Brunner syndrome (BRNRS) is an X-linked recessive disorder characterized by impulsive aggressiveness and mildly impaired intellectual development associated with MAOA deficiency (Brunner et al., 1993).

A rare hyperkinetic movement disorder with characteristics of mild to severe, progressive essential tremor, nystagmus (principally horizontal), duodenal ulceration and a narcolepsy-like sleep disturbance. Refractive errors and cerebellar signs such as gait ataxia and adiadochokinesia may be associated. There have been no further descriptions in the literature since 1976.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the GRIK2 gene.

The nephronophthisis (NPH) phenotype is characterized by reduced renal concentrating ability, chronic tubulointerstitial nephritis, cystic renal disease, and progression to end-stage renal disease (ESRD) before age 30 years. Three age-based clinical subtypes are recognized: infantile, juvenile, and adolescent/adult. Infantile NPH can present in utero with oligohydramnios sequence (limb contractures, pulmonary hypoplasia, and facial dysmorphisms) or postnatally with renal manifestations that progress to ESRD before age 3 years. Juvenile NPH, the most prevalent subtype, typically presents with polydipsia and polyuria, growth retardation, chronic iron-resistant anemia, or other findings related to chronic kidney disease (CKD). Hypertension is typically absent due to salt wasting. ESRD develops at a median age of 13 years. Ultrasound findings are increased echogenicity, reduced corticomedullary differentiation, and renal cysts (in 50% of affected individuals). Histologic findings include tubulointerstitial fibrosis, thickened and disrupted tubular basement membrane, sporadic corticomedullary cysts, and normal or reduced kidney size. Adolescent/adult NPH is clinically similar to juvenile NPH, but ESRD develops at a median age of 19 years. Within a subtype, inter- and intrafamilial variability in rate of progression to ESRD is considerable. Approximately 80%-90% of individuals with the NPH phenotype have no extrarenal features (i.e., they have isolated NPH); ~10%-20% have extrarenal manifestations that constitute a recognizable syndrome (e.g., Joubert syndrome, Bardet-Biedl syndrome, Jeune syndrome and related skeletal disorders, Meckel-Gruber syndrome, Senior-Løken syndrome, Leber congenital amaurosis, COACH syndrome, and oculomotor apraxia, Cogan type).

Microcephaly, short stature, and impaired glucose metabolism-2 (MSSGM2) is an autosomal recessive syndrome characterized by microcephaly associated with impaired intellectual development, and short stature. Patients develop diabetes in the second or third decade of life, and hypothyroidism and delayed puberty have also been reported (Abdulkarim et al., 2015; Kernohan et al., 2015). For a discussion of genetic heterogeneity of microcephaly, short stature, and impaired glucose metabolism, see MSSGM1 (616033).

Hereditary essential tremor-5 (ETM5) is an autosomal dominant neurologic disorder characterized by kinetic, intention, and/or postural tremor mainly affecting the upper limbs. The age at onset and severity are highly variable, even within families (summary by Hor et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of hereditary essential tremor, see ETM1 (190300).

Hereditary essential tremor-6 (ETM6) is an autosomal dominant neurologic disorder characterized by adult-onset kinetic and/or postural tremor usually affecting the upper limbs. Some patients may have involvement of the head, trunk, lower limbs, and/or voice. Additional neurologic features, such as cognitive impairment or pyramidal signs, are usually not observed. Brain imaging does not show cerebellar atrophy or leukodystrophy. Skin biopsy shows intranuclear eosinophilic inclusions in fibroblasts and sweat gland cells, which may be used for diagnosis. There is evidence of genetic anticipation, with progressive earlier age at onset in younger generations. In rare cases, the phenotype may convert to NIID over time (summary by Sun et al., 2020; Ng et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of hereditary essential tremor, see ETM1 (190300).