Even at the biological level, aging is a highly complex phenomenon that can be examined at the genetic, cellular, organ-system, and even the psycho-social level(Aldwin & Gilmer 2004).  Although there are some conflicting findings on where declines occur, for the most part persons aging with SCI exhibit decreases in health status and physical functioning over time (see Table 2), which serve as markers of premature aging.

Table 2: Health Status and Physical Functioning

To address physiological aging after SCI, the identified studies were separated into different body systems, which include the cardiovascular and endocrine systems (see Table 3), immune system (see Table 4), musculoskeletal system (see Table 5), respiratory system (see Table 6), nervous system (see Table 7), skin and subcutaneous tissues (see Table 8), and the genitourinary and gastrointestinal systems (see Table 9).

There is Level 3 (Mitchell et al. 2010) and Level 4 evidence (Hitzig et al. 2010) that health declines over time, while there is Level 4 evidence (Savic et al. 2010) that health may remain stable.

• SCI may represent a model for premature aging.
• There is strong evidence that the endocrine and musculoskeletal systems are prematurely aging, while there is limited evidence for the respiratory, skin and subcutaneous tissues, genitourinary, and gastrointestinal systems.
• There is weak and limited evidence that the immune and nervous system are prematurely aging.