Balance Amongst Patients With Lumbar Spinal Stenosis

Disease Background:

While lumbar spinal stenosis (LSS) is one of the most common degenerative diseases of the spinal column, there is no universally accepted definition of LSS, and no accepted radiologic diagnostic criteria. LSS most often refers to a narrowing in the central canal of the vertebrae, the lateral recess, or the neural foramen. Changes to these can occur due to acquired degenerative spondylosis or spondylolisthesis, or more rarely due to conditions such as ankylosing spondylitis and space-occupying lesions, or congenital abnormalities. LSS can be classified according to anatomical location, etiology or severity of narrowing, though no validated classification has been published.

The lack of concrete definition has caused difficulties in estimating the prevalence of LSS. Studies using community-based sampling has shown a prevalence of acquired LSS, defined as a narrowing of the central canal to ≤ 10mm in the anterior-posterior direction, of 7.3%. The prevalence has been shown to rise with age, from 4.0% affected at < 40 years of age, to 14.3% amongst patients ≥ 60. No significant differences have been observed in overall prevalence according to gender, although there seems to be a slightly higher prevalence amongst elderly females than males.

Although LSS is often asymptomatic, common symptoms of LSS include low back pain, which worsens with prolonged ambulation, lumbar extension and standing, and which is relieved by rest and forward flexion, as well as lumbar radiculopathy. Patients may also complain of poor balance, and physical examination findings may include a wide-based gait and abnormal Romberg results. Symptoms are thought to occur due to compression of microvascular structures in the nerves, allowing for neural ischemia and defects in nerve conduction, and venous pooling resulting in inadequate oxygenation and metabolite accumulation.

Current treatment options for LSS range across both conservative and surgical management strategies. Conservative management has traditionally been regarded as first-line treatment, with a combination approach of physical therapy and pharmacological treatment with NSAIDs and analgesics. Epidural steroid injections have been used for symptom management, though with limited short and long-term benefits. Surgical management is often indicated in patients with ongoing pain despite attempts at conservative management for 3-6 months. Choice of surgical strategy to relieve the pressure on the neural structures depends on the anatomical location of stenosis and number of stenotic segments, as well as the intraoperative assessment of stability.

The effect of surgical decompression on disability, leg pain and back pain has been widely evaluated, but studies of the effect on postural control are sparse. The present study aims to investigate the effect of surgical decompression of symptomatic lumbar spinal stenosis on postural control by assessment of sway measures before and after surgery.

Postural Balance:

Mechanical balance is defined according to Newton's first law, stating that an object is at equilibrium (balanced) when the forces acting upon it are zero. When considering the balance of an object, the objects centre of gravity (COG) and its base of support (BOS) must be examined. For an object to remain balanced, a vertical line drawn from the COG to the objects BOS, the point of intersection defined as the centre of pressure (COP), must lie within the objects BOS. When the COP goes beyond the BOS, objects become unbalanced and may topple. If in a state of imbalance, humans and animals have physiological compensatory mechanisms to ensure that their COP returns to lie within their BOS. The act of keeping the COP within the BOS is what is intuitively know as human balance.

No central definition of postural balance exists, but a widely cited definition is 'the act of maintaining, achieving or restoring a state of balance during any posture or activity'. Postural balance has previously been considered a summation of postural reflexes alongside sensory feedback mechanisms, with a single neural circuit system integrating somatosensory, vestibular, and visual systems to maintain equilibrium. However, efforts have been made to move away from this viewpoint, and the act of postural balance is now seen as an independent motor skill, incorporating complex physiological mechanisms as well as high-level somatosensory information processing, influenced by individual body schema and personal cognitive elements. Maintaining postural balance is now seen as a summation of two simultaneous and overlapping systems; one to control posture through tonic muscle activity, and one to maintain equilibrium when exposed to internal and external perturbations. Balance is hereby maintained using skeletal muscle with both phasic and tonic activity, with signals originating from distinct neural circuits including from the brain stem, cerebellum, motor cortex etc.

As of writing, no gold standard exists to measure balance, which has allowed a wide variety of techniques to be developed to measure postural balance skills amongst patients, and variables associated with postural control, either through instrumented or non-instrumented tests. Regarding the latter, expert consensus points to the use of the Berg Balance Scale or the Mini Balance Evaluation Systems Test when measuring postural balance in adult populations. However, while non-instrumented tests can give a gross indicator of postural balance, there exists a need for more granular analysis, which can be achieved using instrumented tests. The force platform remains the most widely used device for this purpose, allowing for the quantification of positional and dynamic variables associated with COP position, displacement, and trajectory.

The causes of imbalance are many. Any deficit or pathology relating to somatosensory, motor control or cognitive systems associated with postural control can lead to postural imbalance, including a wide range of pathological conditions such as visuo-vestibular and neurodegenerative diseases, as well as orthopaedic and rheumatological diseases of the musculoskeletal system. While aging itself does not seem to be an independent risk factor for postural imbalance, age-related changes in physiology have been linked to degeneration in postural imbalance, including decreased visual acuity, somatosensory loss, vestibular dysfunction, lower extremity muscle weakness and cognitive impairment.

As mentioned, objects which become unbalanced due to COP dislocation beyond the objects BOS may fall. Balance impairment, measured by dynamic variables using force plates as well as non-instrumental tests, have been shown to be predictors of falls amongst elderly. Falls represent a large healthcare burden, having serious risk of related injury, rising prevalence with age and being a significant cause of mortality amongst elderly. Furthermore, falls have been shown to be an independent risk factor of functional decline. As such, fall prevention is a topic of both national and international interest.

Spinal stenosis has shown to be a risk factor for both postural imbalance as well as a risk factor for falls. Previous studies have made efforts to quantify the effects of decompressive surgery on postural imbalance, though have remained limited by number of patients and short follow-up regimes, as well as lacking correlation between measurements of postural imbalance and risk of falls.

Wii Balance Board:

The Wii Balance Board (WBB), first released in 2007, has gained popularity as a low-cost, portable force-plate transducer, to use in postural balance assessment. It contains four transducers used to measure COP, as well as a scale to measure weight, and was originally used as a video game controller for the game WiiFit. Although capable of providing an intrinsic WiiFit Stillness score, this has shown limited usability in previous studies. However, using custom software, the WBB has been extensively validated against 'gold standard' laboratory-grade force plates, and shown to have good within-device and between-device reliability, as well as being a valid tool to measure balance amongst older adults. Work has been done to ensure that the WBB can be calibrated to minimize measurement error, and open-source algorithms exist for the calculation of positional and dynamic variables recorded by the WBB. While not recommended for clinical diagnostic measurements, the use of the WBB in longitudinal monitoring for research purposes remains viable.

Rationale of the study:

This study will be the first to correlate the effects of decompressive surgery in patients with symptomatic LSS with postural balance, and associated quality of life increases. Previous studies have been hampered by low power due to sampling size limitations, and short follow-up regimes, both of which is sought to be managed through multi-centre collaboration and inclusion, and a follow-up regime spanning two years from the time of surgery.

It is expected that the results of this study can facilitate an increased understanding of the role of postural balance when considering surgical management of symptomatic LSS patients, as well as enable targeted treatment of patients with postural imbalance and LSS.

Research question:

Do elderly patients with symptomatic LSS, who have undergone decompressive surgery, show an improvement in postural balance compared to pre-operative values?