The quantification of the tonic and phasic components is of utmost importance in the post-acute stage of SCI for both the choice of therapy and monitoring its effectiveness. A variety of spastic patterns is often observed in the clinical setting with contradictory results obtained for the two components. For instance, an individual with a predominance of hypertonia has his lower limbs stiffened, difficult to stretch or bend, and handling them is not easily accomplished. It limits his/her mobility and ability to perform ADL. On the other hand, an individual with a predominant phasic (dynamic) component of spasticity has highly excitable limbs, the handling of which is not limited by muscle hypertonia but can be associated with sudden rapid spontaneous movements that may increase the risk of falling during ADL. Moreover, spasms can interfere with the ability to relax while resting and thus can contribute to the individual's overall physical and psychological discomfort. In the chronic stage, the individual partly adapts to the spasticity and the quantification of the presentation in the sense of medication adjustment becomes less relevant. However, the assessment is important in case of a change in health status as well as for clinical trials.
Benz et al. presented a scale to assess the dynamic component of spasticity. Their team developed the Spinal Cord Assessment Tool for Spastic Reflexes (SCATS) to rate flexor and extensor spasms and clonus [5]. The validity of the tool was tested by comparing electromyographic and kinematic measurements, and a significant correlation was found between them. This tool has the disadvantage of needing to involve three different tests which yield three different scores ranging from 0 to 3 points. Apart from being more time-consuming, the examination provides results that are difficult to compare not only to each other but also to the MAS outcomes.
Penn et al. designed a scale based on a self-report questionnaire for people after spinal cord injury [6]. Their scale has two parts to assess the frequency and severity of spasms [7]. Mills et al. have tested the reliability of the Penn Spasm Frequency Scale (PSFS) in chronic SCI people and found it “almost perfect” [8]. However, it has the disadvantage of analyzing based on self-reported information, with the individual focusing on spasms associated with specific situations, such as difficulty moving from the wheelchair. Spasm frequency also varies with the individual's activity level over time. Benz et al. found no correlation between the PSFS and SCATS scores, which may be due to the fact that flexor and extensor spasms are triggered during specific ADL [5].
The assessment of the dynamic component of spasticity is also partly included in the SCI-SET questionnaire which reflects the impact of spasticity on ADL in people with chronic SCI. Responders are asked how negatively or positively their spasticity symptoms have affected various aspects of their daily life within the past seven days. The authors report high test-retest reliability, suggesting a stable impact of spasticity on ADL in the chronic stage of SCI. Moreover, a satisfactory correlation with other subjective tests was found [9].
The Muscle Excitability Scale (MES) was designed to complement the Modified Ashworth Scale (MAS), which measures hypertonia, in the clinical assessment of spastic motor behavior in SCI people [10]. MES is a simple clinical tool for quantifying motor response to stimulation on a five-point scoring scale. It has the advantage of being rapid and easy to use. Thus, both MAS and MES scores are available to the investigator, making it possible to characterize the person's spastic motor behavior more comprehensively. In our opinion, assessing the spasticity of individual muscle groups in the lower limbs is irrelevant to the therapeutic considerations. Therefore, in order to assess generalized spinal spasticity we deliberately simplified the examination to passive triple flection and subsequent extension, each lasting one second. In contrast, examining spasticity on both the right and left lower limbs is important due to the often-asymmetric SCI and will allow a response to the higher grade of the two.
After a long evolution and refinement of MES and MAS in a modified examination procedure we proceeded to the selected psychometric tests. Inter-rater and intra-rater reliability for MES was estimated as moderate using weighted kappa. These results are consistent with the high variability of muscle excitability based on many variables. Unlike muscle hypertonia which does not change significantly in response to repeated movement during the examination, muscle excitability often responds to repeated stimuli by a sharp decrease in intensity, even for small stimuli. Therefore, we applied the above-mentioned procedure with the individual being examined in the supine position in the bed without any prior adjustment to avoid the induction of spasms. For the same reason, joint ranges of motion need to be examined in advance. We focused only on passive triple flection and subsequent full extension of the leg. This also provided us with an idea of MAS. The simultaneous examination of both components is supported by other authors [11].
Reliability assessment of MAS for both lower limbs in our cohort showed higher inter-rater and intra-rater reliability, considered substantial, than MES. This indicates less variation in the tonic than the phasic component of spasticity. Other authors have also addressed MAS reliability with discordant results. For example, Haas et al. found only fair inter-rater reliability between a physiotherapist and a physician when assessing different lower limb muscle groups in 30 individuals with SCI [12]. Craven and Morris concluded that MAS has inadequate inter-rater and inter-temporal reliability in assessing lower limb spasticity [11]. On the other hand, Baunsgaard et al. found satisfactory reliability of MAS in 31 participants using weighted kappa [13]. Similarly, Apkinar et al. reported moderate to substantial inter-rater and intra-rater reliability for MAS when assessing spasticity in 65 participants with SCI [14]. Despite these discrepancies in psychometric testing, MAS is the most widely used tool in both routine clinical practice and research studies. The substantial agreement in our assessment of MAS reliability in the context of other studies indicates, among other things, that a well-standardized examination procedure was chosen by us and was applied by well-trained and experienced raters.
We compared the MES outcomes with the subjective PSFS rating. As for spasm frequency, a low correlation was found between MES and PSFS for the two raters. These results were unsurprising because spasm frequency depends on daily activities, as mentioned above. When compared for spasm severity, MES and PSFS showed a higher correlation classified as moderate by one of the raters. Despite the subjective assessment and the unclearly defined scale grades, this part of PSFS reflects the intensity of muscle excitability more accurately.
The muscle excitability assessment shows a disproportionate distribution of grades in favor of grade 2. There are two main reasons for this. Grades 0 and 1 were missing as only individuals who showed signs of muscle excitability were included in the study. The highest prevalence of grade 2 indicates compensated muscle excitability in most people, 26 of whom were on appropriate medical therapy. In contrast, the MAS grades were more evenly distributed with a slight predominance of grades 1 and 0. The representation of grades 3 and 4 was relatively high despite 14 people taking the maximum dose of myorelaxants.
LIMITS OF THE MES
Despite a simple and unequivocal assignment to grades based on the clinical presentation, the MES scores may vary for the same patient. This may be due to the examiner's skillfulness and experience with tactile contact and handling the patient. These influence both the strength of the exteroceptive stimulus and the way of moving the patient's limb. Similarly, the level of clinical experience may play a role in motor response assessment. A patient's muscle excitability may also be influenced by certain factors such as his/her mental condition, fatigue, the weather, preceding activities, etc. These are probably more significant than in the assessment of hypertonia. The response to the first stimulus differs considerably from that to repetitive stimuli. Therefore, it is important to measure the intensity of the phasic component at the beginning of the examination after at least five minutes of relaxation in the supine position.