Study to Evaluate the Long-Term Safety, Tolerability and Efficacy of Dalfampridine.

2 Minute Walk Test (2MinWT) and Change from Baseline by Visit

10 Meter Walk Test (10MWT) and Change from Baseline by Visit

The TUG measures mobility and balance and can predict the risk of falls. This test, which was initially called the Get-up and Go test, is considered a measure of dynamic balance. The subject is asked to stand up from a chair, walk 10 feet at a comfortable pace, turn around and be seated. The Timed Up and Go (TUG) is measured in seconds. There will be one practice test and then the timed test. Only the timed test will be analyzed at each visit time point. Reciprocal transformation may be performed if the time values are markedly skewed.

The Walk-12 is a 12-question questionnaire that asks subjects to rate limitations of their mobility during the preceding two weeks on a 5-point scale (from 1= not at all to 5=extremely). For each visit, the Walk-12 score will be calculated by summing the 12 components and transforming into a scale with a range of 0 to 100. A higher score indicates a greater degree of limitation in walking. A negative change indicates an improvement in walking. 0 = no limitation in mobility to 100 extreme limitation in mobility. Walk-12 Score = 100 * [(Mean of the 12 items) - 1]/(5-1)

The SIS consists of 59 items grouped in 8 domains: strength, hand function, activities of daily living (ADL) / instrumental activities of daily living (IADL), mobility, communication, emotion, memory and thinking, and participation/role function. The subject is asked to rate the level of difficulty in performing each item in the preceding week. Each item is scored on a 5-point scale ranging from 1 (inability to complete the item) to 5 (no difficulty experienced at all). For each domain, the SIS score will be calculated by summing all the items within the domain and transforming into a scale with a range of 0 to 100 as follows: SIS Score = 100 * [(Actual raw score - Lowest possible raw score)/ (Highest possible raw score-Lowest possible raw score)].

The Subject Global Impression (SGI) is single item measure of treatment response that asks the subject to rate the effects of the investigational drug on his or her overall walking ability using a 7 point scale ranging from 1 = "Terrible" to 7 = "Delighted."

The SF-12 v2 (4-week recall) is a general health-related quality-of-life profile measure consisting of 12 items. The SF-12 Physical Component Summary (PCS) and the Mental Component Summary (MCS) scores will be derived and normed to a general United States population for score algorithm. The normalized PCS and MCS scores will be calculated at baseline, Month 12, and subsequent visits.

SF-12 is a Physical and Mental Health Composite Scores (PCS \& MCS) are computed using the scores of twelve questions and range from 0 to 100, where a zero score indicates the lowest level of health measured by the scales and 100 indicates the highest level of health.