Development of the Mental Clutter Scale.

Mental fog is a core symptom of fibromyalgia. Its definition and measurement are central to an understanding of fibromyalgia-related cognitive disability. The Mental Clutter Scale was designed to measure mental fogginess. In an exploratory factor analysis of two different samples (n=128 and n=170), cognitive symptoms of fibromyalgia loaded on 2 dimensions: cognition and mental clarity. The mental clarity factor comprised 8 items with factor loadings greater than .60 and was named the Mental Clutter Scale. The factor stability of the new scale was good, internal consistency was .95, and test-retest reliability over a median of 5 days was .92. The 8-item scale is a quick measure of mental fog that provides clinicians with information about cognitive functioning in fibromyalgia.