Previously, three types of Usher syndrome (USH), were defined based on the age of onset and severity of the clinical symptoms. The three types (USH1, USH2, USH3) are further sub-typed by genetic testing, which is indicated by a letter after the number. Typically, the symptom presentation and progression is similar within each type. Atypical USH occurs when mutations are found in one of the genes known to cause a subtype, but the symptom presentation is different than what is typical for that type.
Mutations in the arylsulfatase G (ARSG) gene have been implicated to cause a fourth type of USH (USH4) with a distinct set of symptoms that includes the late onset of both hearing impairment and retinitis pigmentosa (RP), with no vestibular involvement. Unlike types USH1-3, in which the RP is mostly located in the outer or peripheral region of the retina, causing tunnel-vision, the RP in USH4 is predominantly located in the pericentral and macular regions of the retina and affects central vision (Khateb et al 2018, Abad-Morales et al 2020, Peter et al 2020, Fowler et al 2021, Igelman 2021 et al 2021). Most recently, researchers at Radboud University Medical Center in the Netherlands confirm these clinical findings in additional, unrelated individuals.
What this means for Usher syndrome: This recently-described distinct type of Usher syndrome expands the field of knowledge, which is important for improving diagnosis and management of the symptoms; and vital to the development of a treatment and/or cure for this still little-understood disease.