Longitudinal Trajectory of Adaptive Skills in Phelan-McDermid Syndrome.

Phelan-McDermid syndrome (PMS), caused by SHANK3 haploinsufficiency, lacks natural history data. We report the trajectory of adaptive behavior from a prospective, longitudinal, natural history study. English-speaking people aged 3-21 years with a PMS molecular diagnosis were followed over 2 years. We analyzed longitudinal Vineland Adaptive Behavior Scales, Second Edition domain-level standard scores and subdomain-level growth scale values (GSVs) obtained at baseline, 12 months, and 24 months. We assessed within-subject time effects and cross-sectional age effects using linear mixed effects models. This sample included 99 participants (baseline age = 8.83 ± 4.58 years). Within-subject standard scores decreased/remained constant for all domains: Communication (slope of within-subject mean-centered age = -0.33 [95% CI -1.08, 0.41]; p = 0.38), Socialization (-1.25 [-1.95, -0.56]; p < 0.001), and Daily Living Skills (-0.35 [-1.37, 0.67]; p = 0.50). However, subdomain GSVs showed within-subject growth across several categories. Receptive (5.26 [2.49, 8.02]; p < 0.001) and Written (2.79 [1.11, 4.47]; p = 0.001) Communication GSVs increased. Personal (1.84 [0.81, 2.86]; p < 0.001) and Domestic (2.31 [0.98, 3.64]; p < 0.001) Daily Living Skills GSVs increased. Socialization subdomain GSVs did not change. PMS is characterized by impaired adaptive behavior and slow, small gains in communication and daily living, but not socialization skills, as measured by subdomain GSVs. Unlike standard scores, measuring performance compared to same-age peers, GSVs quantify an individual's progress, emphasizing the need for GSVs in interpreting developmental changes in PMS.