Late diagnosis in severe and mild intellectual disability in adulthood

The diagnosis of intellectual disability (ID) is highly dependent on a comprehensive personal and family medical history, a complete physical examination and a careful developmental assessment of the patient. Our study intended to: (1) classify the aetiology of mild and severe ID in an adult population of 140 Italian subjects; (2) evaluate the frequency of associated medical conditions; (3) evaluate the age of diagnosis in both groups; and (4) underline the importance of aetiological diagnosis for adult ID patients also.

The study involved 140 consecutive adult Italian ID inpatients and outpatients neurologically investigated at the Neurological Institute C. Mondino of Pavia Service for Mental Retardation. A total of 80 patients had mild ID (MID group) (39 females, 41 males), mean age 34 years (range 19-61 years), mean IQ = 64 (range 51-75), and 60 had severe ID (SID group) (32 females, 28 males), mean age 30 years (range 19-69 years). They underwent a complete diagnostic work-up that comprised prenatal, perinatal and postnatal history, physical examinations, laboratory investigations, genetic survey and neuroradiological investigations to determine the aetiology of ID and to evaluate the presence of associated medical conditions.

ID aetiology was classified as prenatal in 34% of the MID and 28% of the SID group. Perinatal and postnatal events were found in 6% of the MID and in 5% of the SID group. Associated medical conditions were found in 97 patients (47% MID and 26% SID). A genetic diagnosis was possible in 6% of patients above 20 years of age and in 5% of patients above 40 years. A diagnosis of cerebral dysgenesis was possible in 5% of patients above 20 years and 4% of patients above 40 years.

A long interval between the diagnosis of ID and the aetiological definition can be observed in a significant percentage (24%) of our population, leading to unfortunate consequences of late diagnosis: late onset of a specific therapeutic program, genetic counselling that is frequently no more useful, and ineffective prenatal diagnosis, leading to the birth of other affected subjects (for familiar ID).