The symptoms of Leigh syndrome are classically described as beginning in infancy and leading to death within a span of several years; however, as more cases are recognized, it is apparent that symptoms can emerge at any age—including adolescence or adulthood—and patients can survive for many years following diagnosis. Symptoms are often first seen after a triggering event that taxes the body's energy production, such as an infection or surgery. The general course of Leigh syndrome is one of episodic developmental regression during times of metabolic stress. Some patients have long periods without disease progression while others develop progressive decline.

Infants with the syndrome have symptoms that include diarrhea, vomiting, and dysphagia (trouble swallowing or sucking), leading to a failure to thrive. Children with early Leigh disease also may appear irritable and cry much more than usual. Seizures are often seen. Excess lactate may be seen in the urine, cerebrospinal fluid, and blood of a person with Leigh syndrome.

As the disease progresses, the muscular system is debilitated throughout the body, as the brain cannot control the contraction of muscles. Hypotonia (low muscle tone and strength), dystonia (involuntary, sustained muscle contraction), and ataxia (lack of control over movement) are often seen in people with Leigh disease. The eyes are particularly affected; the muscles that control the eyes become weak, paralyzed, or uncontrollable in conditions called ophthalmoparesis (weakness or paralysis) and nystagmus (involuntary eye movements). Slow saccades are also sometimes seen. The heart and lungs can also fail as a result of Leigh disease. Hypertrophic cardiomyopathy (thickening of part of the heart muscle) is also sometimes found and can cause death; asymmetric septal hypertrophy has also been associated with Leigh syndrome. In children with Leigh-syndrome associated ventricular septal defects, caused by pyruvate dehydrogenase deficiency, high forehead and large ears are seen; facial abnormalities are not typical of Leigh syndrome.

However, respiratory failure is the most common cause of death in people with Leigh syndrome. Other neurological symptoms include peripheral neuropathy, loss of sensation in extremities caused by damage to the peripheral nervous system.

Hypertrichosis is seen in Leigh syndrome caused by mutations in the nuclear gene SURF1.

Symptoms include:

- intellectual disability (more than half of the patients have an IQ below 50)

- microcephaly

- sometimes pancytopenia (low blood counts)

- cryptorchidism

- low birth weight

- dislocations of pelvis and elbow

- unusually large eyes

- low ears

- small chin

Leigh syndrome (also called Leigh disease and subacute necrotizing encephalomyelopathy) is an under-recognized inherited neurometabolic disorder that affects the central nervous system. It is named after Archibald Denis Leigh, a British neuropsychiatrist who first described the condition in 1951.

Symptoms of M2DS include infantile hypotonia and failure to thrive, delayed psychomotor development, impaired speech, abnormal or absent gait, epilepsy, spasticity, gastrointestinal motility problems, recurrent infections, and genitourinary abnormalities. Many of those affected by M2DS also fit diagnostic criteria for autism. M2DS can be associated with syndromic facies, namely an abnormally flat back of the head, underdevelopment of the midface, ear anomalies, deep-set eyes, prominent chin, pointed nose, and a flat nasal bridge.

Symptoms for the disease include microcephaly, a low birth weight, dwarfism, small teeth, and diabetes. The symptoms of Stimmler syndrome are closely related to a disease studied by Haworth et al. in 1967 as well as Leigh subacute necrotizing encephalopathy with lactic acidosis

The symptoms of Sly syndrome are similar to those of Hurler syndrome (MPS I). The symptoms include:

- in the head, neck, and face: coarse (Hurler-like) facies and macrocephaly, frontal prominence, premature closure of sagittal lambdoid sutures, and J-shaped sella turcica

- in the eyes: corneal opacity and iris coloboma

- in the nose: anteverted nostrils and a depressed nostril bridge

- in the mouth and oral areas: prominent alveolar processes and cleft palate

- in the thorax: usually pectus carinatum or exacavatum and oar-shaped ribs; also a protruding abdomen and inguinal or umbilical hernia

- in the extremities: talipes, an underdeveloped ilium, aseptic necrosis of femoral head, and shortness of tubular bones occurs

- in the spine: kyphosis or scoliosis and hook-like deformities in thoracic and lumbar vertebrate

- in the bones: dysostosis multiplex

In addition recurrent pulmonary infections occur. Hepatomegaly occurs in the gastrointestinal system. Splenomegaly occurs in the hematopoietic system. Inborn mucopolysaccharide metabolic disorders due to β-glucuronidase deficiency with granular inclusions in granulocytes occurs in the biochemical and metabolic systems. Growth and motor skills are affected, and mental retardation also occurs.

