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B. J. H. van Lith et al.
baseline was based on tests with a known perturbation
direction. As a consequence, the perturbation intensities
might have been too high for the participants following
perturbations in unknown directions. This may also
(partly) explain the much lower number of participants
able to sustain perturbations with unknown compared
with known directions. Another reason might be that
reactive steps following unexpected perturbations are
relatively strongly influenced by delayed postural re-
sponses that occur in patients with HSP (4, 26), because
patients cannot compensate by an anticipatory “central
set” if the direction is unknown. Nevertheless, the
improved balance capacity following perturbations in
known directions suggests that hip adductor spasticity
indeed impairs the quality of lateral stepping responses
and, thus, is a relevant treatment target from a frontal-
plane balance perspective.
The fact that the beneficial effects on muscle tone,
gait width, comfortable gait velocity, lateral balance
persisted until 16 weeks after treatment seems to chal-
lenge the common opinion that the biological effects
of BTX-A have worn off after this time interval. It
may, therefore, be that the stretching component of the
treatment protocol was responsible for the observed
long-term effects.
Study limitations and future perspectives
A limitation of the present exploratory study in pa-
tients with HSP is the relatively small sample size
and the lack of a control condition, while the rather
stringent inclusion and exclusion criteria limit the
generalizability of our findings. Nevertheless, this
study provides indications for the beneficial effects
of bilateral BTX-A injections in the adductor longus,
adductor magnus and gracilis muscles and subsequent
stretching of these muscles on gait width, comfortable
gait speed, and reactive lateral stepping in known
perturbation directions, whereas maximal gait speed,
gait endurance, and clinical balance scores appear to
be less responsive in these patients. Future research
should preferably be multi-centred to increase the
number of participants and use a randomized controlled
design. Gait width and gait speed would be valuable
and responsive outcome measures in such trials. The
instrumented balance assessments used in the present
study are, however, less suitable for multi-centre stu-
dies as they require further development of clinically
affordable systems. Clinically applicable assessments
to validly test the quality of (lateral) reactive stepping
responses are therefore urgently needed. As HSP is a
chronic and progressive condition, it would also be
relevant to conduct longitudinal, comparative cohort
studies to investigate whether repetitive cycles of BTX-
www.medicaljournals.se/jrm
A treatment of spastic hip adductors improve lifetime
functional ambulation prognosis in these patients.
ACKNOWLEDGEMENT
This investigation was supported by an unrestricted grant from
Merz Pharmaceuticals.
The authors have no conflicts of interest to declare.
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