MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) is a multisystem disorder with protean manifestations. The vast majority of affected individuals develop signs and symptoms of MELAS between ages two and 40 years. Common clinical manifestations include stroke-like episodes, encephalopathy with seizures and/or dementia, muscle weakness and exercise intolerance, normal early psychomotor development, recurrent headaches, recurrent vomiting, hearing impairment, peripheral neuropathy, learning disability, and short stature. During the stroke-like episodes neuroimaging shows increased T2-weighted signal areas that do not correspond to the classic vascular distribution (hence the term "stroke-like"). Lactic acidemia is very common and muscle biopsies typically show ragged red fibers.

Muckle-Wells syndrome (MWS) is characterized by episodic skin rash, arthralgias, and fever associated with late-onset sensorineural deafness and renal amyloidosis (Dode et al., 2002).

Alström syndrome is characterized by cone-rod dystrophy, obesity, progressive bilateral sensorineural hearing impairment, acute infantile-onset cardiomyopathy and/or adolescent- or adult-onset restrictive cardiomyopathy, insulin resistance / type 2 diabetes mellitus (T2DM), nonalcoholic fatty liver disease (NAFLD), and chronic progressive kidney disease. Cone-rod dystrophy presents as progressive visual impairment, photophobia, and nystagmus usually starting between birth and age 15 months. Many individuals lose all perception of light by the end of the second decade, but a minority retain the ability to read large print into the third decade. Children usually have normal birth weight but develop truncal obesity during their first year. Sensorineural hearing loss presents in the first decade in as many as 70% of individuals and may progress to the severe or moderately severe range (40-70 db) by the end of the first to second decade. Insulin resistance is typically accompanied by the skin changes of acanthosis nigricans, and proceeds to T2DM in the majority by the third decade. Nearly all demonstrate hypertriglyceridemia. Other findings can include endocrine abnormalities (hypothyroidism, hypogonadotropic hypogonadism in males, and hyperandrogenism in females), urologic dysfunction / detrusor instability, progressive decrease in renal function, and hepatic disease (ranging from elevated transaminases to steatohepatitis/NAFLD). Approximately 20% of affected individuals have delay in early developmental milestones, most commonly in gross and fine motor skills. About 30% have a learning disability. Cognitive impairment (IQ <70) is very rare. Wide clinical variability is observed among affected individuals, even within the same family.

A rare genetic disease characterized by childhood onset of bilateral progressive sensorineural hearing loss, ocular anomalies (myopia, cataract, retinitis pigmentosa), central and peripheral nervous system features (dementia, epilepsy, ataxia, peripheral neuropathy), ectodermal features (skin atrophy, alopecia, dental caries), and skeletal anomalies (bone cysts, joint stiffness, scoliosis, kyphosis). Laboratory examination may reveal elevated cerebrospinal fluid protein.

Chronic infantile neurologic cutaneous and articular syndrome (CINCA) is an early-onset, severe, chronic inflammatory disease, characterized by cutaneous symptoms, central nervous system involvement, and arthropathy (Feldmann et al., 2002). See also familial cold autoinflammatory syndrome-1 (FCAS1, CAPS1; 120100), an allelic disorder with a less severe phenotype.

A rare genetic syndromic intellectual disability characterized by global developmental delay, intellectual disability, infantile or childhood onset of progressive ataxia, and bilateral sensorineural hearing impairment. Variable features include signs of upper and lower motor neuron disease, peripheral neuropathy, myopathic facies, lower limb muscle wasting, and heel contractures. There have been no further descriptions in the literature since 1993.

Males with deafness-dystonia-optic neuronopathy (DDON) syndrome have prelingual or postlingual sensorineural hearing impairment in early childhood, slowly progressive dystonia or ataxia in the teens, slowly progressive decreased visual acuity from optic atrophy beginning at approximately age 20 years, and dementia beginning at approximately age 40 years. Psychiatric symptoms such as personality change and paranoia may appear in childhood and progress. The hearing impairment appears to be consistent in age of onset and progression, whereas the neurologic, visual, and neuropsychiatric signs vary in degree of severity and rate of progression. Females may have mild hearing impairment and focal dystonia.

Any autosomal recessive nonsyndromic deafness in which the cause of the disease is a mutation in the GRXCR1 gene.