Though not always present, the cardinal characteristics of this multi-system disorder include: cardiomyopathy (dilated or hypertrophic, possibly with left ventricular noncompaction and/or endocardial fibroelastosis), neutropenia (chronic, cyclic, or intermittent), underdeveloped skeletal musculature and muscle weakness, growth delay, exercise intolerance, cardiolipin abnormalities, and 3-methylglutaconic aciduria.

It can be associated with stillbirth.

Barth syndrome is manifested in a variety of ways at birth. A majority of BTHS patients are hypotonic at birth, show signs of cardiomyopathy within the first few months of life, and experience a deceleration in growth in the first year, despite adequate nutrition. As patients progress into childhood, their height and weight lag significantly behind other children. While most patients express normal intelligence, a high proportion of BTHS patients also express mild or moderate learning disabilities. Physical activity is also hindered due to diminished muscular development and muscular hypotonia. Many of these disorders are resolved after puberty. Growth accelerates during puberty, and many patients reach a normal adult height.

Cardiomyopathy is one of the more severe manifestations of BTHS. The myocardium is dilated, reducing the systolic pump of the ventricles. For this reason, most BTHS patients have left myocardial thickening (hypertrophy). While cardiomyopathy can be life-threatening, it is commonly resolved or substantially improved in BTHS patients after puberty.

Neutropenia is another deadly manifestation of BTHS. Neutropenia is a granulocyte disorder that results in a low production of neutrophils, the body’s primary defenders against bacterial infections. Surprisingly, however, BTHS patients have relatively fewer bacterial infections than other patients with neutropenia.

This syndrome is characterized by overgrowth and advanced bone age. Affected individuals are dysmorphic, with macrodolichocephaly, downslanting palpebral fissures and a pointed chin. The facial appearance is most notable in early childhood. Affected infants and children tend to grow quickly; they are significantly taller than their siblings and peers, and have an unusually large skull and large head. Adult height is usually in the normal range, although Broc Brown has the condition and was named the world's tallest teenager. As of late 2016, he was 7'8" and still growing.

Individuals with Sotos syndrome often have intellectual impairment, and most also have behavioral problems. Frequent behavioral impairments include attention deficit hyperactivity disorder (ADHD), phobias, obsessive compulsive disorder, tantrums, and impulsive behaviors (impulse control disorder). Problems with speech and language are also common. Affected individuals may often have stuttering, difficulty with sound production, or a monotone voice. Additionally, weak muscle tone (hypotonia) may delay other aspects of early development, particularly motor skills such as sitting and crawling.

Other signs include scoliosis, seizures, heart or kidney defects, hearing loss, and problems with vision. Some infants with this disorder experience jaundice and poor feeding. A small number of patients with Sotos syndrome have developed cancer, most often in childhood, but no single form of cancer has been associated with this condition. It remains uncertain whether Sotos syndrome increases the risk of specific types of cancer. If persons with this disorder have any increased cancer risk, their risk is only slightly greater than that of the general population.

Stimmler syndrome is a rare autosomal recessive congenital disorder first described by Stimmler et al. in 1970. It is characterized by dwarfism, diabetes, a small head, and high levels of alanine in the urine.

Acro–dermato–ungual–lacrimal–tooth (ADULT) syndrome is a rare genetic disease. ADULT syndrome is an autosomal dominant form of ectodermal dysplasia, a group of disorders that affects the hair, teeth, nails, sweat glands, and extremities. The syndrome arises from a mutation in the TP63 gene. This disease was previously thought to be a form of ectrodactyly–ectodermal dysplasia–cleft syndrome (EEC), but was classified as a different disease in 1993 by Propping and Zerres.

ADULT syndrome features include ectrodactyly, syndactyly, excessive freckling, lacrimal duct anomalies, dysplastic nails, hypodontia, hypoplastic breasts and nipples, hypotrichosis, hypohidrosis, broad nasal bridge, midfacial hypoplasia, exfoliative dermatitis, and xerosis. The lack of facial clefting and ankyloblepharon are important because they exist in ectrodactyly–ectodermal dysplasia–cleft syndrome (EEC) but not in ADULT syndrome.

