POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

An extremely rare autosomal recessively inherited neuromuscular disease characterised by ocular manifestations such as ptosis and diplopia followed by chronic diarrhoea, malnutrition and intestinal pseudo-obstruction.

Progressive external ophthalmoplegia-4 is an autosomal dominant form of mitochondrial disease that variably affects skeletal muscle, the nervous system, the liver, and the gastrointestinal tract. Age at onset ranges from infancy to adulthood. The phenotype ranges from relatively mild, with adult-onset skeletal muscle weakness and weakness of the external eye muscles, to severe, with a multisystem disorder characterized by delayed psychomotor development, lactic acidosis, constipation, and liver involvement (summary by Young et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

Mungan syndrome (MGS) is characterized by chronic intestinal pseudoobstruction (CIPO), megaduodenum, long-segment Barrett esophagus, and cardiac abnormalities of variable severity (summary by Bonora et al., 2015).

Cardiospondylocarpofacial syndrome (CSCF) is characterized by growth retardation, dysmorphic facial features, brachydactyly with carpal-tarsal fusion, extensive posterior cervical vertebral synostosis, cardiac septal defects with valve dysplasia, and deafness with inner ear malformations (summary by Le Goff et al., 2016).

Mitochondrial encephalo-cardio-myopathy due to <i>TMEM70</i> mutation is characterized by early neonatal onset of hypotonia, hypetrophic cardiomyopathy and apneic spells within hours after birth accompanied by lactic acidosis, hyperammonemia and 3-methylglutaconic aciduria.

CHOPS syndrome is a disorder involving multiple abnormalities that are present from birth (congenital). The name "CHOPS" is an abbreviation for a list of features of the disorder including cognitive impairment, coarse facial features, heart defects, obesity, lung (pulmonary) involvement, short stature, and skeletal abnormalities.\n\nChildren with CHOPS syndrome have intellectual disability and delayed development of skills such as sitting and walking. Characteristic facial features include a round face; thick hair; thick eyebrows that grow together in the middle (synophrys); wide-set, bulging eyes with long eyelashes; a short nose; and down-turned corners of the mouth.\n\nMost affected individuals are born with a heart defect called patent ductus arteriosus (PDA). The ductus arteriosus is a connection between two major arteries, the aorta and the pulmonary artery. This connection is open during fetal development and normally closes shortly after birth. However, the ductus arteriosus remains open, or patent, in babies with PDA. If untreated, this heart defect causes infants to breathe rapidly, feed poorly, and gain weight slowly; in severe cases, it can lead to heart failure. Multiple heart abnormalities have sometimes been found in children with CHOPS syndrome. In addition to PDA, affected individuals may have ventricular septal defect, which is a defect in the muscular wall (septum) that separates the right and left sides of the heart's lower chamber.\n\nPeople with CHOPS syndrome have abnormalities of the throat and airways that cause momentary cessation of breathing while asleep (obstructive sleep apnea). These abnormalities can also cause affected individuals to breathe food or fluids into the lungs accidentally, which can lead to a potentially life-threatening bacterial lung infection (aspiration pneumonia) and chronic lung disease. Affected individuals are shorter than more than 97 percent of their peers and are overweight for their height. They also have skeletal differences including unusually short fingers and toes (brachydactyly) and abnormally-shaped spinal bones (vertebrae).\n\nOther features that can occur in CHOPS syndrome include a small head size (microcephaly); hearing loss; clouding of the lens of the eye (cataract); a single, horseshoe-shaped kidney; and, in affected males, undescended testes (cryptorchidism).

Mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease is characterized by progressive gastrointestinal dysmotility (manifesting as early satiety, nausea, dysphagia, gastroesophageal reflux, postprandial emesis, episodic abdominal pain and/or distention, and diarrhea); cachexia; ptosis/ophthalmoplegia or ophthalmoparesis; leukoencephalopathy; and demyelinating peripheral neuropathy (manifesting as paresthesias (tingling, numbness, and pain) and symmetric and distal weakness more prominently affecting the lower extremities). The order in which manifestations appear is unpredictable. Onset is usually between the first and fifth decades; in about 60% of individuals, symptoms begin before age 20 years.

Leukoencephalopathy, developmental delay, and episodic neurologic regression syndrome (LEUDEN) is characterized by global developmental delay apparent in early childhood, followed by episodic neurologic regression or decompensation associated with systemic stress, such as febrile infection. Affected individuals have hypotonia, gait difficulties or ataxia, poor or absent speech with dysarthria, and variable motor abnormalities, including spasticity, dystonia, extrapyramidal signs, and tremor. Many patients have seizures. Brain imaging shows diffuse white matter abnormalities, poor myelination, thin corpus callosum, and generalized cerebral atrophy with enlarged ventricles. The clinical features of the disorder and the abnormal brain imaging findings are progressive (summary by Mao et al., 2020).

ACTG2 visceral myopathy is a disorder of smooth muscle dysfunction of the bladder and gastrointestinal system with phenotypic spectrum that ranges from mild to severe. Bladder involvement can range from neonatal megacystis and megaureter (with its most extreme form of prune belly syndrome) at the more severe end, to recurrent urinary tract infections and bladder dysfunction at the milder end. Intestinal involvement can range from malrotation, neonatal manifestations of microcolon, megacystis microcolon intestinal hypoperistalsis syndrome, and chronic intestinal pseudoobstruction (CIPO) in neonates at the more severe end to intermittent abdominal distention and functional intestinal obstruction at the milder end. Affected infants (with or without evidence of intestinal malrotation) often present with feeding intolerance and findings of non-mechanical bowel obstruction that persist after successful surgical correction of malrotation. Individuals who develop manifestations of CIPO in later childhood or adulthood often experience episodic waxing and waning of bowel motility. They may undergo frequent abdominal surgeries (perhaps related to malrotation or adhesions causing mechanical obstruction) resulting in resection of dilated segments of bowel, often becoming dependent on total parenteral nutrition (TPN).

Immunodeficiency-77 (IMD77) is an immunologic disorder characterized by recurrent and persistent polymicrobial infections with multiple unusual organisms. Skin and pulmonary infections are the most common, consistent with increased susceptibility to epithelial cell infections. The age at onset is highly variable: some patients have recurrent infections from childhood, whereas others present in late adulthood. The limited number of reported patients are all female, suggesting incomplete penetrance or a possible sex-influenced trait. Patient cells, mainly macrophages, show impaired killing of intracellular bacteria and organisms, including nontubercular mycobacteria, although there is also impaired killing of other organisms, such as Pseudomonas, Candida, and Aspergillus. Treatment with gamma-IFN (IFNG; 147570) may be a therapeutic option (summary by McCormack et al., 2017 and Merselis et al., 2020).

Visceral myopathy-2 (VSCM2) is characterized by gastrointestinal symptoms resulting from intestinal dysmotility and paresis, including abdominal distention, pain, nausea, and vomiting. Some patients exhibit predominantly esophageal symptoms, with hiatal hernia and severe reflux resulting in esophagitis and stricture, whereas others experience chronic intestinal pseudoobstruction. Bladder involvement resulting in megacystis and megaureter has also been observed and may be evident at birth (Dong et al., 2019; Gilbert et al. (2020)).