Characterization of the Effects of Prolonged-release Fampridine on Ambulatory Function in Patients With Multiple Sclerosis

Inclusion Criteria: 1. Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations. 2. Male or female subjects must be 18 to 65 years old, inclusive, at the time of informed consent. 3. Must have a diagnosis of either primary progressive, secondary progressive, progressive remitting, or relapsing remitting MS as defined by the revised McDonald Committee criteria \[Lublin et al. 1996; McDonald et al. 2001; Polman et al. 2005; Section 22.4, Appendix L\] of at least 3 months duration. 4. Must be able to walk at least 50 meters (with or without a walking aid) at each individual 6minWT conducted pre-randomization. 5. Must have an impaired walking function demonstrated by a mild gait ataxia (Functional System (FS) score = 2 in the cerebellar system component of the EDSS) or a FS score of \> 2 in the pyramidal system component of the EDSS based only on evaluation of the lower limbs (hip flexors, knee flexors, knee extensors, and ankle dorsiflexors)or a restricted ambulation (\< 1h walking duration) at the Screening Visit. 6. Female subjects of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception for 30 days after their last dose of study treatment. 7. Subjects must be able to understand the patient information sheet and comply with the requirements of the protocol. Main Exclusion Criteria: * Any history of seizure, epilepsy, or other convulsive disorder, with the exception of febrile seizures in childhood. * Any prior treatment with anti-epileptic medications specifically prescribed for the treatment of epilepsy. * Onset of MS exacerbation within the 60 days prior to the Screening Visit. * Use of mitoxantrone, cyclophosphamide, rituximab, alemtuzumab, daclizumab, cladribine or any other immune suppressant (except FTY720) or antibody (except natalizumab) within 3 months prior to the Screening Visit, or scheduled use during study participation. * Pulsed steroid treatment within the 60 days prior to the Screening Visit, or at any time during the screening period. * Women who are pregnant or breast feeding, * Clinically significant concomitant disease states such as renal failure, (e.g., moderate or severe renal impairment), hepatic dysfunction (e.g., acute or chronic hepatitis), cardiovascular or pulmonary disease, malignant disease (tumor, neoplasia) etc. * Participation in another study with investigational drug within the 30 days preceding and during the present study. * Contraindications to the class of drugs under study, e.g. known allergy to pyridine-containing substances. * Any prior treatment with fampridine (4-aminopyridine; 4 AP) or 3,4-diaminopyridine in any formulation. * Patients with an acute urinary tract infection at the Screening Visit as indicated by symptoms like painful urination/dysuria, urinary frequency, urinary urgency, pollakiuria, suprapubic pain, flank pain, costovertebral angle tenderness or fever \>38°C in combination with a clinically significant pathological finding in the urine analysis (e.g., positive for leukocytes) at the Screening.