This disorder causes neurological problems, including mental retardation, brain atrophy and ventricular dilation, myoclonus, hypotonia, and epilepsy.

It is also associated with growth retardation, megaloblastic anemia, pectus excavatum, scoliosis, vomiting, diarrhea, and hepatosplenomegaly.

Among the signs and symptoms of adenylosuccinate lyase deficiency are the following:

- Aggressive behavior

- Microcephaly

- Autism

- Brachycephaly

- Mild Cerebellar hypoplasia

- Seizures

Generally, the majority of individuals with creatine transporter defect express the following symptoms with varying levels of severity: developmental delay and regression, mental retardation, and abnormalities in expressive and cognitive speech. However, several studies have shown a wider variety of symptoms including, but not limited to attention deficit and hyperactivity with impulsivity, myopathy, hypotonia, semantic-pragmatic language disorder, oral dyspraxia, extrapyramidal movement disorder, constipation, absent speech development, seizures, and epilepsy. Furthermore, symptoms can significantly vary between hemizygous males and heterozygous females, although, symptoms are generally more severe in hemizygous males. Hemizygous males more commonly express seizures, growth deficiency, severe mental retardation, and severe expressive language impairment. Heterozygous females more commonly express mild retardation, impairments to confrontational naming and verbal memory, and learning and behavior problems.

D-Bifunctional protein deficiency (officially called 17β-hydroxysteroid dehydrogenase IV deficiency) is an autosomal recessive peroxisomal fatty acid oxidation disorder. Peroxisomal disorders are usually caused by a combination of peroxisomal assembly defects or by deficiencies of specific peroxisomal enzymes. The peroxisome is an organelle in the cell similar to the lysosome that functions to detoxify the cell. Peroxisomes contain many different enzymes, such as catalase, and their main function is to neutralize free radicals and detoxify drugs, such as alcohol. For this reason peroxisomes are ubiquitous in the liver and kidney. D-BP deficiency is the most severe peroxisomal disorder, often resembling Zellweger syndrome.

Characteristics of the disorder include neonatal hypotonia and seizures, occurring mostly within the first month of life, as well as visual and hearing impairment. Other symptoms include severe craniofacial disfiguration, psychomotor delay, and neuronal migration defects. Most onsets of the disorder begin in the gestational weeks of development and most affected individuals die within the first two years of life.

This defect leads to a multi-systemic disorder of the connective tissue, muscles, central nervous system (CNS), and cardiovascular system. Homocystinuria represents a group of hereditary metabolic disorders characterized by an accumulation of the amino acid homocysteine in the serum and an increased excretion of homocysteine in the urine. Infants appear to be normal and early symptoms, if any are present, are vague.

Signs and symptoms of homocystinuria that may be seen include the following:

People with methylmalonyl CoA mutase deficiency exhibit many symptoms similar to other diseases involving inborn errors of metabolism. Sometimes the symptoms appear shortly after birth, but other times the onset of symptoms is later.

Newborn babies experience with vomiting, acidosis, hyperammonemia, hepatomegaly (enlarged livers), hyperglycinemia (high glycine levels), and hypoglycemia (low blood sugar). Later, cases of thrombocytopenia and neutropenia can occur.

In some cases intellectual and developmental disabilities, such as autism, were noted with increased frequency in populations with methylmalonyl-CoA mutase deficiency.

Depending on the affected gene(s), this disorder may present symptoms that range from mild to life-threatening.

- Stroke

- Progressive encephalopathy

- Seizure

- Kidney failure

- Vomiting

- Dehydration

- Failure to thrive and developmental delays

- Lethargy

- Repeated Yeast infections

- Acidosis

- Hepatomegaly

- Hypotonia

- Pancreatitis

- Respiratory distress

People with hypermethioninemia often do not show any symptoms. Some individuals with hypermethioninemia exhibit learning disabilities, mental retardation, and other neurological problems; delays in motor skills such as standing or walking; sluggishness; muscle weakness; liver problems; unusual facial features; and their breath, sweat, or urine may have a smell resembling boiled cabbage.

Hypermethioninemia can occur with other metabolic disorders, such as homocystinuria, tyrosinemia and galactosemia, which also involve the faulty breakdown of particular molecules. It can also result from liver disease or excessive dietary intake of methionine from consuming large amounts of protein or a methionine-enriched infant formula.

Propionic acidemia is characterized almost immediately in newborns. Symptoms include poor feeding, vomiting, dehydration, acidosis, low muscle tone (hypotonia), seizures, and lethargy. The effects of propionic acidemia quickly become life-threatening.

