Ill or just old? Towards a conceptual framework of the relation between ageing and disease

Abstract

Background: Is this person ill or just old? This question reflects the pondering mind of a doctor while interpreting the complaints of an elderly person who seeks his help. Many doctors think that ageing is a non-disease. Accordingly, various attempts have been undertaken to separate pathological ageing from normal ageing. However, the existence of a normal ageing process distinct from the pathological processes causing disease later in life can be questioned.

Discussion: Ageing is the accumulation of damage to somatic cells, leading to cellular dysfunction, and culminates in organ dysfunction and an increased vulnerability to death. Analogously, chronic diseases initiate early in life and their development is slow before they become clinically apparent and culminate in disability or death. The definition of disease is also subject to current opinions and scientific understanding and usually, it is an act of individual creativity when physical changes are recognised as symptoms of a new disease. New diseases, however, are only rarely really new. Most new diseases have gone undiagnosed because their signs and symptoms escaped recognition or were interpreted otherwise. Many physical changes in the elderly that are not yet recognised as a disease are thus ascribed to normal ageing. Therefore, the distinction between normal ageing and disease late in life seems in large part arbitrary.

Summary: We think that normal ageing cannot be separated from pathological processes causing disease later in life, and we propose that the distinction is avoided.

Figure 1

The biological mechanism underlying ageing The quadrangle (A) represents the human body. Extrinsic and intrinsic stressors cause injury (B). Subsequently, several mechanisms repair the injury, but the repair is not complete because of its high metabolic costs (C). As a consequence, damage accumulates (D-G), and cells, tissues, and organs decline in function.

Figure 2

The distinction between component causes and sufficient causes Sufficient causes I and II have five component causes each and sufficient cause III has seven component causes. The component causes are depicted as A to J. All three sufficient causes share the component causes A and B, that can be interpreted as shared risk factors if the sufficient causes have distinct effects (adapted from Rothman, 1976 [9]).

Figure 3

Conceptual scheme of ageing as the accumulation of component causes throughout life Ageing starts with the accumulation of component causes A–E. The presence of these five component causes completes sufficient cause I, resulting in effect I, e.g. unsteadiness. In the following period, the addition of component causes F–H completes sufficient cause II, resulting in effect II, e.g. a gait disorder. The further accumulation of component causes I and J completes sufficient cause III, resulting in effect III, e.g. death (see also the description of the example).