Do patients with ankylosing spondylitis have poorer balance than normal subjects?
H. C. Murray 1,2,
C. Elliott 1,,
S. E. Barton 2 and
A. Murray1
1 Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne, and
2 Division of Physiotherapy, Bradford University, Bradford, UK
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Abstract
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Objectives. To investigate whether patients with ankylosing spondylitis have poorer balance than normal subjects, and to study the relationship between balance and posture.
Methods. Balance was studied in 30 ankylosing spondylitis subjects using sway magnetometry, making quantitative measurements of movement at the hips with eyes open and eyes closed. The results were compared with data from 58 normal subjects. Balance was also compared with quantitative measurements of posture.
Results. The numbers of patients with poor balance, above the 95th percentile for normal, were significantly greater than expected; 18% for eyes open
(P = 0.03) and 23% for eyes closed
(P = 0.004). No significant relationships between balance
and any of the quantitative descriptions of posture were demonstrated.
Conclusion. A significant proportion of ankylosing spondylitis
patients have poorer balance than normal subjects.
KEY WORDS: Ankylosing spondylitis, Balance, Sway, Posture, Magnetometry.
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Introduction
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Ankylosing spondylitis (AS) is a rheumatic disease with a prevalence at 1% of the general population [1]. It is commonly associated with a young population, the average age of diagnosis being 24 yr [2]. Pain and stiffness progress with advancing disease. Following inflammation at the margins of the vertebra, calcification ensues. This can result in the fusion of the affected joints [3], with the advanced stage producing a rigid spine.
The fusion of joints and the adoption of a less painful position can cause shortening of soft tissues and the development of a stooping posture [4]. Forward flexion tends to afford the AS sufferer the greatest relief of pain, although it heightens the formation of soft tissue contractures and calcification of entheses [5]. Once this progression to a forward flexed posture has begun, it requires the patient to use more energy to prevent leaning further forward [5]. Physiotherapy and self-management aim to prevent a stooping posture [6]. Viitanen and Suni [7] in their review of the literature concluded that physiotherapy benefited AS patients by reducing stiffness and correcting posture. Balance problems relate to poor posture [3]. Postural changes have also been implicated in balance impairment in other clinical areas [8, 9]. Russell [2] remarked on the loss of balance in AS
patients being associated with severe joint deformities and falls, and Viitanen and Suni [7] on the changes in spinal curvature being associated with changes in equilibrium. However, there are no quantitative measurement data on balance in
AS patients.
The aims of this study were to quantify balance in AS patients, determine the proportion with abnormal balance and assess the relationship between balance and posture.
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Balance measurement techniques
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Force platforms are a common method of measuring balance [10]. They measure the position of the centre of force with strain gauges. However, the accuracy of force platforms for measuring balance has been questioned [11]. Sway magnetometry has also been used. This technique measures movement at the hips in the horizontal plane using electromagnetic transmitter and receiver coils. FitzGerald et al. [12] concluded that sway magnetometry had statistically greater sensitivity than the force platform, both for eyes open and for eyes closed. This is perhaps due to the changes in movement being recorded directly at the hips in sway magnetometry. Sway magnetometry has been used to assess 58 subjects with no known balance problems, from 15 to 64 yr of age [13]. This has provided normative data for comparison with patient data for eyes open and eyes closed. There was no statistical difference between sway data for males and females, and no detectable age effect in this normal group.
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Methods
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Patients
All patients studied had been referred to a hospital physiotherapy department for assessment and rehabilitation. Thirty-one AS patients consented to take part. Only one patient, who was pregnant, was excluded. There were no other exclusion criteria. Of the remaining 30, 21 were males and nine were females between the ages of 27 and 67 yr (average age 44 yr). They had been diagnosed with AS for between 4 months and 24 yr (average 10 yr). The severity of AS was classified as mild, moderate or severe, using the definitions in Table 1
. The overall classification was defined as the median grade for each patient. None of the patients had any known coexisting balance disorder due to vestibular or middle ear abnormality.
Measurement of posture
Measurements of posture were taken on one occasion prior to the balance test. Shoes and tops were removed for accurate measurement. The measurements made are described below.
Macrae's modification of Shober's test [3].
The measurement was started with the subject in an upright posture. The posterior superior iliac spines (dimples of venus) were palpated to determine the level of the 5th vertebrae and the lumbosacral junction. From this level, 10 cm was measured superiorly and marked, and 5 cm inferiorly and also marked. The subject was asked to flex their spine. The distance between the two marks was recorded.
Tragus to the wall [2].
The subject was asked to stand upright with heels against the wall. A measurement was then taken from the tragus to the wall using a tape measure.
Chest expansion [14].
A tape measure was placed around the thorax at the level of the fourth rib. The subject was asked to expire fully followed by a maximum inspiration. The difference between the two readings was taken as the measurement of chest expansion.
