Overview and Description
Understanding of the neurologic consequences of healthy aging is important to Physical Medicine & Rehabilitation providers for a number of reasons. First, many physiatrists treat older adults in their practices. Second, distinguishing neurologic changes of healthy aging from those of age-related disease influences decisions about the need for evaluation and treatment. Third, as the population ages, PM&R physicians will see increasing numbers of healthy older adults who desire recommendations for physical activities they can participate in for health maintenance and disease prevention. Physiatrists are uniquely qualified to provide such guidance, which takes into account the unique characteristics of the healthy older adult.
Information from human research is presented in this article. However, some information is only available from animal research, due to the difficulty of performing invasive anatomical and physiological studies in humans.
Prior to presenting details, several introductory remarks are in order regarding normal biological aging.
- For the purposes of this section, biological aging is defined as a process of progressive, intrinsic, and cumulative cellular and organ changes that result in decrease of the organism’s ability to withstand stressors. These changes eventually result in the organism’s decreased ability to survive. Biological aging is also characterized by universality across populations, cultures, and physical environments.1
- The exact causes of aging remain unknown. Many theories of aging are supported by experimental evidence, but a single unifying framework has yet to be found. Prominent themes in these theories include accumulation of free radical damage, imbalance between cellular damage vs. repair, and disorders of mitochondrial function.2
- A well-known characteristic of biological aging is increased variability in the normal presentation, whether it be in populations, organ systems, or tissues.3 Not all cells in a tissue age at the same rate, nor do all tissues and organs age at the same rate within an organism. Likewise, in populations of healthy older adults, there is a wide range of what can be considered to be normal function.4,5 This variability at times can blur the distinction between healthy aging and disease of aging. Therefore, the clinician may benefit from observation of patients over time, in order to better determine the nature and rate of change in the neurologic findings.3
Relevance to Clinical Practice
In this section, the framework of a standard neurologic examination will be used to briefly summarize common neurologic changes of healthy aging. By definition, these changes are not sufficient to impair an individual’s ability to function well, commensurate with their age. They are not subclinical presentations of diseases of aging. As noted above, the considerable variability in aging populations will result in a wide range of presentations within these categories.
Mental status, cognition 5-10
The most consistently found change in cognition of healthy older adults is slowed processing speed, especially for new or complex information. Experts have noted that processing speed is such a pervasive part of cognition that it is difficult to test other aspects of cognition without involving a component of processing speed. Other common changes of cognitive aging include: decreased mental flexibility, decreased maintenance of sustained and/or divided attention, and decrements in working memory (ability to manipulate information held in short term memory/fluid intelligence). Not decreased, however, are semantic memory (vocabulary, recall of facts learned in past/crystallized intelligence), and ability to learn new skills. Aging changes in other sensory systems can influence testing performance, and so need to be accounted for.
The anatomic findings associated with these cognitive changes are not uniform throughout the brain. Findings from human MRI studies include decreased volume of gray matter in the prefrontal cortex (associated with executive functioning) and in the hippocampus (associated with memory). This decrease is not thought to be due to large amounts of neuron loss, but rather to decrease in neuron soma size, decrease in synapse number and decrease in complexity of dendrite branching. MRI studies of older brains have also reported decrease in white matter volume. This finding is related to increased myelin degeneration, decreased myelin repair, and shortening of myelinated nerve fibers. These changes in myelination are associated with slowed nerve conduction velocities. Due to the volume loss of cortex and white matter, ventricles may enlarge slightly. Accumulation of neurofibrillary tangles also occurs in brains of healthy older persons.11
Depression is not considered a requisite neurological change of aging. However, with the potential multiple stresses that older adults may experience (retirement, death of spouse, loss of physical health, etc.), depression is an important condition to recognize and treat. The incidence and prevalence of depression in the geriatric age group is projected to increase as the world’s population ages.12 Theories regarding the etiology of depression include decrease in neurotransmitter function and alterations in cell signaling,13 factors that are also influenced by aging.7
Age-related stiffening of the lens results in decreased lens accommodation for near vision (presbyopia). Other vision changes of aging include: slowed adjustment to low light, decreased color discrimination, decreased depth perception, and decreased ability to detect contrast. Visual acuity may also decrease, and upward gaze can be limited. Many of these visual changes are related to a drop-out of photoreceptors and retinal ganglion cells. Slowed adjustment to dark is also related to slowed pupillary reflexes, and color discrimination is affected by yellowing of the lens with aging.
