Krabbe disease comprises a spectrum ranging from infantile-onset disease (i.e., onset of extreme irritability, spasticity, and developmental delay before age 12 months) to later-onset disease (i.e., onset of manifestations after age 12 months and as late as the seventh decade). Although historically 85%-90% of symptomatic individuals with Krabbe disease diagnosed by enzyme activity alone have infantile-onset Krabbe disease and 10%-15% have later-onset Krabbe disease, the experience with newborn screening (NBS) suggests that the proportion of individuals with possible later-onset Krabbe disease is higher than previously thought. Infantile-onset Krabbe disease is characterized by normal development in the first few months followed by rapid severe neurologic deterioration; the average age of death is 24 months (range 8 months to 9 years). Later-onset Krabbe disease is much more variable in its presentation and disease course.

Intellectual developmental disorder and retinitis pigmentosa (IDDRP) is characterized by mild to moderate intellectual disability and typical features of RP. Patients experience reduced night vision, constriction of visual fields, and reduced visual acuity; optic disc pallor, attenuated retinal blood vessels, and bone-spicule pigmentation are seen on funduscopy. Attention-deficit/hyperactivity disorder is observed in some patients (Tatour et al., 2017).