Functional Systems Scores (FSS) and Expanded Disability Status Scale (EDSS)
The FSS and EDSS constitute one of the oldest and probably the most widely utilized assessment instruments in MS (Kurtzke, 1983). Based on a standard neurological examination, the 7 functional systems (plus "other") are rated. These ratings are then used in conjunction with observations and information concerning gait and use of assistive devices to rate the EDSS. Each of the FSS is an ordinal clinical rating scale ranging from 0 to 5 or 6. The EDSS is an ordinal clinical rating scale ranging from 0 (normal neurologic examination) to 10 (death due to MS) in half-point increments. The FSS and EDSS have been used in virtually every major clinical trial that has been conducted in MS during the last four decades and in numerous other clinical studies.
Administration time will vary depending upon the condition of the patient and the skill of the examiner. Although the FSS and EDSS themselves can be rated in a few minutes, the neurological examination that is needed to make the ratings can take anywhere from 15 minutes to a half-hour.
The FSS and EDSS are administered in person by a trained examiner, most often a neurologist. However, nurse practitioners with the proper training can also complete the neurological examination and rate the FSS and EDSS.
The FSS and EDSS are ordinal clinical rating scales that are rated on the basis of the judgment of the examiner. Each of the FSS and the EDSS are single-item scales and there is no composite or summed score. The FSS include pyramidal, cerebellar, brainstem, sensory, bowel and bladder, visual, cerebral (or mental), and other.
Download the FSS Form (PDF) and the EDSS Form (PDF).
The FSS and EDSS were developed in the 1950's and refined in the 1980's to provide a standardized measure of global neurological impairment in MS. Although some clinicians use these measures they are utilized primarily in clinical studies, especially clinical trials. The EDSS has frequently been used as a component of the primary or secondary outcomes in clinical trials. However, dissatisfaction with the psychometric characteristics of the FSS and EDSS have led investigators to develop other measures for clinical studies in MS, for example, the MSFC. However, since the EDSS represents a familiar and widely used albeit imperfect standard, it will probably remain an important part of clinical assessment in MS for the foreseeable future.
Both test-retest reliability and inter-rater agreement have varied considerably from study to study with some studies finding high values and other studies unacceptably low figures (Coulthard-Morris, 2000). In most studies, the distribution of scores on the EDSS forms a bimodal distribution with peaks in the lower and upper ranges and a trough in the middle. (Hohol et al, 1995) Scores on the lower end of the EDSS are more dependent upon nuances in the neurological examination; those in the middle range are more dependent upon gait, while those in the upper (more impaired) range are also dependent upon activities of daily living. Since the EDSS is an ordinal rating scale, a 1-point difference in one part of the scale does not represent the same interval as a 1-point difference in another part of the scale, thus making change or group differences difficult to interpret. Most importantly, there is evidence that the EDSS lacks adequate sensitivity to fluctuations in MS-related impairment. (Rudick et al, 1996)
Many investigators have attempted to correct some of the problems mentioned above by making changes in the FSS and EDSS. See for example Goodkin et al (1992). As a result, there have been several different versions of the FSS and EDSS, many of which have been used in clinical trials but not published. The version presented here is the one originally published by Kurtzke (1983) with some explanatory material added when the FSS and EDSS were incorporated in the Minimal Record of Disability by the World Health Organization. (Haber and LaRocca, 1985)