Classical homocystinuria, also known as cystathionine beta synthase deficiency or CBS deficiency, is an inherited disorder of the metabolism of the amino acid methionine, often involving cystathionine beta synthase. It is an inherited autosomal recessive trait, which means a child needs to inherit a copy of the defective gene from both parents to be affected.

Adenylosuccinate lyase deficiency, also called adenylosuccinase deficiency, is a rare autosomal recessive metabolic disorder characterized by the appearance of succinylaminoimidazolecarboxamide riboside (SAICA riboside) and succinyladenosine (S-Ado) in cerebrospinal fluid, urine.These two succinylpurines are the dephosphorylated derivatives of SAICA ribotide (SAICAR) and adenylosuccinate (S-AMP), the two substrates of adenylosuccinate lyase (ADSL), which catalyzes an important reaction in the de novo pathway of purine biosynthesis. ADSL catalyzes two distinct reactions in the synthesis of purine nucleotides, both of which involve the β-elimination of fumarate to produce aminoimidazole carboxamide ribotide (AICAR) from SAICAR or adenosine monophosphate (AMP) from S-AMP.

Methylmalonic acidemia (MMA), also called methylmalonic aciduria, is an autosomal recessive metabolic disorder. It is a classical type of organic acidemia. The result of this condition is the inability to properly digest specific fats and proteins, which in turn leads to a buildup of a toxic level of methylmalonic acid in the blood.

Methylmalonic acidemia stems from several genotypes, all forms of the disorder usually diagnosed in the early neonatal period, presenting progressive encephalopathy, and secondary hyperammonemia. The disorder can result in death if undiagnosed or left untreated. It is estimated that this disorder has a frequency of 1 in 48,000 births, though the high mortality rate in diagnosed cases make exact determination difficult. Methylmalonic acidemias are found with an equal frequency across ethnic boundaries.

Creatine transporter defect (CTD) is an inborn error of creatine metabolism in which creatine is not properly transported to the brain and muscles due to defective creatine transporters. CTD is an X-linked disorder caused by mutations in the SLC6A8 gene. The SLC6A8 gene is located on the short arm of the sex chromosome, Xq28. Hemizygous males with CTD express speech and behavior abnormalities, intellectual disabilities, development delay, seizures, and autistic behavior. Heterozygous females with CTD generally express fewer, less severe symptoms. CTD is one of three different types of cerebral creatine deficiency (CCD). The other two types of CCD are guanidinoacetate methyltransferase (GAMT) deficiency and deficiency. Clinical presentation of CTD is similar to that of GAMT and AGAT deficiency. CTD was first identified in 2001 with the presence of a hemizygous nonsense mutation in the SLC6A8 gene in a male patient.

Hypermethioninemia is an excess of the amino acid methionine, in the blood. This condition can occur when methionine is not broken down properly in the body.

Defined as those seen in any macrocytic, megaloblastic anemia:

- Anemia: causing fatigue, conjuctival pallor, pale complexion, and in some cases, a mild icterus (yellowing of the eye).

- Glossitis ("shiny tongue"): shiny, glossy tongue.

- Cheilosis (stomatitis): Inflammation of the edges of the lips and the oral mucosa.

- Tabes dorsalis ("subacute combined degeneration of the spinal cord"): This involves the posterior section of the spinal cord and therefore involves proprioception (sense of position), touch, sense of vibration and in severe cases the lateral corticospinal tract, causing spastic paralysis of the limbs.

- Peripheral neuropathy: tingling sensation in the arms and legs.

- Pancytopenia: decreased number of blood cells of all lineages (RBCs, leucocytes, platelets), due to decreased bone marrow production.

- Methylmalonyl CoA-emia: defined as blood having an unusually high concentration of methylmalonyl CoA.

- Peripheral findings such as hypersegmented neutrophils and large RBCs on high field view of the blood smears.

- Laboratory findings indicating increased MCV (Mean Corpuscular Volume), decreased Hgb/Hct (indicating anemia), and decreased value of vitamin B in the blood.

- Proteinuria: protein found in the urine detected by analysis or by dipstick.

- Reversal of all symptoms except neurological symptoms, by IV injection of vitamin B.

- Schilling test indicating no radioactive vitamin B in the urine. (This test has dropped out of favor and should not be tried in patients with any form of renal failure).

Another common symptom of copper deficiency is peripheral neuropathy, which is numbness or tingling that can start in the extremities and can sometimes progress radially inward towards the torso. In an Advances in Clinical Neuroscience & Rehabilitation (ACNR) published case report, a 69-year-old patient had progressively worsened neurological symptoms. These symptoms included diminished upper limb reflexes with abnormal lower limb reflexes, sensation to light touch and pin prick was diminished above the waist, vibration sensation was lost in the sternum, and markedly reduced proprioception or sensation about the self’s orientation. Many people suffering from the neurological effects of copper deficiency complain about very similar or identical symptoms as the patient. This numbness and tingling poses danger for the elderly because it increases their risk of falling and injuring themselves. Peripheral neuropathy can become very disabling leaving some patients dependent on wheel chairs or walking canes for mobility if there is lack of correct diagnosis. Rarely can copper deficiency cause major disabling symptoms. The deficiency will have to be present for an extensive amount of time until such disabling conditions manifest.

