<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by adult-onset leukoencephalopathy with axonal spheroids and pigmented glia ? [/INST] ALSP is thought to be a rare disorder, although the prevalence is unknown. Because it can be mistaken for other disorders with similar symptoms, ALSP may be underdiagnosed. </s>