ATP1A3-related disorder consists of heterogenous overlapping clinical findings that pertain to the four most common historically defined phenotypes: alternating hemiplegia of childhood (AHC); cerebellar ataxia, areflexia, pes cavus, optic atrophy, sensorineural hearing loss (CAPOS) syndrome; relapsing encephalopathy with cerebellar ataxia (RECA) / fever-induced paroxysmal weakness and encephalopathy (FIPWE); and rapid-onset dystonia-parkinsonism (RDP). These phenotypes exist on a spectrum and should be regarded as classifications of convenience. AHC is characterized by onset prior to age 18 months of paroxysmal hemiplegic episodes, predominately involving the limbs and/or the whole body, lasting from minutes to hours to days (and sometimes weeks) with remission only during sleep, only to resume after awakening. Although paroxysmal episodic neurologic dysfunction predominates early in the disease course, with age increasingly persistent neurologic dysfunction predominates, including oculomotor apraxia and strabismus, dysarthria, speech and language delay, developmental delay, and impairment in social skills. Other system involvement may include cardiovascular (cardiac conduction abnormalities) and gastrointestinal (constipation, vomiting, anorexia, diarrhea, nausea, and abdominal pain) manifestations. CAPOS syndrome presents in infancy or childhood (usually ages 6 months to 5 years) with cerebellar ataxia during or after a fever. The acute febrile encephalopathy may include hypotonia, flaccidity, nystagmus, strabismus, dysarthria/anarthria, lethargy, loss of consciousness, and even coma. Usually, considerable recovery occurs within days to weeks; however, persistence of some degree of ataxia and other manifestations is typical. RECA/FIPWE primarily presents with fever-induced episodes (infancy to age 5 years); however, first episodes can occur occasionally in young adults during illnesses such as mononucleosis. Recurrent fever-induced episodes may be ataxia-dominated RECA-like motor manifestations or FIPWE-like non-motor manifestations (encephalopathy) and can vary among affected individuals. Notably, RECA-like and FIPWE-like manifestations can occur in the same individual in different episodes. In some individuals episodes seem to decrease in frequency and severity over time, whereas others might experience worsening of manifestations. RDP presents in individuals ages 18 months to 60 years and older with dystonia that is typically of abrupt onset over hours to several weeks, though some individuals report gradual onset over the course of months. A stress-related trigger is identifiable in up to 75% of individuals. Dystonia rarely improves significantly after onset; some individuals report mild improvement over time, whereas others can experience subsequent episodes of abrupt worsening months to years after onset. Limbs are usually the first to be affected, although by the time of diagnosis – typically many years after onset – individuals most commonly display a bulbar-predominant generalized dystonia. Exceptions are common and a rostrocaudal gradient is rare rather than typical. Migraines and seizures are also observed.

Any autosomal dominant nonsyndromic deafness in which the cause of the disease is a mutation in the GSDME gene.

WFS1 spectrum disorder (WFS1-SD) comprises classic WFS1 spectrum disorder and nonclassic WFS1 spectrum disorder. Classic WFS1-SD, a progressive neurodegenerative disorder, is characterized by onset of diabetes mellitus and optic atrophy before age 16 years. Additional complications may include one or more of the following: variable hearing impairment / deafness, diabetes insipidus, neurologic abnormalities, neurogenic bladder, and psychiatric abnormalities. Nonclassic WFS1-SD is less common than classic WFS1-SD. Phenotypes that appear to be milder than classic WFS1-SD include: optic atrophy and hearing impairment; neonatal diabetes, profound congenital deafness, and cataracts; isolated diabetes mellitus; isolated congenital cataracts; and isolated congenital, slowly progressive, and low-frequency (<2000 Hz) sensorineural hearing loss.

Any autosomal dominant nonsyndromic deafness in which the cause of the disease is a mutation in the MYH14 gene.

Autosomal dominant deafness-41 (DFNA41) is characterized by onset of progressive sensorineural hearing loss usually in the second decade. The hearing loss is severe and ultimately affects all frequencies. Exposure to noise exacerbates the hearing loss, particularly at high frequencies (summary by Yan et al., 2013).

For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

DFNX2, also known as DFN3, is an X-linked recessive disorder characterized by progressive conductive and sensorineural hearing loss and a pathognomonic temporal bone deformity that includes dilatation of the inner auditory canal and a fistulous connection between the internal auditory canal and the cochlear basal turn, resulting in a perilymphatic fluid 'gusher' during stapes surgery (summary by de Kok et al., 1995 and Song et al., 2010). See also choroideremia, deafness, and mental retardation (303110), a contiguous gene deletion syndrome involving the POU3F4 and CHM (300390) genes on Xq21; isolated choroideremia (303100) is caused by mutation in the CHM gene.