Measurement of balance
The calibration of the sway magnetometry equipment was checked at the beginning of each session. This involved moving the receiver coil a known distance of 160 mm and ensuring the measured distance had an error of <5 mm, and ensuring that with no movement of the coils the measured distance indicated due to noise was not more than 5 mm. Calibration was well within tolerance for all test sessions. The subject was asked to stand on a footprint template, which separated the heels by 4 cm and angled the feet at 30 degrees. The footprints were placed at approximately 1 m from a wall. All measurements were taken with shoes off. Two receiver coils were placed on a belt and the belt attached just inferior to the iliac crests. One coil was placed centrally in a posterior position to record anterior/posterior sway and the other coil placed laterally, to record lateral sway. The belt was tightened to prevent it slipping. Subjects were asked to keep their knees straight, hold their arms loosely by their sides, look straight ahead, focusing on a cross on the wall when eyes open, and remain as still as possible for the 30 s measurement period. Distractions and interruptions were avoided, and curtains were used to prevent visual distractions. A full description of the measurement technique has been reported [12].
Measurements with eyes open and eyes closed were completed twice on the same occasion. The average sway path lengths at the hips for both conditions were calculated. When compared with other measurements, such as mean distance from the sway centre, area of movement and area of a circle enclosing the movement locus, path length had previously been shown to give the smallest inter-subject variability for eyes open and eyes closed, and the most reliable detection of the small increase in sway on eye closure [15].
Statistical analysis
To determine whether AS patients have poorer balance than normal subjects, the chi-squared test was used to test the differences in the proportion of AS subjects above the limits of normal, compared with normative data. These data and the limits of normality were derived from a previous completely independent study [13].
To determine whether the proportion of AS subjects with abnormal balance was related to severity (mild, moderate or severe), a chi-squared test was used to test the proportion of AS subjects in each of the severity classifications that exceeded the normal limit against the expected proportion. This was performed for each of the three methods of posture measurement independently and for the combined (median) result of the three methods.
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Results
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Balance in AS subjects compared with normal subjects
Path lengths for eyes open and eyes closed are plotted against patient age in Fig. 1
. The percentage and number of AS subjects with normal and abnormal balance compared with the normative data are given in Table 2
. The numbers of AS subjects above the limits of normal for eyes open and eyes closed were statistically significant for both eyes open (P = 0.03) and eyes closed (P = 0.004). There was no significant difference between the means of the two groups (Table 3
).

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FIG. 1. Path length of sway for measurements over a period of 30 s with eyes open and eyes closed, plotted against age for all 30 AS patients. There was no relationship with age. Data are presented in this way for comparison with published normal data. The 95th percentiles for normal data are drawn.
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TABLE 2. The percentage (and number) of subjects with poor balance with eyes open and eyes closed in each classification
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AS severity and balance
For each severity subgroup classified by a combination (median value) of the three posture measurement methods, the measurements of path length for eyes open and eyes closed are given in Table 3
, and the percentage (and number) of subjects with poor balance given in Table 2
. There were no statistically significant differences in balance between the mild, moderate and severe subgroups. This also applied when any of the three individual measurement methods was used.
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Discussion
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Poor balance was not a problem for all AS individuals, and the majority were within normal limits. However, a significant proportion of AS patients demonstrated balance impairment both with eyes open (P = 0.03) and eyes closed (P = 0.004). Although there is no previous quantitative research connecting AS and balance, it was expected that some AS patients would deviate from normal. Khan [3] related compromised balance and injuries to a poor posture in AS patients, and in general the importance of posture and its relationship to balance is widely accepted [9].
Poor posture, decreased range of movement and pain are commonly associated with balance impairment. It has been confirmed by this study that AS patients are more likely to have poor balance than normal subjects. The AS subjects with poor balance were distributed throughout the three classifications of mild, moderate and severe. The statistical power of the study does not permit a weak association with severity to be excluded, and more patients would need to be studied to detect such a relationship.
Clinically, our findings indicate the need for awareness of potential balance disorders and the need for monitoring of balance impairment in some AS patients. Since balance problems increase and spinal mobility decreases with age in a normal population [7], balance monitoring and re-education may have greater clinical importance with increasing age. The test is easy to perform, and can help identify patients with balance impairment. This would allow the progression of the disease in individual patients to be studied and the effect of treatment to be assessed. Sway magnetometry proved to be a useful tool in researching balance. In longer term studies it should be able to monitor improvement or deterioration over time.
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Acknowledgments
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We thank physiotherapists Ms Jenny Ross, Ms Bronia Smallman, Ms Alison McClelland and Consultant Rheumatologist Dr I. Griffiths for help with this study. This work was undertaken by the first author as part of the final year requirements of the BSc Degree in Physiotherapy at the University of Bradford with the support of the University Undergraduate Projects Committee.
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Notes
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Correspondence to: C. Elliott, Audiology Department, Freeman Hospital, Freeman Road, Newcastle upon Tyne NE7 7DN, UK. 
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References
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Submitted 2 June 1999;
revised version accepted 19 November 1999.