Infants with this disorder appear normal at birth but usually develop signs and symptoms during the first year of life or in early childhood. The characteristic features of this condition, which can range from mild to life-threatening, include feeding difficulties, recurrent episodes of vomiting and diarrhea, excessive tiredness (lethargy), and weak muscle tone (hypotonia). If untreated, this disorder can lead to delayed development, seizures, and coma. Early detection and lifelong management (following a low-protein diet and using appropriate supplements) may prevent many of these complications. In some cases, people with gene mutations that cause 3-methylcrotonyl-CoA carboxylase deficiency never experience any signs or symptoms of the disorder.

The characteristic features of this condition are similar to those of Reye syndrome, a severe disorder that develops in children while they appear to be recovering from viral infections such as chicken pox or flu. Most cases of Reye syndrome are associated with the use of aspirin during these viral infections.

The symptoms are visible within the first week of life and if not detected and diagnosed correctly immediately consequences are fatal.

Signs and symptoms of CTLN1 in infants are caused by increasing levels of ammonia in the blood and cerebrospinal fluid and include excessive vomiting, anorexia, refusal to eat, irritability, increased intracranial pressure, and worsening lethargy, seizures, hypotonia, respiratory distress, hepatomegaly, and cerebral edema. These symptoms appear within days of birth in the more severe forms of the disease with complete deficiency of the enzyme. As ammonia accumulates further, the affected infant may enter a hyperammonemic coma, which indicates neurological damage and can cause developmental delays, cognitive disabilities, cerebral palsy, hypertonia, spasticity, ankle clonus, seizures, and liver failure.

Milder forms of the disease are caused by partial arginosuccinate synthetase deficiency and may manifest in childhood or in adulthood. Symptoms of mild CTLN1 include failure to thrive, avoidance of high-protein foods, ataxia, worsening lethargy, and vomiting. Hyperammonemic coma can still develop in these people. CTLN1 can also develop in the perinatal period.

Hartnup disease manifests during infancy with variable clinical presentation: failure to thrive, photosensitivity, intermittent ataxia, nystagmus, and tremor.

Nicotinamide is necessary for neutral amino acid transporter production in the proximal renal tubules found in the kidney, and intestinal mucosal cells found in the small intestine. Therefore, a symptom stemming from this disorder results in increased amounts of amino acids in the urine.

Pellagra, a similar condition, is also caused by low nicotinamide; this disorder results in dermatitis, diarrhea, and dementia.

Hartnup disease is a disorder of amino acid transport in the intestine and kidneys; otherwise, the intestine and kidneys function normally, and the effects of the disease occur mainly in the brain and skin. Symptoms may begin in infancy or early childhood, but sometimes they begin as late as early adulthood. Symptoms may be triggered by sunlight, fever, drugs, or emotional or physical stress. A period of poor nutrition nearly always precedes an attack. The attacks usually become progressively less frequent with age. Most symptoms occur sporadically and are caused by a deficiency of niacinamide. A rash develops on parts of the body exposed to the sun. Mental retardation, short stature, headaches, unsteady gait, and collapsing or fainting are common. Psychiatric problems (such as anxiety, rapid mood changes, delusions, and hallucinations) may also result.

Multiple carboxylase deficiency is a form of metabolic disorder involving failures of carboxylation enzymes.

The deficiency can be in biotinidase or holocarboxylase synthetase.

These conditions respond to biotin.

Forms include:

- Holocarboxylase synthetase deficiency - neonatal;

- Biotinidase deficiency - late onset;

If left untreated, the symptoms can include feeding problems, decreased body tone, generalized red rash with skin exfoliation and baldness, failure to thrive, seizure, coma, developmental delay, foul smelling urine, lactic acidosis, and high levels of ketones and ammonia in the blood.

PDCD is generally presented in one of two forms. The metabolic form appears as lactic acidosis. The neurological form of PDCD contributes to hypotonia, poor feeding, lethargy and structural abnormalities in the brain. Patients may develop seizures and/or neuropathological spasms. These presentations of the disease usually progress to mental retardation, microcephaly, blindness and spasticity.

Females with residual pyruvate dehydrogenase activity will have no uncontrollable systemic lactic acidosis and few, if any, neurological symptoms. Conversely, females with little to no enzyme activity will have major structural brain abnormalities and atrophy. Males with mutations that abolish, or almost abolish, enzyme activity presumably die in utero because brain cells are not able to generate enough ATP to be functionally viable. It is expected that most cases will be of mild severity and have a clinical presentation involving lactic acidosis.

