Assessment of postural control after spinal cord injury or disease: A narrative review

Introduction

Following spinal cord injury or disease (SCI/D), physical rehabilitation is focused on regaining independence for community living and maximizing quality of life. The multi-disciplinary rehabilitation team facilitates the retraining of daily activities (e.g., bathing, feeding) and mobility (e.g., walking, wheeling, transferring), and assists with the reintegration of vocational and recreational activities. The priorities of rehabilitation are also informed by people living with SCI/D, who have identified improving upper limb and trunk strength, sexual function, bowel and bladder function, and walking ability as being most important for life satisfaction ( ). Common to the safe and successful completion of all of these tasks is the requirement of postural control. Without postural control, movement is limited and independence compromised.

The term postural control is often used interchangeably with the terms balance or balance control. Indeed, these are similar and inter-related constructs. Postural control refers to one’s ability to maintain, achieve, or restore a state of balance at any given time ( ). From a biomechanical perspective, balance refers to the relationship between one’s center of mass and one’s base of support ( ). When the vertical projection of one’s center of mass moves outside of one’s base of support, muscle activity is required to change the base of support or reposition the center of mass within the base of support. Without this corrective muscle activity, a fall would result. Achieving a state of balance, or postural control, is required when maintaining a posture (e.g., sitting or standing), moving voluntarily, or reacting to an external perturbation (e.g., trip, push) ( ).

For many individuals living with SCI/D, postural control is an ongoing challenge due to the sensorimotor deficits associated with their injury. Falling is common after SCI/D; 78% (95% confidence interval: 73%–83%) of ambulators fall each year, while 69% (95% confidence interval: 60%–76%) of wheelchair users fall ( ). Falls have a significant detrimental impact on those living with SCI/D ( ; ). Up to 62% of ambulators and 38% of wheelchair users have reported a physical injury from a fall, with most injuries being minor in nature (i.e., cuts, bruises) ( ). The most common serious injuries include fractures and head trauma with a loss of consciousness ( ). More than 50% of chronic wheelchair users with SCI/D experience osteoporosis ( ); hence, these individuals are at high risk of sustaining a fall-related fracture ( ).

In addition to physical injury, those who fall or who have a high fall risk may develop a fear of falling, defined as an enduring concern about falling that causes one to avoid activities despite being physically able to perform them ( ). About 47% to 63% of people living with SCI/D report a fear of falling ( ; ). This fear often leads to a “post-fall syndrome” characterized by dependence, loss of autonomy, depression, anxiety, reduced mobility and restricted participation in daily activities ( ). Individuals living with SCI/D describe the harmful psychosocial impact of falls, including feelings of vulnerability and embarrassment, lost work productivity, interference with parenting, and a restriction of meaningful recreational activities ( ). The constant threat of falling, and fear of the potential consequences of a fall, can have a significant impact on quality of life ( ).

Chapter objectives

Postural control is inherent to our ability to move and function safely, but is commonly impaired following SCI/D. Hence, improving postural control is a target of rehabilitation assessment and intervention across the continuum of care (i.e., from acute care to community rehabilitation). Regardless of neurological level of injury, severity of the SCI/D (i.e., as indicated by the American Spinal Injury Association Impairment Scale (AIS)), time since injury or mobility status, an evaluation of postural control is warranted for all individuals living with SCI/D. Yet, clinicians and researchers lack guidance on how to approach the assessment of postural control after SCI/D, especially given the heterogeneity in clinical presentation of this population. The primary aim of this narrative review is to outline an individualized and comprehensive approach to the assessment of postural control after SCI/D. The secondary aims are to: (1) describe common impairments in postural control following SCI/D, and (2) describe the current state of postural control assessment for individuals with SCI/D.