Hearing 5,15, 16
High frequency hearing loss (presbycusis) becomes more common with increasing age. This problem is often related to atrophy of the capillary bed in the stria vascularis, or damage to cochlear hair cells, but can also be significantly influenced by environmental noise exposure. High frequency hearing loss can affect ability to discern specific speech sounds, such as consonants. Older adults may also have difficulty understanding speech in noisy environments. This problem is thought to be due not only to hearing loss, but also to altered auditory processing. The increased cognitive effort of listening may cause the older adult with hearing loss to show decreased memory for spoken information.
Motor system (strength, tone, posture and gait) 5,17,18,19
Sarcopenia is the age-associated loss of muscle mass and function, which results in decreased muscle strength and power. Factors related to sarcopenia include decreased physical activity, altered hormone status, decreased total caloric and protein intake, increased inflammatory mediators, and altered protein synthesis.17 Sarcopenia is also associated with increased motor unit size. Animal studies have also identified age-related alterations in neuromuscular junction structure, including increased branching of the pre-synaptic nerves, and receptor redistribution in the post-synaptic membrane.
There are several aging-related changes in posture and movement. Posture may become mildly forward flexed, gait speed may be slowed, and arm swing decreased. Step length shortens and duration of double support phase increases. These changes may be influenced also by the presence of age related-diseases such as osteoarthritis. Taken together, such changes can predispose the older adult to the serious problem of falls.
Deep tendon reflexes at the ankles may be decreased in up to ~30% of older adults by the age of 80 years. Other deep tendon reflexes are usually preserved. So-called pathological reflexes (palmo-mental, glabellar, snout, etc.) may be present in the healthy older adult for unknown reasons. Observation of the patient over time could be helpful in determining whether or not they may indicate sub-clinical neurologic disease.
The most common sensory change in older adults is a decrease in lower extremity vibratory sensation (in about 30% of persons over age 60 yrs.). Upper extremities tend not to show this decrement. Sensation for light touch and pain are generally preserved.
Due to the enlargement of motor units with aging, there can be a decline in accuracy of fine motor task performance. Healthy aging is not associated with ataxia.
Older adults experience shorter sleep cycles, more frequent night-time waking, and earlier morning waking.
EMG/NCV 22, 23, 24 Aging motor and sensory peripheral nerves show loss of neurons. These changes result in slowed nerve conduction velocities, and decreased action potential amplitudes. Nerve conduction velocities decrease by 0.5-4.0 m/s per decade in persons over age 60 years old; action potential amplitude may decrease by as much as 50% by age 70 years old.24 Due to anterior horn cell loss, with reinnervation, aging motor unit action potentials (MUAPs) have longer durations, more phases, and lower amplitudes than in younger adults.
Pertinence to patient care: 25
Care of older adults may be enhanced by implementing the following suggestions:
- Cognition and communication: allow adequate time for information processing and learning, use less complex sentence structure, present one task at a time; encourage participation in measures to maintain good health in physical, cognitive and social domains.8
- Depression: consider exercise an important part of the prevention and treatment of depression. Aerobic and strength training have both been shown to improve mood.13
- Vision: provide adequate lighting without glare, identify potential environmental hazards (i.e. stairs) with high-contrast visual cues.14
- Hearing: provide quiet environments for conversation, present spoken information slowly and clearly, position listeners so they can see the speaker’s face and lips, and encourage hearing aid use as appropriate. However, hearing aids will not assist with auditory processing, given its relationship to aging changes in the central nervous system.
