Systemic Primary Carnitine Deficiency

Clinical characteristics.

Systemic primary carnitine deficiency (CDSP) is a disorder of the carnitine cycle that results in defective fatty acid oxidation. It encompasses a broad clinical spectrum including the following:

• Metabolic decompensation in infancy typically presenting between age three months and two years with episodes of hypoketotic hypoglycemia, poor feeding, irritability, lethargy, hepatomegaly, elevated liver transaminases, and hyperammonemia triggered by fasting or common illnesses such as upper respiratory tract infection or gastroenteritis
• Childhood myopathy involving heart and skeletal muscle with onset between age two and four years
• Pregnancy-related decreased stamina or exacerbation of cardiac arrhythmia
• Fatigability in adulthood
• Absence of symptoms

The latter two categories often include mothers diagnosed with CDSP after newborn screening has identified low carnitine levels in their infants.

Diagnosis/testing.

Plasma carnitine levels are extremely reduced in CDSP. The diagnosis is established by identification of biallelic pathogenic variants in SLC22A5 or demonstration of reduced fibroblast carnitine transport.

Management.

Treatment of manifestations: Metabolic decompensation and skeletal and cardiac muscle function improve with 100-400 mg/kg/day oral levocarnitine (L-carnitine) if it is started before irreversible organ damage occurs. Hypoglycemic episodes are treated with intravenous dextrose infusion; cardiomyopathy requires management by specialists in cardiology.

Prevention of primary manifestations: Maintain appropriate plasma carnitine concentrations with oral L-carnitine supplementation; prevent hypoglycemia with frequent feeding and avoiding fasting. Hospitalization for intravenous glucose administration for individuals who are required to fast for a procedure or who cannot tolerate oral intake due to illness such as gastroenteritis.

Prevention of secondary complications: Oral metronidazole and/or decreasing the carnitine dose usually results in the resolution of the fishy odor due to L-carnitine supplementation.

Surveillance: Echocardiogram and electrocardiogram: annually during childhood and less frequently in adulthood; monitor plasma carnitine concentration frequently until levels reach the normal range, then, measure three times a year during infancy and early childhood, twice a year in older children, and annually in adults; evaluate serum creatine kinase concentration and liver transaminases during acute illnesses.

Agents/circumstances to avoid: Fasting longer than age-appropriate periods.

Evaluation of relatives at risk: Measure plasma carnitine levels in sibs of an affected individual.

Pregnancy management: Pregnant women with CDSP require close monitoring of plasma carnitine levels and increased carnitine supplementation as needed to maintain normal plasma carnitine levels.

Genetic counseling.

CDSP is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal diagnosis for pregnancies at increased risk are possible if the SLC22A5 pathogenic variants in the family are known.

Suggestive Findings

Systemic primary carnitine deficiency (CDSP) should be suspected in the following clinical situations:

• Infant with positive newborn screening
• Infants with hypoketotic hypoglycemic episodes that may be associated with hepatomegaly, elevated transaminases, and hyperammonemia
• Children with skeletal myopathy and/or elevated serum concentration of creatine kinase (CK)
• Children with cardiomyopathy
• Adults with unexplained fatigability

Preliminary Testing

Newborn screening using tandem mass spectrometry (MS/MS) detects low levels of free carnitine (C0) and can identify infants with CDSP and mothers with CDSP. Because carnitine is transferred from the placenta to the fetus during pregnancy, an infant’s carnitine levels during the neonatal period can reflect those of the mother. Thus, unaffected infants born to affected mothers can have low carnitine levels shortly after birth [Schimmenti et al 2007, El-Hattab et al 2010, Lee et al 2010].

The ACMG recommends that total and free carnitine be determined in infants who screen positive. Extremely reduced plasma free, acylated, and total (i.e., the sum of free and acylated) carnitine levels (i.e., <10% of controls) are diagnostic of this disorder [Longo et al 2006].

In addition, plasma carnitine levels should be measured in all mothers of infants found to have low free carnitine levels on newborn screening in order to determine if the mother (rather than the infant) has CDSP, or if both mother and infant have CDSP [Schimmenti et al 2007, El-Hattab et al 2010, Lee et al 2010].

Other presentations. In infants, children, and adults with other presentations plasma carnitine levels remain the mainstay of the initial laboratory diagnosis.

Of note, other biochemical studies may also have been done to address a broader differential diagnosis:

• Plasma acylcarnitine profile. Because of the low level of all acylcarnitine, this study may well be unsuccessful. If a profile can be generated, there is generally no specific elevation of any acylcarnitine species.
• Urine organic acid analysis. Nonspecific dicarboxylic aciduria, common in the acute decompensation of many fatty acid oxidation disorders, has been reported in some affected individuals with CDSP as well.

Establishing the Diagnosis

The diagnosis of CDSP is established in a proband by identification of biallelic pathogenic variants in SLC22A5 on molecular genetic testing (see Table 1) or, if biallelic pathogenic variants cannot be identified, by use of a skin biopsy to assess carnitine transport in cultured fibroblasts.

