Understanding speech and language in KIF1A-associated neurological disorder.

KIF1A-associated neurological disorder (KAND) is a genetic condition characterised by motor, cognitive and ophthalmologic features. The speech and language phenotype have not been systematically analysed. Here, we assess speech and language using observer- and clinician-reported outcomes, and performance outcome measures. 44 individuals (25 female) with KAND (median age 7 years, range 1-60 years) participated. Median age at diagnosis was 4 years (range 0.5-58 years). KIF1A variants were missense (41/44 individuals, 93%), intragenic deletion (2/44, 5%) and splice site (1/44, 2%). Age at first words was delayed (>12 months) in 38/44 (86%) individuals. At assessment, 28/44 (64%) combined words into sentences and all of the 20 individuals assessed had dysarthria. Apraxic speech features and phonological impairments occurred in children aged under 8 years. 36/37 (97%) participants had language impairment, with expressive language skills stronger than receptive (p = 0.02) and written (p = 0.03) language on the Vineland Adaptive Behaviour Scales. 7/32 (22%) caregivers reported speech and language regression. Mild to severe intellectual disability occurred in 31/33 (94%) individuals. 22/44 (50%) participants had used augmentative and alternative communication, such as key word sign or speech generating devices. Individuals had average social motivation skills in contrast to moderately impaired social cognition, communication and awareness on the Social Responsiveness Scale (p < 0.05). 16/44 (36%) had epilepsy and 40/44 (91%) had visual impairment, namely nystagmus (16/44, 36%), optic nerve atrophy and strabismus (both 12/44, 27%). Individuals with KAND frequently have speech and language disorders necessitating early and targeted speech and language interventions.