The Incidence and Evolution of Parkinsonian Rigidity in Rett Syndrome: A Pilot Study

Background: Patients with Rett syndrome (RTT) may demonstrate parkinsonian features. Here, we report a preliminary cross-sectional and prospective evaluation of the evolution, regional distribution, and eventual incidence of rigid tone in a cohort of MECP2 mutation-positive patients. Methods: In 51 participants, muscle tone rigidity in extremity regions and neck plus hypomimia were quantified using an RTT rigidity distribution (RTTRD) score with a range of 0 to 15. RTTRD scores were correlated with age, ability to walk and speak, mutation type, and, in a small subgroup (n = 9), cerebrospinal fluid (CSF) homovanillic acid (HVA) and 5-hydroxyindole-acetic acid levels. Results: Participant ages ranged from 2 years and 5 months, to 54 years. Rigidity was found in 43/51 (84.3%); it appeared as early as age 3, increased in extent with age, and was present in all participants aged >= 13. Ankle region rigidity appeared first, followed by proximal legs, arms, neck, and face. Ambulatory participants (n = 21) had lower RTTRD scores than nonambulatory (n = 30; p = 0.003). We found a trend to lower scores in participants with retained speech (n = 13) versus those with none (n = 38; p = 0.074), and no difference in scores for those with truncating (n = 25) versus missense mutations (n = 22; p = 0.387). RTTRD scores correlated negatively with CSF HVA levels (R = -0.83; p = 0.005), but not with 5-hydroxyindole- acetic acid levels (R = -0.45; p = 0.22). Conclusions: Although assessment of muscle tone is somewhat subjective and the RTTRD has not been validated, this study nevertheless suggests that parkinsonian rigidity in RTT is common and frequently increases in extent with age; its severity correlates directly with impaired ambulation and inversely with CSF HVA levels.