Impairment is defined by WHO as organ or system disorders that potentially will impair functioning at the organism level. Disability arises from the inability to perform an action in the manner consistent with the purpose of an organ or organ system. Successfully achieving the goal of preventing or delaying disability depends on understanding the etiology of the disability process. Above the waterline is dependence, the visible effect of disability. Below the waterline lie impairment and preclinical disability, both not readily visible because independence is maintained.
Preclinical disability represents the transitional state between impairment and disability.
It is hypothesized to be the critical point or threshold below which the transition to disability can be modified. Fried et al. In the first, tasks are performed without modification but general activity level gradually declines. The individual is unaware of the impact on function until he or she crosses a threshold of severity. For example, an individual with coronary artery disease might not recognize his decreased functional ability until his exercise tolerance is reduced to the point that he is having noticeable difficulty walking or climbing stairs.