Comprehensive assessment of postural control

More than 30 measures of postural control have been used in research studies with the SCI/D population ( ). Hence, the challenge of postural control assessment post-SCI/D is not the availability of measures, but rather deciding which measure(s) to use. In their systematic review, considered the psychometric properties, clinical utility and comprehensiveness of each measure. A measure’s comprehensiveness is a relatively new concept that reflects the complexity of postural control. Postural control requires the complex interaction of numerous musculoskeletal and neurophysiological mechanisms, which are effectively outlined in the Systems Framework for Postural Control ( ; ). In the original framework, described six systems that contribute to postural control and suggested that appropriate evaluation of each system would enable identification of an individual’s specific impairments and compensations, promoting individualized treatments. In 2015, Sibley and colleagues led a revision of the Framework, resulting in the identification of nine contributing systems (see Table 1 ); functional stability limits, underlying motor systems, static stability, verticality, reactive postural control, anticipatory postural control, dynamic stability, sensory integration, and cognitive influences. This revised Systems Framework for Postural Control provides a basis for a comprehensive assessment of postural control for individuals with SCI/D.

Following SCI/D, impairments in postural control are expected. The spinal cord contains numerous descending motor spinal pathways that are involved in the regulation of postural control through activation of postural synergies ( ; ). There are also a number of ascending pathways, which carry somatosensory information to the brain and allow the adaptation of postural control to the environment ( ). The extent of the impairments in postural control due to the SCI/D depends on the neurological level and severity of injury. The resulting impairments are more pronounced in a sub-set of the postural control systems (see Table 1 and Fig. 1 ). All individuals with SCI/D are likely to experience impairments in functional stability limits, underlying motor systems (e.g., muscle strength and coordination), static stability and sensory integration. Research involving individuals with motor incomplete SCI/D (i.e., AIS C and D) demonstrated deficits in reactive postural control ( ; ) and cognitive influences ( ; ). For dynamic stability, the overall findings are variable. Individuals with AIS D SCI/D show variability in gait parameters, suggesting compromised dynamic stability ( ; ); however, they also show markers of greater stability during walking than age- and sex-matched able-bodied adults ( ; ). proposed that increased stability when walking may be a compensatory strategy to avoid losing balance and having to rely on an impaired reactive postural control system to prevent a fall. There do not appear to be impairments in anticipatory postural control for those individuals with the least severe SCI/D (i.e., AIS D) ( ; ), but deficits may be evident in those with greater sensorimotor deficits. Lastly, SCI/D does not affect verticality ( ), although deficits may occur in individuals with a relevant co-morbidity, such as a vestibular disorder or brain injury.

Not only do the above-mentioned physiological factors influence postural control, but so do psychological factors, such as balance confidence ( ). Balance confidence or self-efficacy refers to an individual’s perceived self-confidence in maintaining a state of balance during relatively non-hazardous activities ( ). The Systems Framework for Postural Control does not account for the effect of this psychological construct on postural control. However, due to the associations between balance confidence and postural control after SCI/D ( ; ) (see Table 1 and Fig. 1 ), a measure of balance confidence should be included as the tenth component of a comprehensive assessment of postural control (see Fig. 2 ). This inclusion allows the clinician or researcher to evaluate the alignment between an individual’s perceived balance confidence and physical abilities. For example, perceived balance confidence may be high while the physical ability to maintain a state of balance is low. These findings may suggest an individual lacks insight into his/her postural control deficits, resulting in a high fall risk due to an increased likelihood of risk-taking behaviors. Conversely, someone with low balance confidence, but capable of maintaining a state of balance during daily activities, may needlessly restrict their mobility and participation due to an exaggerated concern about falling. Hence, balance confidence adds a unique dimension to the comprehensive assessment of postural control.

Current state of postural control assessment in clinical settings

Survey studies involving more than 400 Canadian physical therapists have shed light on the postural control assessment practices for adult populations ( ; ). While the researchers reported variation in how postural control was assessed clinically, they noted that several postural control systems were regularly evaluated (i.e., by > 80% of respondents): static stability, dynamic stability and underlying motor systems ( ). In contrast, reactive postural control was evaluated by only 41% of respondents ( ). The most commonly used measures of postural control included movement observation, the Berg Balance Scale and the single-leg stance test ( ; ). In another survey by , 94% of physical therapists agreed that quantifying impairments in postural control was critical for patient care; however, only 43% of respondents agreed that existing measures of postural control were adequate for clinical practice. Moreover, even fewer (21%) indicated that these measures evaluated postural control in a comprehensive way ( ), highlighting a key gap in current approaches to the assessment of postural control.