Arakawa's syndrome II is an autosomal dominant metabolic disorder that causes a deficiency of the enzyme tetrahydrofolate-methyltransferase; affected individuals cannot properly metabolize methylcobalamin, a type of Vitamin B.

It is also called Methionine synthase deficiency, Tetrahydrofolate-methyltransferase deficiency syndrome, and N5-methylhomocysteine transferase deficiency.

Copper deficiency can cause a wide variety of neurological problems including, myelopathy, peripheral neuropathy, and optic neuropathy.

Imerslund–Gräsbeck syndrome, is a rare autosomal recessive, familial form of vitamin B deficiency caused by malfunction of the ""Cubam"" receptor located in the terminal ileum. This receptor is composed of two proteins, amnionless (AMN), and cubilin. A defect in either of these protein components can cause this syndrome. This is a rare disease, with a prevalence about 1 in 200,000, and is usually seen in patients of European ancestry.

Vitamin B is an important vitamin needed for bone marrow functioning, the deficit of which causes decreased marrow output and anemia. Vitamin B has two forms, one of which, along with folate, is important in DNA synthesis. Vitamin B is sensitive to acid deformation in the stomach, so a molecule called haptocorrin (R-factor), protects it in the stomach. In the small bowel, a molecule named intrinsic factor (IF), allows vitamin B to be absorbed in the ileum. IGS is caused by a mutation in the receptors located in the terminal portion of ileum. This is a very rare, and unlikely cause of vitamin B deficiency but is a cause nonetheless.

People with guanidinoacetate methyltransferase deficiency have a wide spectrum of neurological symptoms. In addition to developmental disability and muscle weakness, some children with this disorder experience seizures. They may also develop autistic behaviors that affect communication and social interaction. Some affected children exhibit certain involuntary movements such as tremors or facial tics.

Guanidinoacetate methyltransferase deficiency is a very rare disorder. Only a few dozen affected individuals have been reported worldwide. Of these, approximately one third are of Portuguese origin.

The common MTHFR deficiencies are usually asymptomatic, although the 677T variant can cause a mildly increased risk of some diseases.

For individuals homozygous in the 677T variant, there is a mildly elevated risk of thromboembolism (odds ratio 1.2), and stroke (odds ratio 1.26). There is also an elevated risk of neural tube defects among children of individuals with the C677T polymorphism (odds ratio 1.38).

For cardiovascular risk, common MTHFR deficiencies were once thought to be associated but meta-analyses indicate that correlation this was an artifact of publication bias.

Methylenetetrahydrofolate reductase (MTHFR) deficiency is the most common genetic cause of elevated serum levels of homocysteine (hyperhomocysteinemia). It is caused by genetic defects in MTHFR, which is an important enzyme in the methyl cycle.

Common variants of MTHFR deficiency are asymptomatic and have only minor effects on disease risk. Severe variants (from nonsense mutations) are vanishingly rare.

Propionic acidemia, also known as propionic aciduria, propionyl-CoA carboxylase deficiency and ketotic glycinemia, is an autosomal recessive metabolic disorder, classified as a branched-chain organic acidemia.

The disorder presents in the early neonatal period with progressive encephalopathy. Death can occur quickly, due to secondary hyperammonemia, infection, cardiomyopathy, or basal ganglial stroke.

Propionic acidemia is a rare disorder that is inherited from both parents. Being autosomal recessive, neither parent shows symptoms, but both carry a defective gene responsible for this disease. It takes two faulty genes to cause PA, so there is a 1 in 4 chance for these parents to have a child with PA.

This disorder usually appears in the first few months of life, when development of new motor and cognitive skills becomes delayed or stops. Eventually, affected children may lose previously acquired skills such as head control or the ability to sit unsupported.

Methylmalonyl-CoA mutase is a mitochondrial homodimer apoenzyme (EC. 5. 4.99.2) that focuses on the catalysis of methylmalonyl CoA to succinyl CoA. The enzyme is bound to adenosylcobalamin, a hormonal derivative of vitamin B12 in order to function. Methylmalonyl-CoA mutase deficiency is caused by genetic defect in the MUT gene responsible for encoding the enzyme. Deficiency in this enzyme accounts for 60% of the cases of methylmalonic acidemia.