CHAPTER 79: MODIFIED ASHWORTH SCALE

Description

The modified Ashworth Scale is designed to measure an individual’s muscle tone, which is generally defined as the resistance of muscle as it is being passively lengthened or stretched and occurs as a result of the intrinsic synergies of muscle, tendon, and connective tissue as well as the active contraction of muscle (Blackburn, van Vliet, & Mockett, 2002). As increased resistance to passive stretch (hypertonus) is common in a number of disorders, such as stroke or cerebral palsy, its assessment and management is a major component of many rehabilitation protocols and the modified Ashworth Scale quantifies that resistance (Blackburn et al., 2002). The Ashworth Scale was originally developed by Ashworth in 1964 as a taxonomy to assess the anti-spastic effects of carisoprodol in multiple sclerosis subjects. Through his research he described a 5-point ordinal scale for grading the muscle resistance he encountered when moving a limb through the range of motion during passive stretching of the muscle. Relative to spasticity, the assessment grades the subject’s level of muscle tone as follows: (0) normal muscle tone; (1) slight increase in muscle tone, “catch” when limb moved; (2) more marked increase in muscle tone but limb is easily flexed; (3) considerable increase in muscle tone; and (4) limb rigid in flexion or extension. The Ashworth Scale was modified by Bohannon and Smith (1987) by adding the grade of 1+ as well as slightly modifying definitions of muscle tone to render the scale more discrete. The observational assessment takes less than 5 minutes to complete as the person is graded on the ability of his or her limb to move passively through the range of motion.

Psychometrics

Bohannon and Smith (1987) found inter-rater reliability of manual testing of elbow flexor muscle spasticity of 30 patients with intracranial lesions to be r = 0.87. Comparing the original and modified assessments using Kappa values, which indicate the measure of agreement corrected for chance (1.0 = total agreement between raters, 0 = no agreement), Haas, Bergstrom, Jamous, and Bennie (1996) found Kappa values ranged between 0.21 and 0.61 (mean 0.37) with the original scale being slightly more reliable though not statistically significant. A study of 20 patients with chronic spinal cord injury found that intrarater reliability ranged from fair to almost perfect (0.20 to 1.0) and differed significantly between raters and inter-rater reliability was poor-to-moderate (0.60) for all muscle groups. Inter-session reliability for a single rater was found to be only fair-to-good (0.40 to 0.75) for all muscle groups (Craven & Morris, 2010). While a study by Li, Wu, and Li (2014), using 2 raters, found that kappa values were 0.66 and 0.69 for elbow flexors and 0.48 and 0.48 for plantar flexors in study of 51 subjects with hemiplegic stroke, suggesting that alternative or complimentary measures be considered in conjunction with the modified Ashworth Scale.

Advantages

Both the original and modified versions of the Ashworth Scale are relatively simple and easy assessments to administer and there is a fair amount of research in support of their use in clinical practice. A unique aspect of both versions is that they allow the clinician to isolate the muscle being tested and each muscle test can be completed in only a couple of minutes. Information purveyed from results can also be used to gauge effects of interventions as well as establishing a baseline assessment of an individual’s muscle tone.

Disadvantages

Lack of agreement between research studies have been noted, as problems have arisen with the ability to reproduce adequate intra- and inter-rater reliability results (those > 0.85) in many studies. Others have noted that the modified Ashworth Scale scoring remains ambiguous and is less reliable than the original version of the assessment and there is some research to suggest that the assessments appear to be more reliable for the upper extremities. (Pandyan et al., 1999). There may be several reasons for this, such as the assessment’s construct or that resistance to passive movement is a complex measure that may be influenced by many factors.

Administration

During assessment the subject is placed in the supine position. If testing a muscle that primarily flexes, the joint is placed in a maximally flexed position and moved to a position of maximal extension over a 1 second count of “one thousand one,” while keeping in mind to grade the ability of the joint to move through the passive range of motion relative to the patient’s muscle tone while considering the inclusion/exclusion criteria of the levels of the scale.

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