- Sarcopenia: include resistance training in exercise recommendations to address decreased physical activity and weakness; consider adding power training commensurate with patient ability.19 Protein supplementation may also have an important role in addressing decreased dietary protein intake and altered protein synthesis. Adequate protein intake is vital to building and maintaining muscle mass.26
- Falls risk, posture and gait changes: include exercise in the prevention or treatment plan. Options include specific balance exercises, aerobic and resistance training. tai chi and/or function-based exercises. 27
Additionally, in order to evaluate and follow an older adult’s motor function and balance, a number of standardized assessment tools are available for use in the clinic. Such tools include the 6 Minute Walk Test, the Timed Up and Go test, and the Berg Balance Assessment (Rehabmeasures.org).
- Sleep: recognize the influence that inadequate sleep can have on cognitive function.
Cutting Edge/ Unique Concepts/ Emerging Issues
The extreme complexity of the nervous system leaves much that is yet to be studied. New technology (functional MRI, positron emission tomography, etc.) enables new accessibility to non-invasive and quantitative study of anatomic and physiological parameters in the human brain. Findings to date appear to show re-organization of some brain networks with aging, which may be related to maintenance of cognitive functions.28 Such techniques may not only provide additional knowledge about normal neurologic function, but could also potentially provide data to allow for earlier diagnosis of neurologic disease, thereby assisting in the provision of early therapies.
Another research use of imaging technology is the employment of computed tomography (CT) to determine skeletal muscle cross-sectional area as an index for sarcopenia. Such a determination if often used in research studies to predict outcomes. For example, sarcopenia has been associated with poor perioperative outcomes, especially in geriatric patients.29
Gaps in Knowledge/ Evidence Base
The relationship between exercise, physical fitness, and cognition is a new and growing area of study. Quantitative MRI techniques have documented increased gray matter volumes in pre-frontal cortex and hippocampus in individuals who are at least moderately physically active.30 However, the relationships between cortical volume and cognitive function are just beginning to be understood. Much more research is necessary before firm conclusions can be reached about the contribution of exercise to cognitive health in aging.
Fatigue in older adults is another topic that merits further study.31 Fatigue is a common complaint in the geriatric population, and oftentimes an etiology is not easily determined. Treatment therefore is difficult. Greater understanding of fatigue in aging may assist in decreasing morbidity.
- da Costa JP, Vitorino R, Silva G, Vogel C, et al. A synopsis on aging-Theories, mechanisms and future prospects. Ageing Res Rev. 2016;29:90-112.
- Lipsky M, King M. Biological theories of aging. Disease-a-month. 2015;61:460-466.
- Fedarko N. The biology of aging and frailty.Clin Geriatr Med. 2011;27(1):27-37.
- Schott, JM. The neurology of ageing: what is normal? Pract Neurol. 2017;17:172-182.
- Galvin JE. Neurologic signs in older adults. In: Fillit HM, Rockwood K, Young J, Eds. Brocklehurst’s textbook of geriatric medicine and gerontology, 8th ed. Philadelphia, PA: Saunders Elsevier; 2017:105-108.
- Grajauskas LA, Siu W, Medvedev G, Guo H, et al. MRI-based evaluation of structural degeneration in the ageing brain: Pathophysiology and assessment. Ageing Res Rev.2019;49:67-82.
- Liu H, Yang Y, Yuguo X, Zhu W, et al. Aging of cerebral white matter. Ageing Res Rev. 2017;34:64-76.
- Martin J, Li C. Normal cognitive aging. In: Fillit HM, Rockwood K, Young J, Eds. Brocklehurst’s textbook of geriatric medicine and gerontology, 8th ed. Philadelphia, PA: Saunders Elsevier; 2017: 171-178.
- Oschwald J, Guye S, Liem F, Rast P, et al. Brain structure and cognitive ability in healthy aging: a review on longitudinal correlated change. Rev Neurosci. 2020;31(1):1-57.
- Seraji-Bzorgzad N, Paulson H, Heidebrink J. Neurologic examination in the elderly. Handb Clin Neurol. 2019;167:73-88.
- Crary JF, Trojanowski JQ, Schneider JA, Abisambra JF, et al. Primary age-related tauopathy (PART): a common pathology associated with human aging. Acta Neuropathol. 2014;128:755–766.
- Eyre H, Baune B, Lavretsky H. Clinical advances in geriatric psychiatry. A focus on prevention of mood and cognitive disorders. Psychiatr Clin N Am. 2015:495-514.
