NCD) are approximately 1%—2% at age 65 years and as high as 30% by age 85 years. The prevalence of mild NCD is very sensitive to the definition of the disorder, particularly in community settings, where evaluations are less detailed. In addition, in contrast with clin- ical settings, where cognitive concern must be high to seek and locate care, there may be a less clear decline from baseline functioning. Estimates of the prevalence of mild cognitive impairment (which is substantially congruent with mild NCD) among older individuals are fairly variable, ranging from 2% to 10% at age 65 and 5% to 25% by age 85.
The course of NCD varies across etiological subtypes, and this variation can be useful in differential diagnosis. Some subtypes (e.g., those related to traumatic brain injury or stroke) typically begin at a specific time and (at least after initial symptoms related to in- ﬂammation or swelling subside) remain static. Others may fluctuate over time (although if this occurs, the possibility of delirium superimposed on NCD should be considered).
NCDs due to neurodegenerative diseases like Alzheimer's disease or frontotemporal lobar degeneration typically are marked by insidious onset and gradual progression, and the pattern of onset of cognitive deficits and associated features helps to distinguish among them.
NCDs with ohset in childhood and adolescence may have broad repercussions for so- cial and intellectual development, and in this setting intellectual disability (intellectual nosed to capture the full diagnostic picture and ensure the provision of a broad range of services. In older individuals, NCDs often occur in the setting of medical illnesses, frailty, and sensory loss, which complicate the clinical picture for diagnosis and treatment.
When cognitive loss occurs in youth to midlife, individuals and families are likely to seek care. NCDs are typically easiest to identify at younger ages, although in some settings malingering or other factitious disorders may be a concern. Very late in life, cognitive symptoms may not cause concern or may go unnoticed. In late life, mild NCD must also be distinguished from the more modest deficits associated with “normal aging,” although a substantial fraction of what has been ascribed to normal aging likely represents prodromal phases of various NCDs. In addition, it becomes harder to recognize mild NCD with age because of the increasing prevalence of medical illness and sensory deficits. It becomes harder to differentiate among subtypes with age because there are multiple potential sources of neurocognitive decline.
subtypes. Some subtypes are distributed throughout the lifespan, whereas others occur exclusively or primarily in late life. Even within the NCDs of aging, the relative prevalence varies with age: Alzheimer's disease is uncommon before age 60 years, and the prevalence increases steeply thereafter, while the overall less common frontotemporal lobar degener- ation has earlier onset and represents a progressively smaller fraction of NCDs with age.
Genetic and physiological. The strongest risk factor for major and mild NCDs is age, primarily because age increases the risk of neurodegenerative and cerebrovascular dis- ease. Female gender is associated with higher prevalence of dementia overall, and especially
Alzheimer’s disease, but this difference is largely, if not wholly, attributable to greater lon- gevity in females.
Individuals’ and families’ level of awareness and concern about neurocognitive symp- toms may vary across ethnic and occupational groups. Neurocognitive symptoms are more likely to be noticed, particularly at the mild level, in individuals who engage in com- plex occupational, domestic, or recreational activities. In addition, norms for neuropsy- chological testing tend to be available only for broad populations, and thus they may not be easily applicable to individuals with less than high school education or those being evaluated outside their primary language or culture.