Molecular genetic testing approaches can include single-gene testing, use of a multigene panel, and more comprehensive genomic testing:

• Single-gene testing. Sequence analysis of SLC22A5 is performed first and followed by gene-targeted deletion/duplication analysis if only one or no pathogenic variant is found.
• A multigene panel that includes SLC22A5 and other genes of interest (see Differential Diagnosis) may also be considered. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview; thus, clinicians need to determine which multigene panel is most likely to identify the genetic cause of the condition at the most reasonable cost while limiting identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype. (3) In some laboratories, panel options may include a custom laboratory-designed panel and/or custom phenotype-focused exome analysis that includes genes specified by the clinician. (4) Methods used in a panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
  For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.
• More comprehensive genomic testing (when available) including exome sequencing, mitochondrial sequencing, and genome sequencing may be considered if serial single-gene testing (and/or use of a multigene panel that includes SLC22A5) fails to confirm a diagnosis in an individual with features of CDSP. Such testing may provide or suggest a diagnosis not previously considered (e.g., mutation of a different gene or genes that results in a similar clinical presentation).
  For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Fibroblast carnitine transport (uptake). Carnitine transport in skin fibroblasts from affected individuals is typically reduced below 10% of control rates [Longo et al 2006].

Clinical Description

The clinical manifestations of systemic primary carnitine deficiency (CDSP) can vary widely with respect to age of onset, organ involvement, and severity of symptoms. The CDSP phenotype encompasses a broad clinical spectrum including metabolic decompensation in infancy, cardiomyopathy in childhood, fatigability in adulthood, or absence of symptoms. CDSP has typically been associated with infantile metabolic presentation in about half of affected individuals and childhood myopathic presentation in the other half. However, adults with CDSP who have mild or no symptoms have been reported. Such milder phenotypes are expected to be underdiagnosed; therefore, it is difficult to determine the relative prevalence of different phenotypes associated with CDSP [Longo et al 2006, El-Hattab & Scaglia 2015].

Infantile metabolic (hepatic) presentation. Affected children can present between age three months and two years with episodes of metabolic decompensation triggered by fasting or common illnesses such as upper-respiratory tract infection or gastroenteritis. These episodes are characterized clinically by poor feeding, irritability, lethargy, and hepatomegaly. Laboratory evaluations usually reveal hypoketotic hypoglycemia (hypoglycemia with minimal or no ketones in urine), hyperammonemia, and elevated liver transaminases. If affected children are not treated with intravenous dextrose infusion during episodes of metabolic decompensation (see Management), they may develop coma and die [Longo et al 2006, El-Hattab & Scaglia 2015].

Childhood myopathic (cardiac) presentation. The average age of myopathic presentation is between age two and four years, indicating that the myopathic manifestations of CDSP may develop over a longer period of time. Myopathic manifestations include dilated cardiomyopathy, hypotonia, skeletal muscle weakness, and elevated serum creatine kinase (CK). Death from cardiac failure can occur before the diagnosis is established, indicating that this presentation can be fatal if not treated. Older children with the infantile presentation may also develop myopathic manifestations including elevated CK, cardiomyopathy, and skeletal muscle weakness [Longo et al 2006, El-Hattab & Scaglia 2015].

Adulthood presentation. Several women have been diagnosed with CDSP after newborn screening identified low carnitine levels in their infants. About half of those women complained of fatigability, whereas the other half were asymptomatic. One woman was found to have dilated cardiomyopathy and another had arrhythmias [Schimmenti et al 2007, El-Hattab et al 2010, Lee et al 2010]. An asymptomatic adult male with CDSP has also been reported [Spiekerkoetter et al 2003].

Pregnancy-related symptoms. Pregnancy is a metabolically challenging state because energy consumption significantly increases. In addition, during pregnancy plasma carnitine levels are physiologically lower than those of non-pregnant controls. Affected women can have decreased stamina or worsening of cardiac arrhythmia during pregnancy, suggesting that CDSP may manifest or exacerbate during pregnancy [Schimmenti et al 2007, El-Hattab et al 2010].

Atypical manifestations. Other manifestations reported in individuals with CDSP include the following:

• Anemia [Cano et al 2008]
• Proximal muscle weakness and global developmental delays [Wang et al 2001]
• Respiratory distress [Erguven et al 2007]
• Arrhythmias and electrocardiographic (ECG) abnormalities [Schimmenti et al 2007, Lee et al 2010], including long QT syndrome [De Biase et al 2012]

Heterozygous carriers. Heterozygous carriers are asymptomatic [Amat di San Filippo et al 2008].

Prognosis. Infantile metabolic and childhood myopathic presentations of CDSP can be fatal if untreated (see Management). The long-term prognosis is favorable as long as affected individuals remain on carnitine supplements. Repeated attacks of hypoglycemia or sudden death from arrhythmia have been described in affected individuals discontinuing carnitine supplementation [Longo et al 2006].

Genotype-Phenotype Correlations

Fibroblast carnitine transport is reduced in all affected individuals. However, it has been demonstrated that carnitine transport is higher in the fibroblasts of asymptomatic individuals than in the fibroblasts of symptomatic individuals. Nonsense and frameshift variants in SLC22A5 are typically associated with lower carnitine transport and are more prevalent in symptomatic individuals whereas missense variants and in-frame deletions may result in protein with retained residual carnitine transport activity and are more prevalent in asymptomatic individuals [Rose et al 2012].

