Epidemiologic Perspective

Epidemiologic observational studies of older adults in the community and in acute and chronic care institutions have identified risk factors for falls; all suggest that risk increases as the number of risk factors increases. Table 103-1 summarizes risk factor profiles by the setting in which older adults were studied. Across settings, altered mobility and cognition are major risk factors for falls. Interestingly, it is possible to be too immobile to fall.¹¹ For example, in one chronic care setting, residents with fair standing balance had the highest fall rates, those with good standing balance had intermediate fall rates, and persons with poor standing balance had the lowest fall rates.¹² Risk factor patterns and appropriate preventive interventions may differ substantially by overall mobility capacity; active older adults may experience falls and injuries for reasons very different than those for persons who stand and walk with difficulty, who in turn may fall for very different reasons than those who cannot stand.

Risk factors for injurious falls may differ from risk factors for all falls. In a study of older adults in residential care facilities, fracture risk among fallers was higher in those with better balance and no history of falls, perhaps suggesting that in some persons, activity increases the risk of producing sufficient force to fracture.¹³ Risk factor profiles are also limited in that they generally only identify chronic and stable risk factors, sometimes called predisposing risk factors. Many falls might occur because an individual with a predisposing risk has additional acute precipitating factors.¹⁴ For example, an older adult with limited mobility and cognition might not become a faller until he or she develops diarrhea, becomes dehydrated and dizzy, and tries to rush to the bathroom. Because risk factor studies have rarely accounted for these more transient and dynamic precipitating contributors, much less is known about them.

Risk factors for falls overlap substantially with risk factors for other geriatric syndromes.¹⁵ Older age and impairments in cognition, mobility, and function are risk factors for falls, incontinence, delirium, and frailty.¹⁵ Thus, there is a population of older adults with multiple impairments who are at risk for numerous geriatric syndromes, including falls and injuries. There may be other populations of older adults who are at risk for different types of falls; active older adults may fall and injure themselves during demanding activities, whereas very immobile older adults may fall out of bed and chairs or even be dropped by caregivers. In the future, more distinct risk factor profiles might be defined based on overall mobility status.

Epidemiologic studies have also identified environmental risk factors for falls. Virtually all falls can be considered the result of interactions between a person and her or his environment. The individual has some level of ability to move, and the environment has some level of challenge. The issue is the context in which the person and environment interact. The risk factors identified in Table 103-1 are sometime called intrinsic factors because they relate to the individual, and risk factors associated with the environment are sometimes called extrinsic factors. The typical older faller experiences a fall in an environment and while performing tasks that would not cause a healthy young person to fall. This phenomenon of falling under a low challenge is the rationale for believing that many falls in older people are due largely to intrinsic factors. If an older adult has many intrinsic risk factors, it is possible that only modest problems with the environment or modest degrees of challenge in a task will precipitate a fall.

Table 103-2 lists environmental risk factors for falls. Commonly reported indoor environmental factors are uneven walkways, loose rugs, absence of grab bars in the bathroom, and poor lighting.^16,17 Outdoor hazards are less frequently assessed but might be important, especially for more active older adults. Other extrinsic elements, such as the availability of help, may be an important factor in falls among persons living in the community and are even more likely to contribute to falls in an institutional setting.¹⁰ Additional risk factors for falls include psychological and attitudinal characteristics such as risk preference.^11,18 Other risk factors for injury include osteoporosis, low body weight, and fall direction.¹⁹

From a physiologic perspective, balance dysfunction results from impairment in one or more of the following systems: peripheral sensory receptors for input, central nervous system (CNS) structures for processing sensory input and planning motor output, and effector organs to carry out the movement plan (Box 103-1). In many situations, it is the combination of deficits across these systems that produces instability and falls.

There are three main sensory systems used for balance—vision, somatosensation, and vestibular function. Visual functions such as acuity, depth perception, dark adaptation, contrast sensitivity, and peripheral vision help determine body position and trajectory in space and monitor the environment.²⁰ Because bifocal glasses prioritize two focal lengths, one in the lower field at about 20 inches for reading and one in the upper visual field at about 20 feet for distance, bifocals can limit acuity in the critical zone in front of the feet while walking.²¹ Diseases of the aging eye that affect multiple visual functions are common; these include glaucoma, macular degeneration, and cataracts. Medications that cause miosis, or constriction of the pupil, can reduce dark adaptation.