Several organizations have produced clinical recommendations regarding the assessment of postural control. The American Physical Therapy Association and the Academy of Neurological Physical Therapy supported the development of a core set of outcome measures for adults with neurological conditions ( ). The recommended measures of postural control include the Berg Balance Scale for static and dynamic sitting and standing, the Functional Gait Assessment for dynamic balance, and the Activities-specific Balance Confidence (ABC) Scale for balance confidence. More specific to the SCI/D population, the Canadian Standing and Walking Module Group produced the Standing and Walking Assessment Tool (SWAT) ( ). The SWAT is a SCI/D-specific, staged approach to the standardized assessment of standing and walking ability post-SCI/D. Within the SWAT, the Berg Balance Scale is recommended for the majority of individuals with SCI/D (i.e., those who have at minimum the ability to stand with assistance) while the ABC Scale is recommended for those who ambulate, whether therapeutically with assistance or independently in the community ( ). In sum, these two practice guidelines are aligned and highlight appropriate measures of postural control for the SCI/D population. However, as outlined below, the Berg Balance Scale and ABC Scale on their own do not fulfill the requirement of a comprehensive and individualized approach to the assessment of postural control after SCI/D.

concluded their systematic review with recommendations for the clinical assessment of postural control after SCI/D. They concluded that of the > 30 measures reviewed “no single measure had high clinical utility, strong psychometric properties and comprehensiveness” ( ). The Berg Balance Scale and Functional Reach Test were highlighted for their well-established psychometric properties in the SCI/D population, while the mini-Balance Evaluation Systems Test (mini-BESTest) was identified as the most comprehensive measure that has been used with individuals with SCI/D. The mini-BESTest was developed from the original Systems Framework of Postural Control, and is one of the only clinical measures that evaluates reactive postural control ( ; ). Since completed the database searching for their review, the mini-BESTest has been shown to be a valid and reliable measure of postural control for individuals with motor incomplete SCI/D ( ; ). Given the comprehensiveness, strong psychometric properties and high clinical utility of the mini-BESTest, its use among the SCI/D population is expected to increase.

Instrumented assessments of postural control

In addition to the above-mentioned clinical measures, there are instrumented assessments of postural control involving force plates and inertial measurement units (IMU) that have primarily been used in research settings. These instruments can be adapted to facilitate assessment of the components of postural control, with the exception of balance confidence. In their review, highlighted the use of instrumented assessments of postural control among the SCI/D population. Center of pressure-based measures of postural sway, quantified with force plates, were most commonly used ( ). Force plates consist of a rigid surface instrumented with multiple force transducers allowing the measurement of forces in three dimensions. Force plate-based measures of postural sway are considered the gold standard measure of postural control in standing ( ). They are a valid and reliable tool for the evaluation of standing postural control among individuals with incomplete SCI/D ( ). More specifically, force plates have been used to quantify the performance of functional stability limits ( ; ), underlying motor systems ( ), static stability ( ), anticipatory postural control ( ), dynamic stability ( ), sensory integration ( ) and cognitive influences ( ). More recently, force plates have been used to assist with the identification of spatiotemporal deficits in reactive postural control ( ), with some metrics showing adequate validity and reliability ( ).

IMU are being increasingly used to quantify postural control during sitting ( ), standing ( ) and walking ( ). IMU typically include accelerometers to measure linear acceleration and gyroscopes to measure angular velocity, and may include magnetometers to measure the amplitude and direction of magnetic fields ( ). Information characterizing postural control in static standing can be collected with a single IMU ( ); however, multiple sensors are needed to extract information about more complex and dynamic postural tasks ( ; ). Their reliability and/or validity for use during sitting, standing and walking with individuals with SCI/D have been established ( ; ; ). IMU can be used in a variety of contexts and for a variety of tasks, and therefore address a number of the components of postural control. For example, among the SCI/D population, IMU have been used to quantify underlying motor systems ( ), static stability ( ), dynamic stability ( ), sensory integration ( ; ) and cognitive influences ( ).

Although IMU and force plates can facilitate a comprehensive assessment of postural control, these tools tend to have low clinical utility due to their high cost, high level of skill required to administer and interpret the measure, and in the case of force plates, their low portability ( ). As low-cost alternatives to these instrumented tools are developed, their use in clinical settings is likely to increase.