Prevalence

CDSP has a frequency of 1:20,000-1:70,000 in the United States [Magoulas & El-Hattab 2012], 1:40,000 in Japan [Koizumi et al 1999], and 1:120,000 in Australia [Wilcken et al 2003]. The disease is very common in the Faroe Islands, where the prevalence is 1:300 [Rasmussen et al 2014].

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs of an individual diagnosed with systemic primary carnitine deficiency (CDSP), the following evaluations are recommended:

• Echocardiogram and electrocardiogram
• Serum creatine kinase (CK) concentration
• Liver transaminases
• Pre-prandial blood glucose concentration
• Consultation with a clinical geneticist and/or genetic counselor

Treatment of Manifestations

L-carnitine supplementation. The main treatment for CDSP is oral levocarnitine (L-carnitine) supplementation. Typically, a high dose (100-400 mg/kg/day, divided in 3 doses) is required. Individuals with CDSP respond well if oral L-carnitine supplementation is started before irreversible organ damage occurs. Metabolic decompensation and skeletal and cardiac muscle function improve with L-carnitine supplementations.

Oral L-carnitine supplementation in infants with CDSP identified through newborn screening results in slow normalization of the plasma carnitine concentration. The carnitine dose needs to be adjusted according to the plasma carnitine concentrations, which should be measured frequently.

L-carnitine supplementation has relatively few side effects:

• High doses of oral L-carnitine can cause increased gastrointestinal motility, diarrhea, and intestinal discomfort.
• Oral L-carnitine can be metabolized by intestinal bacteria to produce trimethylamine, which has a fishy odor. Oral metronidazole at a dose of 10 mg/kg/day for 7-10 days and/or decreasing the carnitine dose usually results in the resolution of the odor [Longo et al 2006].

Note: (1) An unaffected infant born to a mother with CDSP can have low carnitine levels detected on newborn screening; in these infants oral L-carnitine supplementation is followed by a rise in plasma carnitine concentration within days or a few weeks [Schimmenti et al 2007, El-Hattab et al 2010]. (2) Asymptomatic adults with CDSP have been reported; however, the limited literature and the lack of follow up make it unclear whether these individuals have potential health risks. Because some fatty acid oxidation defects such as medium chain acyl CoA dehydrogenase (MCAD) deficiency can remain asymptomatic until they results in sudden death or another acute presentation during stress, it is prudent to treat asymptomatic individuals with CDSP with L-carnitine supplementation to prevent the possibility of decompensation during intercurrent illness or stress [El-Hattab et al 2010].

Other

• Hypoglycemic episodes are treated with intravenous dextrose infusion.
• Cardiomyopathy requires management by specialists in cardiology.

Prevention of Primary Manifestations

Maintaining appropriate plasma carnitine concentrations through oral L-carnitine supplementation (see Treatment of Manifestations) and preventing hypoglycemia (with frequent feeding and avoiding fasting) typically eliminate the risk of metabolic, hepatic, cardiac, and muscular complications.

Note: Hospitalization to administer intravenous glucose is recommended for individuals with CDSP who are required to fast because of medical or surgical procedures or who cannot tolerate oral intake because of an illness such as gastroenteritis.

Prevention of Secondary Complications

L-carnitine supplementation is well tolerated and has relatively few side effects: increased gastrointestinal motility, diarrhea, and a fishy odor. Oral metronidazole and/or decreasing the carnitine dose usually results in the resolution of the odor.

Surveillance

The following evaluations have been suggested [Magoulas & El-Hattab 2012]:

• Echocardiogram and electrocardiogram. Perform annually during childhood and less frequently in adulthood. Individuals with cardiomyopathy require management and follow up by specialists in cardiology.
• Plasma carnitine concentration. Monitor frequently until levels reach the normal range, thereafter, measure three times a year during infancy and early childhood, twice a year in older children, and annually in adults.
• Serum CK concentration and liver transaminases. Consider measuring during acute illnesses.

Agents/Circumstances to Avoid

Individuals with CDSP should avoid fasting longer than age-appropriate periods.

Evaluation of Relatives at Risk

Sibs of affected individuals should be tested by measuring plasma carnitine concentrations. If the carnitine levels are low, further evaluation for CDSP is recommended by molecular genetic testing if the SLC22A5 pathogenic variants have been identified in the family or fibroblast carnitine transport assay.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Pregnancy Management

Pregnancy is a metabolically challenging state because energy consumption significantly increases. In addition, plasma carnitine levels are physiologically lower during pregnancy than those of non-pregnant controls. Affected women can have decreased stamina or worsening of cardiac arrhythmia during pregnancy, suggesting that CDSP may manifest or exacerbate during pregnancy [Schimmenti et al 2007, El-Hattab et al 2010]. Therefore, all pregnant women with CDSP, including those who are asymptomatic, require close monitoring of plasma carnitine levels and increased carnitine supplementation as needed to maintain normal plasma carnitine levels.

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.