We thank Amiram Catz and colleagues for providing an opportunity to clarify any misunderstandings about our recent paper [1] that addressed how to better measure a treatment effect in a clinical trial after spinal cord injury (SCI). To increase the acceptance of SCAR by clinical investigators, the tool evolved through Rasch analysis of current assessment tools that could be modified to track changes in voluntary motor activities during recovery from SCI [1]. Although it incorporates some transformed items from the Spinal Cord Independence Measure (SCIM), as well as upper extremity motor scores (UEMS), it was not intended to replace SCIM [2]. SCIM is a valuable tool for describing many body functions after SCI, but it is a multidimensional ordinal assessment and was not designed to be an objective primary endpoint for a SCI trial.

The ultimate tool for any trial measurement is an interval scale that is unidimensional with the differences between scale scores being known and unchanging (i.e., linear). The clearest examples are measurements of time or distance. Understandably, such measures do not lend themselves to comprehensively describe body functions after SCI. Assessment of CNS trial endpoints has relied on subjective multidimensional ordinal scales (e.g., UEMS, SCIM), which were developed as descriptive classification tools and not intended as clinical outcome measures as differences between item scores are unequal and often a subjective evaluation based on the expertise and experience of the evaluator. However, it is possible to transform ordinal data by methods such as maximum likelihood estimation algorithms (e.g., Rasch analysis) to measure and test an underlying unidimensional trait. Our goal was to develop a unidimensional measure for acute and sub-acute SCI trials that would enable investigators to repeatedly and reliably track one dimension of SCI recovery, volitional motor behavior, as a metric for the effectiveness of an interventional treatment.

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