Hurler syndrome was first described to predominantly involve the skeletal system9. Dysostosis multiplex refers to the complex of orthopaedic abnormalities resulting due to defective membranous and endochondral growth10. The effect of these orthopaedic abnormalities on locomotor ability of the affected child is profound. Children with Mucopolysaccharidosis 1 have below average gross motor abilities11 and delayed maturation of gait12. Orthopaedic abnormalities including thoracolumbar kyphosis in this group can be progressive and surgical intervention may be justified. Thoracolumbar kyphosis with a Cobb’s angle of more than 40 degrees in children with Hurler syndrome is most likely to progress13. Our justification for spinal surgical intervention in this cohort of patients is based on clinical practise guidelines by International consensus procedure developed using a modified Delphi approach7 with the eventual goal of treatment is to maintain or improve the quality of life.
There are seven types of Mucopolysaccharidosis and three subtypes of Mucopolysaccharidosis 1 (A-Hurler syndrome, B-Hurler-Scheie syndrome and C-Scheie syndrome). This study is the first attempt to assess functional outcome after Spinal deformity correction in patients with MPS of any subtype. Data obtained from GAITRiteTM electronic walkway assessed spatio-temporal parameters of gait in this group of children before and after correction of thoracolumbar kyphosis. GAITRiteTM walkway has been proven to have good reliability with immediate re-test, 2week re-test and Intraclass correlation coefficients >0.75 for measuring spatio-temporal parameters of gait14 along with strong concurrent validity15.
The nearest pre-operative and post-operative gait assessment data within 9 to 24 months of the index surgery were used for analysis. Children were found to be recovering from the index spinal surgery when assessed within 9 months of surgery. Therefore, time was given for the child to recover from the spinal surgery and then gait assessment was done. After 24 months there was a possibility of deterioration of gait due to the progression of disease in the hip and knee joints affecting global locomotor ability. Physiological maturation of gait is another potential confounding variable in growing children. Gait in the MPS study group may be expected to mature more slowly than the unaffected child. Restricting the post op assessment time frame to 9 months to 24 months helps us identify changes in the gait which can be attributed to spinal surgery and not due to physiological development. Gait analysis data of one patient alone was obtained 35 months after index spine surgery as that was the only available post-operative assessment data for that patient.
Walking distance is a good indicator of the overall ability of bipedal locomotion to propel the body from point A to point B. Walking distance or distance travelled on the walkway is measured in centimetres on the horizontal axis from heel centre of first foot print to the heel centre of last foot print8. This parameter is not affected by the disease process in the joints of the lower extremities or individual footfalls. Parameters such as stance phase can be affected if the disease process has resulted in antalgic gait from an affected hip joint. Similarly, step length can be recorded as a negative value if the child fails to bring the landing foot heel point forwards of the stationary foot making parameters of individual footfalls unreliable. Nine out of eleven patients in the study group exhibited a statistically significant increase in walking distance postoperatively (mean postoperative increase of 232.06 cms, P-value 0.05). There is no literature evidence to standardize normal walking distance according to age in this cohort.
Cadence is calculated as the number of steps taken per minute. Reducing cadence in a growing child is an indicator of maturing gait16. All the children in our study were assessed within 9 to 24 months from the index spine surgery, which was a cross-sectional analysis in a surgical cohort and not a linear assessment of gait maturation. We observed that there was a trend towards reduction of cadence postoperatively in six out of the eleven patients in the study group, which was not statistically significant (P-value 0.79). Therefore, physiological development did not have significant impact on gait during our assessment time frame postoperatively.
Gait velocity was calculated on the walkway after dividing the distance travelled by ambulation time. Measuring gait velocity reflected the ability or the ease with which the children walked after Spinal surgery. Gait velocity in our study increased in seven out of the eleven patients postoperatively. However, the mean increase in gait velocity by 8.73 cms/sec postoperatively was also not statistically significant (P-value 0.32). Cadence and gait velocity showed only a marginal improvement postoperatively.
Sagittal spinal curvatures help in maintaining global sagittal spinal balance and aid bipedal locomotion. A balanced spine requires minimal muscular effort to maintain an upright posture. Thoracolumbar kyphosis alters spinal biomechanics by resulting in an anterior shift of the truncal mass with increased flexion moment arm. An increased sagittal spinal curvature with increased flexion moment arm alters physiological loading of compression and shear forces across spinal segments17. Thoracolumbar kyphosis renders the normally strong dorsal extensor paraspinal musculature weak, affecting the ability to maintain an upright posture. The anterior truncal shift stretches the dorsal extensor paraspinal musculature beyond a point where the length-tension relationship is altered18,19.
Compensatory effect of sagittal spinal deformity is commonly observed in the neck and in the lower limbs. A backward tilt of the pelvis, flexion of the knees and dorsiflexion of the ankles are some of those seen in the lower extremity. The effort required for the lower limb to function in conjunction with the compensatory mechanism results in early fatigue with poor exercise tolerance. Children in this cohort have joints in lower extremities affected by the same disease process, adding to the burden of poor gait. Therefore, changes in sagittal spinal balance and poor lower limb function can both affect gait. Surgery to correct thoracolumbar kyphosis in our cohort is an attempt to restore sagittal spinal alignment which in turn has shown a functional improvement of gait. The reported improvement of gait in our study was attributed to restoration of sagittal spinal alignment as none of the children in the study group had any intervention to address the disease process in the joints of lower extremities within the post-operative assessment timeframe. Three children in our study group had hemiepiphysiodesis of knee joint done more than year before the index spinal surgical intervention. This will not have any impact on our study as we have taken the nearest pre-operative gait assessment as baseline.
Spine is the first component to get activated during gait and lumbar spine is the key component which drives the pelvis. EMG studies have shown early truncal muscle activation anticipating propulsion and gait initiation 20,21,22 proving that functioning of lower limbs follow spinal motion during gait. We did not use the standard 6 minute walk test as the joints of the lower limbs were affected by the same disease process which may result in early fatigue. Gaitrite walkway is a validated tool 14,15 in assessing spatiotemporal parameters of gait and we believe walking distance of one minute better reflects gait efficiency before lower limb fatigue sets in.
The process of maturation of gait in children with Mucopolysaccharidosis 1 is delayed when compared to normally developing children11. Improvement in cadence postoperatively was not statistically significant proving that physiological maturation had minimal effect on gait in the post-operative assessment timeframe. Therefore, the trend towards improved gait observed within the postoperative window (9 to 24 months) was due to surgical correction of the spinal deformity rather than developmental maturation of gait.
A limitation of our study was small sample size. Eleven patients from a single sub type (MPS1) is a large number, but remains a statistically small sample size. Studies with small sample size have a higher margin of Type 2 error and can over estimate the magnitude of association. However, we did manage to find out a statistically significant result (walking distance p-value 0.05) in one of the parameter analysed. Therefore, the positive association between spinal surgery to correct thoracolumbar kyphosis and gait should be considered a surrogate end point to design further multi centre studies with larger sample size. A larger sample size may yield statistically significant results in other parameters as well,
HhHowever, we do reflect the population of the largest centre for treatment of severe MPS1 in Europe and multi centre studies of surgical outcomes have proven challenging due to the variability in indication and procedure used in this group.