Children with Weaver syndrome tend to look similar and have distinctive physical and craniofacial characteristics, which may include several, but not all of the following features:

- Macrocephaly

- Large bifrontal diameter

- Flattened occiput

- Long philtrum

- Retrognathia

- Round face in infancy

- Prominent chin crease

- Large ears

- Strabismus

- Hypertelorism

- Epicanthal folds

- Downslanting palpebral fissures

Other features may include loose skin, thin deep-set nails, thin hair, short ribs, limited elbow and knee extension, camptodactyly, and a coarse, low-pitched voice. Delayed development of motor skills such as sitting, standing, and walking are commonly exhibited in early childhood. Patients with Weaver syndrome typically have mild intellectual disability with poor coordination and balance. They also have some neurological abnormalities such as speech delay, epilepsy, intellectual disability, hypotonia or hypertonia, and behavioral problems.

MECP2 Duplication Syndrome (M2DS) is a rare disease that is characterized by severe intellectual disability and impaired motor function. It is an X-linked genetic disorder caused by the overexpression of MeCP2 protein.

Sly syndrome, also called mucopolysaccharidosis type VII (MPS 7), is an autosomal recessive lysosomal storage disease characterized by a deficiency of the enzyme β-glucuronidase, a lysosomal enzyme. Sly syndrome belongs to a group of disorders known as mucopolysaccharidoses, which are lysosomal storage diseases. In Sly syndrome, the deficiency in β-glucuronidase leads to the accumulation of certain complex carbohydrates (mucopolysaccharides) in many tissues and organs of the body.

It was named after its discoverer William S. Sly, an American biochemist who has spent nearly his entire academic career at Saint Louis University.

Pipecolic acidemia, also called hyperpipecolic acidemia or hyperpipecolatemia, is a very rare autosomal recessive metabolic disorder that is caused by a peroxisomal defect.

Pipecolic acidemia can also be an associated component of Refsum disease with increased pipecolic acidemia (RDPA), as well as other peroxisomal disorders, including both infantile and adult Refsum disease, and Zellweger syndrome.

The disorder is characterized by an increase in pipecolic acid levels in the blood, leading to neuropathy and hepatomegaly.

Affected males develop generalized reticular hyper pigmentation in early childhood.

Hair often looks bedraggled or brushed backwards, hanging low on the forehead.

Among the associated extracutaneous manifestations are described:

- Respiratory infections

- Dyskeratosis corneal photophobia

- Hypohidrosis with large deficit of thermoregulation

- Growth retardation

- Gastrointestinal disorders

- Kidney disease

- Kidney stones

- Urinary infections

- Webbed feet or hands

- Electrolyte imbalance

- Retinitis pigmentosa

- Lymphoedema

- Thyroid abnormalities

Each patient shows some of the symptoms listed above. Not every sick person will show all of the listed symptoms.

In females the disease is characterized by skin rashes linear hyper pigmentation following the Blaschko's lines, morphologically similar to stage 3 pigment incontinence. There are no systemic manifestations associated with XLPDR in females.

Barth syndrome (BTHS), also known as 3-Methylglutaconic aciduria type II, is an X-linked genetic disorder. The disorder, which affects multiple body systems, is diagnosed almost exclusively in males. It is named after Dutch pediatric neurologist Masa Barth.

The severity of the disorder can vary within the same family, with symptoms ranging from so mild as to go unnoticed to severe heart and/or liver disease requiring transplantation. It is difficult to predict a given patient's prognosis, but there are a few known indicators of earlier death.

Sotos syndrome (cerebral gigantism or Sotos-Dodge syndrome) is a rare genetic disorder characterized by excessive physical growth during the first years of life. Excessive growth often starts in infancy and continues into the early teen years. The disorder may be accompanied by autism, mild intellectual disability, delayed motor, cognitive, and social development, hypotonia (low muscle tone), and speech impairments. Children with Sotos syndrome tend to be large at birth and are often taller, heavier, and have relatively large skulls (macrocephaly) than is normal for their age. Signs of the disorder, which vary among individuals, include a disproportionately large skull with a slightly protrusive forehead, large hands and feet, large mandible, hypertelorism (an abnormally increased distance between the eyes)(large inter-pupillary distance), and downslanting eyes. Clumsiness, an awkward gait, and unusual aggressiveness or irritability may also occur. Although most cases of Sotos syndrome occur sporadically, familial cases have also been reported. It is similar to Weaver syndrome.

