Objective: To adapt and to combine the self-report Upper Extremity Functional Index and Lower Extremity Function Scale, for the assessment of disability severity in patients with a neuromuscular disease and to examine its psychometric properties in order to make it suitable for indicating disease severity in neuromuscular diseases.
Design: A cross-sectional postal survey study was performed among patients diagnosed with a neuromuscular disease.
Methods: Patients completed both adapted extremity function scales, questionnaires for psychometric evaluation, and disease-specific questions. Confirmatory factor analysis was performed, and reliability and validity were examined.
Results: Response rate was 70% (n = 702). The Extremity Function Index model with a two-factor structure - for upper and lower extremities - showed an acceptable fit. The Extremity Function Index scales showed good internal consistency (alphas: 0.97-0.98). The known-groups validity test confirmed that Extremity Function Index scales discriminate between categories of "Extent of limitations" and "Quality of Life." Convergent and divergent validity tests confirmed that Extremity Function Index scales measure the physical impact of neuromuscular diseases. Relative validity tests showed that the Extremity Function Index scales performed well in discriminating between subgroups of patients with increasing "Extent of limitations" compared to concurrent measurement instruments.
Conclusion: The Extremity Function Index proved to be a sound and easy to apply self-report disability severity measurement instrument in neuromuscular diseases. Implications for rehabilitation The Extremity Function Index reflects the functioning of all muscles in the upper and lower extremities involved in activities of daily living. The Extremity Function Index is an easy to administer and patient-friendly disability severity measurement instrument that has the ability to evaluate differences in disability severity between relevant neuromuscular disease subgroups. The Extremity Function Index is a valid and reliable disability severity measurement instrument for neuromuscular diseases.
Keywords: Disability; activities; disability severity; disease severity; extent of limitations; extremity function; lower extremity functioning; neuromuscular disease; psychometric evaluation; questionnaire; upper extremity functioning.