Prenatal onset may present with non-specific signs such as low Apgar scores and small for gestational age. Metabolic disturbances may also be considered with poor feeding and lethargy out of proportion to a mild viral illness, and especially after bacterial infection has been ruled out. PDH activity may be enhanced by exercise, phenylbutyrate and dichloroacetate.

The clinical presentation of congenital PDH deficiency is typically characterized by heterogenous neurological features that usually appear within the first year of life. In addition, patients usually show severe hyperventillation due to profound metabolic acidosis mostly related to lactic acidosis. Metabolic acidosis in these patients is usually refractory to correction with bicarbonate.

The signs and symptoms of holocarboxylase synthetase deficiency typically appear within the first few months of life, but the age of onset varies. Affected infants often have immunodeficiency diseases, difficulty feeding, breathing problems, a skin rash, hair loss (alopecia), and a lack of energy (lethargy). Immediate treatment and lifelong management (using biotin supplements) may prevent many of these complications. If left untreated, the disorder can lead to delayed development, seizures, and coma. These medical problems may be life-threatening in some cases.

Citrullinemia type I (CTLN1), also known as arginosuccinate synthetase deficiency, is a rare disease caused by a deficiency in argininosuccinate synthetase, an enzyme involved in excreting excess nitrogen from the body. There are mild and severe forms of the disease, which is one of the urea cycle disorders.

Carbamoyl phosphate synthetase I deficiency often becomes evident in the first few days of life. An infant with this condition may be lacking in energy (lethargic) or unwilling to eat, and have a poorly controlled breathing rate or body temperature. Some babies with this disorder may experience seizures or unusual body movements, or go into a coma. Complications of carbamoyl phosphate synthetase I deficiency may include developmental delay and mental retardation.

In some affected individuals, signs and symptoms of carbamoyl phosphate synthetase I deficiency may be less severe, and may not appear until later in life.

3-Methylcrotonyl-CoA carboxylase deficiency (3MCC deficiency), also known as 3-Methylcrotonylglycinuria or BMCC deficiency is an inherited disorder in which the body is unable to process certain proteins properly. People with this disorder have inadequate levels of an enzyme that helps break down proteins containing the amino acid leucine. This condition affects an estimated 1 in 50,000 individuals worldwide.

N-Acetylglutamate synthase (or synthetase) deficiency is an autosomal recessive urea cycle disorder.

Glutathione synthetase deficiency can be classified into three types: mild, moderate and severe.

- "Mild" glutathione synthetase deficiency usually results in the destruction of red blood cells (hemolytic anemia). Rarely, affected people also excrete large amounts of a compound called 5-oxoproline (also called pyroglutamic acid, or pyroglutamate) in their urine (5-oxoprolinuria). This compound builds up when glutathione is not processed correctly in cells.

- Individuals with "moderate" glutathione synthetase deficiency may experience symptoms beginning shortly after birth including hemolytic anemia, 5-oxoprolinuria, and elevated acidity in the blood and tissues (metabolic acidosis).

- In addition to the features present in moderate glutathione synthetase deficiency, individuals affected by the "severe" form of this disorder may experience neurological symptoms. These problems may include seizures; a generalized slowing down of physical reactions, movements, and speech (psychomotor retardation); intellectual disability; and a loss of coordination (ataxia). Some people with severe glutathione synthetase deficiency also develop recurrent bacterial infections.[citation missing]

Holocarboxylase synthetase deficiency is an inherited metabolic disorder in which the body is unable to use the vitamin biotin effectively. This disorder is classified as a multiple carboxylase deficiency, a group of disorders characterized by impaired activity of certain enzymes that depend on biotin. Symptoms are very similar to biotinidase deficiency and treatment – large doses of biotin – is also the same.

Males show more serious symptoms than females affected by this disorder.

The symptoms for males are:

1. Profound sensorineural hearing loss i.e, a complete or almost complete loss of hearing caused by abnormalities in the inner ear.

2. Weak muscle tone - Hypotonia.

3. Impaired muscle coordination - Ataxia.

4. Developmental delay.

5. Intellecual disability.

6. Vision loss caused by optic nerve atrophy in early childhood.

7. Peripheral neuropathy.

8. Recurrent infections, especially in the respiratory system.

9. Muscle weakness caused by recurrent infections.

Symptoms for females:

Very rarely seen hearing loss that begins in adulthood (age > 20 years) combined with ataxia and neuropathy. Optic atrophy and retinitis pigmentosa observed in some cases too.

Infants with LPI are usually symptom-free when breastfed because of the low protein concentration in human milk, but develop vomiting and diarrhea after weaning. The patients show failure to thrive, poor appetite, growth retardation, enlarged liver and spleen, prominent osteoporosis and osteopenia, delayed bone age and spontaneous protein aversion. Forced feeding of protein may lead to convulsions and coma. Mental development is normal if prolonged episode of hyperammonemia can be avoided. Some patients develop severe pulmonary and renal complications. High levels of plasma glutamine and glycine are observed.

