The primary feature of the neurocognitive disorders (NCDs) is a decline in cognitive function. Other conditions such as schizophrenia and depression can have associated cognitive impairment, but for the NCDs it is the defining characteristic (e.g., Laughren, 2011). The name – neurocognitive disorders – arguably hides the fascinating diversity within this disorder class. It includes conditions such as Alzheimer's disease with its long, slow, progressive cognitive decline and uncertain aetiology, and Huntington's disease with its relatively rapid course and clear genetic basis. NCDs primarily affect adults, especially older adults. As the population of Australia and New Zealand (NZ) ages, NCDs are likely to become more prevalent. The NCDs are also not without controversy. Some people question whether they should be classified as a mental disorder (Robles et al., 2015). They are unique in that, unlike other diagnoses in the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5; American Psychiatric Association, 2013), the pathology, and in some cases the aetiology of the conditions, is largely known (Ganguli et al., 2011).
The hallmark of a person with a neurocognitive disorder is a decline in one or more formally defined areas of cognition. In the DSM-5, these are complex attention, executive function, learning and memory, language, perceptual-motor, or social cognition. The extent of decline separates the two main divisions within this disorder class, of major or mild neurocognitive disorder, as per the following example.
• Major NCD refers to disorders previously referred to as dementia or delirium where there is significant decline from previous functioning in one or more of the six areas of cognitive functioning. This decline is: 1. noticeable, it is evident to the affected person (they describe it) or a close other (e.g., a knowledgeable informant); 2. demonstrable, when formally assessed; and 3. impactful, it interferes with the person's capacity to do everyday tasks.
• Mild NCD differs in extent only. Mild NCD refers to cognitive difficulties at a level where the person needs to use strategies to function in everyday life.
We all experience cognitive decline as we age, so drawing the line between normal and abnormal decline can be difficult. Some consider the threshold suggested by the DSM-5 too low.