Infantile Refsum disease is one of three peroxisome biogenesis disorders which belong to the Zellweger spectrum of peroxisome biogenesis disorders (PBD-ZSD). The other two disorders are Zellweger syndrome (ZS) and neonatal adrenoleukodystrophy (NALD). Although they share a similar molecular basis for disease, Infantile Refsum disease is less severe than Zellweger syndrome.

Infantile Refsum disease is a developmental brain disorder. In addition, patients can show a reduction in central nervous system (CNS) myelin (particularly cerebral), which is referred to as (hypomyelination). Myelin is critical for normal CNS functions. Patients can also show postdevelopmental sensorineuronal degeneration that leads to a progressive loss of hearing and vision.

Infantile Refsum disease can also affect the function of many other organ systems. Patients can show craniofacial abnormalities, hepatomegaly (enlarged liver), and progressive adrenal dysfunction. Newborns may present with profound hypotonia (low muscle tone), and a poor ability to feed. In some patients, a progressive leukodystrophy has been observed that has a variable age of onset.

The Kocher–Debré–Semelaigne syndrome is hypothyroidism in infancy or childhood characterised by lower extremity or generalized muscular hypertrophy, myxoedema, short stature and cretinism. The absence of painful spasms and pseudomyotonia differentiates this syndrome from its adult form, which is Hoffmann syndrome.

The syndrome is named after Emil Theodor Kocher, Robert Debré and Georges Semelaigne.

Also known as Debre–Semelaigne syndrome or cretinism-muscular hypertrophy, hypothyroid myopathy, hypothyroidism-large muscle syndrome, hypothyreotic muscular hypertrophy in children, infantile myxoedema-muscular hypertrophy, myopathy-myxoedema syndrome, myxoedema-muscular hypertrophy syndrome, myxoedema-myotonic dystrophy syndrome.

Kocher-Debre-Semelaigne syndrome gives infant a Hercules appearance.

The Seckel syndrome or microcephalic primordial dwarfism (also known as bird-headed dwarfism, Harper's syndrome, Virchow-Seckel dwarfism, and Bird-headed dwarf of Seckel) is an extremely rare congenital nanosomic disorder.

Inheritance is autosomal recessive.

It is characterized by intrauterine growth retardation and postnatal dwarfism with a small head, narrow bird-like face with a beak-like nose, large eyes with down-slanting palpebral fissures , receding mandible and intellectual disability.

A mouse model has been developed. This mouse model is characterized by a severe deficiency of ATR protein. These mice suffer high levels of replicative stress and DNA damage. Adult Seckel mice display accelerated aging. These findings are consistent with the DNA damage theory of aging.

8p23.1 duplication syndrome is a rare genetic disorder caused by a duplication of a region from human chromosome 8. This duplication syndrome has an estimated prevalence of 1 in 64,000 births and is the reciprocal of the 8p23.1 deletion syndrome. The 8p23.1 duplication is associated with a variable phenotype including one or more of speech delay, developmental delay, mild dysmorphism, with prominent forehead and arched eyebrows, and congenital heart disease (CHD).

Weaver syndrome (also called Weaver-Smith syndrome) is an extremely rare congenital disorder associated with rapid growth beginning in the prenatal period and continuing through the toddler and youth years. It is characterized by advanced osseous maturation, and distinctive craniofacial, skeletal, and neurological abnormalities. It was first described by Dr. David Weaver in 1974. It is similar to Sotos syndrome.

Signs and symptoms arising from liver damage in Alagille syndrome may include a yellowish tinge in the skin and the whites of the eyes (jaundice), itching (pruritus), pale stools (acholia), an enlarged liver (hepatomegaly), an enlarged spleen (splenomegaly) and deposits of cholesterol in the skin (xanthomas). A liver biopsy may indicate too few bile ducts (bile duct paucity) or, in some cases, the complete absence of bile ducts (biliary atresia). Bile duct paucity results in the reduced absorption of fat and vitamins (A, D, E and K), which may lead to rickets or a failure to thrive in children. Around 15% of patients will experience liver cirrhosis in the course of their disease. Hepatocellular cancer has been reported in a number of cases.