Aging: CNS changes

Structural changes:

• Loss of neural tissue
• Thickened leptomeninges in spinal cord
• Decreased number of serotonin, acetylcholine, and dopamine receptors

Functional changes:

• Reduction in cerebral blood flow (decreases about 10-20%, in proportion to neuronal loss)
• Decline in memory, reasoning, perception
• Disturbed sleep/wake cycle
• Increased threshold for many sensory modalities including touch, temperature, sensation, proprioception, hearing, and vision

As one ages, there is a gradual decrease in brain size, thought to be due to decreased neuronal size. This results in widened sulci and enlarged ventricular volume. This “shrinkage” of the brain leads to a larger subdural “space,” which explain why elderly patients are at risk for a subdural hematoma.

The number of neuroreceptors and neurotransmitters decrease even in the absence of dementia or recognized neuro-degenerative disease. Most significant declines are in acetylcholine and serotonin in the cortex, dopamine receptors in the neostriata, and dopamine levels in the substantia nigra and neostriata.

Common Anesthetic Medication Classes and CNS Effects on Aging

• Volatile anesthetics: Elderly patients require decreased inspired concentrations of volatile anesthetics. MAC decreases by approximately 6-7% every decade after age 20.
• Propofol: Both induction doses and maintenance doses of propofol are decreased. Propofol’s hemodynamic effects can be greatly exaggerated in older patients as well.
• Opioids: Elderly patients require smaller doses for similar effect and have an increased incidence of respiratory depression and increased duration of systemic and neuraxial effects. This is due to decreased volume of distribution of these agents, among other reasons.
• Benzodiazepines: Display a significant increase in duration of action. Long-acting benzodiazepines such as lorazepam and diazepam have been associated with delirium in the elderly.
• Atropine: dosing may need to be increased to achieve a given heart rate response in the aging patient.
• Neuromuscular Blockers: Aging in and of itself does NOT increase sensitivity to muscle relaxants at the NM junction, however, co-morbidities often associated with aging (i.e. renal dysfunction) may do so.
• Neuraxial anesthesia in the elderly can be problematic as exaggerated spread of local anesthetic can occur in the epidural space. Conversely, a longer duration of action is expected from a spinal anesthetic.

Postoperative Cognitive Dysfunction

Postoperative cognitive dysfunction (POCD) is a cognitive disorder unique to patients after anesthesia. Unlike delirium, patients with POCD are not acutely confused or agitated. In some studies 10% of older patients (age > 60) developed POCD in the 3 months following noncardiac surgery. Its occurrence has been associated with increased mortality rate. Its etiology is likely multifactorial and includes drug effects, pain, underlying dementia, hypothermia, and metabolic disturbances.