cardiopulmonary resuscitation (CPR) training (cases of apnea or near-miss SIDS).
Family members should be taught to recognize early signs and symptoms of hypoglycemia and should be instructed to provide either glucose gel or glucagon injection while waiting for emergency aid.
Educate family members about frequent feedings and avoidance of fasting in general. If oral intake is decreased or poor, the child should be seen immediately at pediatrician's office or rushed to the emergency room.
Educate family members about the importance of continuing carnitine supplementation.
Educate family members about adhering to fat-restricted diet in fatty acid oxidation disorders or special protein-restricted diet in organic acidemias causing secondary carnitine deficiency.
Refer parents for genetic counseling and discussion of recurrence risk for future pregnancies.
Educate family members about the possibility of prenatal diagnosis. If the molecular defect has been established in the proband (primary carnitine deficiency or medium-chain acyl-CoA dehydrogenase [MCAD] deficiency), molecular analysis can be performed.
What is carnitine deficiency?What is the pathophysiology of carnitine deficiency?What is the prevalence of carnitine deficiency in the US?What is the global prevalence of carnitine deficiency?What is the mortality and morbidity associated with carnitine deficiency?What are the racial predilections of carnitine deficiency?What are the sexual predilections of carnitine deficiency?Which age groups have the highest prevalence of carnitine deficiency?Which clinical history findings are characteristic of primary carnitine deficiency?Which clinical history findings are characteristic of muscle carnitine deficiency?Which clinical history findings are characteristic of secondary carnitine deficiency?Which physical findings are characteristic of primary carnitine deficiency?Which physical findings are characteristic of muscle carnitine deficiency?Which physical findings are characteristic of secondary carnitine deficiency?What causes primary carnitine deficiency?What causes muscle carnitine deficiency?What causes secondary carnitine deficiency?What are the differential diagnoses for Carnitine Deficiency?What is the role of lab tests in the workup of carnitine deficiency?What is the role of genetic testing in the workup of carnitine deficiency?What is the role of imaging studies in the workup of carnitine deficiency?What is the role of ECG in carnitine deficiency?How are newborns screened for carnitine deficiency?What is the role of biopsy in the workup of carnitine deficiency?Which histologic findings are characteristic of carnitine deficiency?How is carnitine deficiency treated?Which specialist consultations are beneficial to patients with carnitine deficiency?Which dietary modifications are used in the treatment of carnitine deficiency?Which activity modifications are used in the treatment of carnitine deficiency?What is the role of medications in carnitine deficiency treatment?Which medications in the drug class Dietary supplements are used in the treatment of Carnitine Deficiency?What is included in long-term monitoring of carnitine deficiency?When is inpatient care indicated for carnitine deficiency?When is included in inpatient care for carnitine deficiency?Which medications are used in carnitine deficiency treatment?When is patient transfer required for carnitine deficiency treatment?How is carnitine deficiency treated?What are the possible complications of carnitine deficiency?What is the prognosis of primary carnitine deficiency?What is the prognosis of secondary carnitine deficiency?What is included in patient education about carnitine deficiency?
Fernando Scaglia, MD, FACMG, Professor of Genetics, Department of Molecular and Human Genetics, Baylor College of Medicine and Texas Children's Hospital
Disclosure: Nothing to disclose.
Specialty Editors
Mary L Windle, PharmD, Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference
Disclosure: Nothing to disclose.
Lois J Starr, MD, FAAP, Assistant Professor of Pediatrics, Clinical Geneticist, Munroe Meyer Institute for Genetics and Rehabilitation, University of Nebraska Medical Center
Disclosure: Nothing to disclose.
Chief Editor
Maria Descartes, MD, Professor, Department of Human Genetics and Department of Pediatrics, University of Alabama at Birmingham School of Medicine
Disclosure: Nothing to disclose.
Additional Contributors
Christian J Renner, MD, Consulting Staff, Department of Pediatrics, University Hospital for Children and Adolescents, Erlangen, Germany
Roe C, Coates P. Mitochondrial fatty acid oxidation disorders. Scriver CR, et al, eds. The Metabolic and Molecular Basic of Inherited Disease. 7th ed. New York, NY: McGraw-Hill, Health Professions Division;.; 1995. 1501-1533.