Peripheral sensation is important for balance. These sensors provide information about the position of the body relative to the support surface and gravity and reflect the relationship of one body part to another during rest and movement. Peripheral sensation is the most important system for monitoring the characteristics of the weight-bearing surface and distribution of body weight onto the feet. Peripheral sensory loss is common in older adults due to diabetes and peripheral vascular disease. In case of peripheral sensory loss, visual systems can compensate by supplementing information about body position. Thus, a combination of vision loss and peripheral sensory loss can create serious problems with the ability to monitor body position.

The vestibular system detects the position of the head with respect to gravity, monitors linear and angular acceleration of the head, and coordinates head and eye movements to maintain gaze and visual field stability while moving. Vestibular system impairments can occur with usual aging and are affected by ischemia or head trauma.²² Several widely recognized vestibular conditions, such as benign paroxysmal vertigo and perhaps Meniere disease, are common in older adults. Others are also common but less well recognized, such as chronic bilateral vestibular hypofunction.²³

The CNS has numerous structures that contribute to balance. Structures in the brainstem and spinal cord are considered central pattern generators that produce stepping behaviors. Balance is further controlled by higher level brain structures, including the frontal cortex, basal ganglia, cerebellum, and motor cortex. Degeneration of brain regions can affect balance, as in the basal ganglia in Parkinson disease and in cerebellar degeneration. All brain processes depend on adequate levels of brain perfusion, so any threats to perfusion affect central processes of gait and balance. Thus, many of the conditions that produce syncope or presyncope can cause transient cerebral hypoperfusion and falls, such as orthostatic hypotension, tachyarrhythmias, bradyarrhythmias, and critical aortic stenosis.²⁴

There has been increased awareness of the role of more diffuse microvascular brain disease as a contributor to balance disorders and falls.²⁵ This mechanism is posited to involve ische­mia in vulnerable brain areas, especially the frontal lobes, and in important white matter tracts connecting the frontal lobes to critical subcortical areas. Radiologically, this ischemia is manifested as leukoaraiosis (white matter disease) on magnetic resonance imaging scans. White matter disease has been associated with specific patterns of cognitive dysfunction involving psychomotor slowing and altered attention and executive functions, such as sequencing and visual spatial organization.^25–27 These cognitive abnormalities have been associated with alterations in gait, especially excessively variable length and timing of stepping, and with falls.^28,29 Thus, there is an emerging concept of altered balance and gait due to abnormal nonamnestic cognitive functions and movement planning produced by regional microvascular brain disease. This condition may be manifest by irregular walking patterns and exacerbated by placing stress on cognition and movement. Tasks that simultaneously stress cognition and movement are part of an emerging conceptual approach to balance assessment termed dual tasking.^30,31 Older adults whose performance worsens substantially when asked to walk and solve cognitive problems simultaneously may be at increased risk for falls.³²

The CNS also operates a multisynaptic righting reflex that produces automatic correcting movements when balance is lost. Typically appearing as a stepping response that occurs much more quickly than what can be done voluntarily, this righting reflex is lost in many CNS disorders, including extrapyramidal diseases and other forms of multisystem atrophy. Sedation, clinical or subclinical, may further reduce alertness and attention. Thus, both sleepiness and sedative medications have been found to increase fall risk.^33,34 Less specific but potentially important are psychological factors, such as fear of falling and risk preferences.^11,35