Hyperprolinemia type II results in proline levels in the blood between 10 and 15 times higher than normal, and high levels of a related compound called pyrroline-5-carboxylate. This rare form of the disorder may appear benign at times, but often involves seizures, convulsions, and intellectual disability.

Hyperprolinemia can also occur with other conditions, such as malnutrition or liver disease. In particular, individuals with conditions that cause elevated levels of lactic acid in the blood, such as lactic acidemia, are likely to have elevated proline levels, because lactic acid inhibits the breakdown of proline.

It is difficult to determine the prevalence of hyperprolinemia type I, as many people with the condition are asymptomatic.

People with hyperprolinemia type I have proline levels in their blood between 3 and 10 times the normal level. Some individuals with type I exhibit seizures, intellectual disability, or other neurological problems.

Pyruvate dehydrogenase deficiency (also known as pyruvate dehydrogenase complex deficiency or PDCD) is one of the most common neurodegenerative disorders associated with abnormal mitochondrial metabolism. PDCD is an X-linked disease that shows heterogeneous characteristics in both clinical presentation and biochemical abnormality. The pyruvate dehydrogenase complex (PDC) is a multi-enzyme complex that plays a vital role as a key regulatory step in the central pathways of energy metabolism in the mitochondria.

Glutathione synthetase deficiency is a rare autosomal recessive metabolic disorder that prevents the production of glutathione. Glutathione helps prevent damage to cells by neutralizing harmful molecules generated during energy production. Glutathione also plays a role in processing medications and cancer-causing compounds (carcinogens), and building DNA, proteins, and other important cellular components.

Lysinuric protein intolerance (LPI), also called hyperdibasic aminoaciduria type 2,cationic aminoaciduria or familial protein intolerance, is an autosomal recessive metabolic disorder affecting amino acid transport.

About 140 patients have been reported, almost half of them of Finnish origin. Individuals from Japan, Italy, Morocco and North Africa have also been reported.

Arts syndrome is a rare metabolic disorder that causes serious neurological problems in males due to a malfunction of the PRPP synthetase 1 enzyme. Arts Syndrome is part of a spectrum of PRPS-1 related disorders with reduced activity of the enzyme that includes Charcot–Marie–Tooth disease and X-linked non-syndromic sensorineural deafness.

Symptoms may differ greatly, as apparently modifiers control to some degree the amount of FX that is produced. Some affected individuals have few or no symptoms while others may experience life-threatening bleeding. Typically this bleeding disorder manifests itself as a tendency to easy bruising, nose bleeding, heavy and prolonged menstruation and bleeding during pregnancy and childbirth, and excessive bleeding after dental or surgical interventions. Newborns may bleed in the head, from the umbilicus, or excessively after circumcision. Other bleeding can be encountered in muscles or joints, brain, gut, or urine

While in congenital disease symptoms may be present at birth or show up later, in patients with acquired FX deficiency symptoms typically show up in later life.

Hartnup disease (also known as "pellagra-like dermatosis" and "Hartnup disorder") is an autosomal recessive metabolic disorder affecting the absorption of nonpolar amino acids (particularly tryptophan that can be, in turn, converted into serotonin, melatonin, and niacin). Niacin is a precursor to nicotinamide, a necessary component of NAD+.

The causative gene, "SLC6A19", is located on chromosome 5.

Carbamoyl phosphate synthetase I deficiency (CPS I deficiency) is an autosomal recessive metabolic disorder that causes ammonia to accumulate in the blood due to a lack of the enzyme carbamoyl phosphate synthetase I. Ammonia, which is formed when proteins are broken down in the body, is toxic if the levels become too high. The nervous system is especially sensitive to the effects of excess ammonia.

The most common clinical history in patients with glycogen-storage disease type 0 (GSD-0) is that of an infant or child with symptomatic hypoglycemia or seizures that occur before breakfast or after an inadvertent fast. In affected infants, this event typically begins after they outgrow their nighttime feeds. In children, this event may occur during acute GI illness or periods of poor enteral intake.

Mild hypoglycemic episodes may be clinically unrecognized, or they may cause symptoms such as drowsiness, sweating, lack of attention, or pallor. Uncoordinated eye movements, disorientation, seizures, and coma may accompany severe episodes.

Glycogen-storage disease type 0 affects only the liver. Growth delay may be evident with height and weight percentiles below average. Abdominal examination findings may be normal or reveal only mild hepatomegaly.Signs of acute hypoglycemia may be present, including the following: