Frailty and outcome after traumatic brain injury

In older age, frailty is defined as a homoeostatic imbalance due to multiple subclinical conditions that, together with the occurrence of ageing stressors, can exacerbate a functional decline in physiological reserves of systems.1 After two decades of clinical research, there is no consensus about a single operational definition of frailty, nor any universally accepted assessment tool, reference standard, clinical criteria, or biological markers. Three main frailty models are commonly used (figure).