- Sadock BJ, Sadock VA, Ruiz P. Kaplan & Sadock’s Concise Textbook of Clinical Psychiatry. 4th ed. Philadelphia, PA: Wolters Kluwer/Lippincott Williams & Wilkins; 2017: 60-63, 907.
- Whiteside MM, Wallhagen MI, Pettengill E. Sensory impairment in older adults: part 2: vision loss. Am J Nursing. 2006;106(11):52-61.
- Pichora-Fuller MK, MacDonald E. Sensory aging: hearing. In: Hof PR, Mobbs CV, Eds. Handbook of the neuroscience of aging. Burlington, MA: Academic Press Elsevier; 2009:193-198.
- Stine-Morrow EAL, Shake MC. Language in aged persons. In: Hof PR, Mobbs CV, Eds. Handbook of the neuroscience of aging. Burlington, MA: Academic Press Elsevier; 2009:287-292.
- Doherty TJ. Invited review: aging and sarcopenia. J Appl Physiol.2003;95(4):1717-27.
- Hepple RT, Rice CL. Innervation and neuromuscular control in ageing skeletal muscle. J Physiol. 2016;594 (8):1965-1978.
- McKinnon NB, Connelly DM, Rice CL, Hunter SW, Doherty TJ. Neuromuscular contributions to the age-related reduction in muscle power: Mechanisms and potential role of high velocity power training. Ageing Res Rev. 2017;35:147-154.
- Aagaard P, Suetta C, Caserotti P, Magnusson SP, Kjaer M. Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure. Scand J Med Sci Sports. 2010;20(1):49-64.
- Feinsilver SH, Hernandez AB. Sleep in the elderly: unanswered questions. Clin Geriatr Med.2017; 33:579-596.
- Freeman TL, Johnson EW, Freeman ED, Brown DP, Lin L. Electrodiagnostic medicine and clinical neuromuscular physiology. In: Cuccurulo SJ, Lee J, Bagay L, Eds. Physical Medicine and Rehabilitation Board Review, 4th ed. New York NY: Demos Medical Publishing; 2020:355.
- Howard JE, McGill KC, Dorfman LJ. Age Effects on Properties of Motor Unit Action Potentials: ADEMG Analysis. Ann Neurol.1988;24:207-213.
- Preston DC, Shapiro BE. Artifacts and technical factors. In: Preston DC, Shapiro BE, Electromyography and neuromuscular disorders. Philadelphia PA: Elsevier; 2021:80-81.
- Rodriguez V, Rakar M. Normal versus abnormal exam. In:Chun A, Ed. Geriatric practice: a competency based approach to caring for older adults. Cham, Switzerland: Springer Nature Switzerland, 2020:50.
- Beasley JM, Shikany JM, Thomson CA. The role of dietary protein intake in the prevention of sarcopenia of aging. Nutr Clin Pract. 2013;28(6):684-690.
- Stubbs B, Brefka S, Denkinger MD. What works to prevent falls in community-dwelling older adults? Umbrella review of meta-analyses of randomized controlled trials. Phys Ther. 2015;95:1095-1110.
- Gonzalez-Escamilla G, Muthuraman M, Chirumamilla VC, Vogt J, Groppa S. Brain Networks Reorganization During Maturation and Healthy Aging-Emphases for Resilience. Front. Psychiatry 2018; 9:601. doi: 10.3389/fpsyt.2018.00601
- Jones KI, Doleman B, Scott S, Lund JN, Williams JP. Simple psoas cross‐sectional area measurement is a quick and easy method to assess sarcopenia and predicts major surgical complications. Colorectal Dis. 2015;17(1):O20-O26.
- Erickson KI, Leckie RL, Weinstein AM. Physical activity, fitness, and gray matter volume. Neuro Biol Aging. 2014;35:S20-S28. 31. Zengarini E, Ruggiero C, Pérez-Zepeda MU, Hoogendijk EO, et al. Fatigue: Relevance and implications in the aging population. Exp Gerontol. 2015; 70:78-83.
Original Version of the Topic
LeAnn Snow, MD. Neurological consequences of aging. Published 10/2/2015
LeAnn Snow, MD
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