Muscles and joints are effector organs critical for balance and mobility. Muscle weakness is widespread in older adults and can be due to primary muscle mass loss (sarcopenia), disorders of peripheral nerves or neuromuscular junctions, or inactivity. Specific muscle diseases such as inflammatory myositis or steroid myopathy can produce proximal muscle and trunk weakness that presents with falls. Lower motor neuron conditions, radicular nerve deficits due to spinal stenosis, or peripheral nerve damage can result in localizing strength deficits that affect specific muscle groups and more distinct functional movements. For example, a foot drop due to damage to the peroneal nerve prevents the forefoot from lifting to clear obstacles and can induce trips and stumbles. Evidence has suggested that low vitamin D levels may contribute to muscle weakness and falls.³⁶ Low testosterone levels have been found to be associated with falls in men younger than 80 years.³⁷ Other common conditions such as arthritis can reduce range of motion and produce pain that alters stepping and weight bearing.³⁸ Deformities distort the weight-bearing surfaces of the foot and cause pain. Fatigue, generalized or localized to muscle, can contribute to loss of balance. Thus, acute conditions such as overworked muscles or chronic conditions associated with fatigue such as anemia³⁹ or congestive heart failure might increase risk for falls. Although traditional assessments of strength have focused on the extremities, more recent thinking has emphasized the role of trunk or core strength involving the abdomen and pelvis as critical to postural control, which is a novel avenue to pursue interventions.⁴⁰

When fall onset is abrupt or there is a major change in balance, the likelihood is higher that there is a medical event that has affected the CNS. Any illness or episode that reduces cerebral perfusion through hypoxia, decreased oxygen-carrying capacity, or hypotension could present as dizziness, lightheadedness, unsteadiness, and falls. Toxic or metabolic abnormalities due to medications, infection, or electrolyte disorders could present as unsteadiness and falls through effects on attention. New focal neurologic deficits due to stroke can also present as unsteadiness. Older adults with predisposing subclinical balance disorders could be more vulnerable to such precipitating factors. These types of physiologic acute, and sometimes transient, changes are harder to include in research studies because stable chronic effects are usually the focus of an investigation.

Biomechanical Perspective

A biomechanical approach to fall risk is based on the concepts of mass, force, momentum, and acceleration of the body as a whole, as well as of body segments. The standing human body is a long tall column that rests over a small base of support. The main task of movement is to displace and recover this column while the base of support changes. Therefore, the assessment of balance includes two main conditions: (1) static balance, defined as steadiness of the fixed column over a constant base of support; and (2) dynamic balance, defined as control of the column and supporting structures during movement. The most essential and classic dynamic balance task is walking. Walking involves alternating the use of one leg to support the body while the other swings from behind to in front of the body. There is an extensive knowledge base about normal and abnormal walking based on a set of biomechanical characteristics; it uses a specialized terminology. Walking can be characterized by the pattern of steps using spatial factors such as step length, stride length (distance between two heel contacts from the same foot), and step width (Figure 103-1). Walking can also be characterized by temporal factors such as double support time (the duration of the stride when both feet are on the ground at the same time) and cadence (step frequency). Walking has been described as controlled falling because the body column moves forward past the base of support, and the feet must be timed to contact the support surface at the right location and time in anticipation of the moving location of the trunk.⁴¹ When this timing is altered by disease, gait becomes irregular, and trunk movement can be altered. Walking can also be characterized by changes in other body segments and joints. During normal walking, the foot begins a step with a push-off from the toe, lifts the foot to swing through, and ends with a heel strike and forward rolling foot contact to initiate the next push-off. The knee is in full extension at the time of toe push off, swings forward with slight flexion to aid foot clearance and then returns to full extension at the point of heel contact. The hip is in extension behind the trunk at push off and swings into flexion at heel contact. The arms swing alternately and in a sequence opposite to the step sequence of the legs. Biomechanical factors that have been found to be abnormal in fallers encompass both static and dynamic balance. Abnormal static balance associated with falling is manifested as increased sway during quiet standing. Some studies have suggested that increased sway to the side, or mediolateral plane, is especially associated with falls.⁴² Alterations in dynamic balance among fallers are diverse, depending on the underlying cause. Frequently observed abnormalities include prolonged double support time during walking, increased step width, increased trunk sway during movement, increased or decreased toe clearance, reduced hip extension, abnormal lateral stepping, and delayed correcting movements at the hip or ankle when the trunk is displaced.^43,44