Autosomal recessive deafness-30 (DFNB30) is characterized by progressive hearing loss with onset in the second decade of life. The high frequencies are initially affected; by age 50, hearing loss is severe in high and middle frequencies and moderate in low frequencies. Vision and balance are normal (Walsh et al., 2002).

This syndrome has characteristics of gingival fibromatosis associated with progressive sensorineural hearing loss. It has been described in two families (with at least 16 affected members spanning five generations in one of the families and five affected members spanning three generations in the other family). It is transmitted as an autosomal dominant trait.

Any autosomal dominant nonsyndromic deafness in which the cause of the disease is a mutation in the ACTG1 gene.

Any autosomal recessive nonsyndromic deafness in which the cause of the disease is a mutation in the TPRN gene.

Any autosomal dominant nonsyndromic deafness in which the cause of the disease is a mutation in the MYO6 gene.

Any autosomal recessive nonsyndromic deafness in which the cause of the disease is a mutation in the SYNE4 gene.

Syndromic optic atrophy, also known as DOA+ syndrome, is a neurologic disorder characterized most commonly by an insidious onset of visual loss and sensorineural hearing loss in childhood with variable presentation of other clinical manifestations including progressive external ophthalmoplegia (PEO), muscle cramps, hyperreflexia, and ataxia. There appears to be a wide range of intermediate phenotypes (Yu-Wai-Man et al., 2010).

An X-linked retinal dystrophy characterized by choroideremia, causing in affected males progressive nyctalopia and eventual central blindness. Obesity, moderate intellectual disability and congenital mixed (sensorineural and conductive) deafness are also observed. Female carriers show typical retinal changes indicative of the choroideremia carrier state.

Perrault syndrome is characterized by sensorineural hearing loss (SNHL) in males and females and ovarian dysfunction in females. SNHL is bilateral and ranges from profound with prelingual (congenital) onset to moderate with early-childhood onset. When onset is in early childhood, hearing loss can be progressive. Ovarian dysfunction ranges from gonadal dysgenesis (absent or streak gonads) manifesting as primary amenorrhea to primary ovarian insufficiency (POI) defined as cessation of menses before age 40 years. Fertility in affected males is reported as normal (although the number of reported males is limited). Neurologic features described in some individuals with Perrault syndrome include learning difficulties and developmental delay, cerebellar ataxia, and motor and sensory peripheral neuropathy.

Autosomal dominant deafness-50 is a form of nonsyndromic hearing loss. Hearing impairment shows postlingual onset and is progressive (summary by Mencia et al., 2009).

Autosomal dominant deafness-70 (DFNA70) is a form of nonsyndromic sensorineural hearing loss. Hearing impairment shows postlingual onset and is slowly progressive (Gao et al., 2015).

Birk-Landau-Perez syndrome (BILAPES) is an autosomal recessive syndromic developmental disorder characterized by global developmental delay apparent from infancy or early childhood. Some patients have developmental regression with loss of speech and motor skills, whereas other patients never achieve these milestones. More variable features may include hypotonia, poor overall growth, ataxia, dystonia, abnormal eye movements, and renal insufficiency (Perez et al., 2017; Kleyner et al., 2022).

An atypical form of Usher syndrome, here designated type IV (USH4), is an autosomal recessive disorder characterized by late onset of retinitis pigmentosa and usually late-onset of progressive sensorineural hearing loss without vestibular involvement (summary by Khateb et al., 2018). For a discussion of genetic heterogeneity of Usher syndrome, see 276900.

DFNB111 is characterized by early-onset, moderate to severe sensorineural hearing loss with no vestibular involvement (Wesdorp et al., 2018; Bademci et al., 2018).

Autosomal dominant deafness-79 (DFNA79) is a nonsyndromic form of progressive sensorineural hearing loss with age of onset ranging from 20 years to 65 years. Affected females appear to have milder hearing loss than males (Lu et al., 2020).

Branchial arch abnormalities, choanal atresia, athelia, hearing loss, and hypothyroidism syndrome (BCAHH) is an autosomal dominant disorder characterized by choanal atresia, athelia or hypoplastic nipples, branchial sinus abnormalities, neck pits, lacrimal duct anomalies, hearing loss, external ear malformations, and thyroid abnormalities. Additional features may include developmental delay, impaired intellectual development, and growth failure/retardation (summary by Cuvertino et al., 2020 and Baldridge et al., 2020).