The main purpose of this study is to determine the effectiveness and safety of one dose of botulinum toxin type A (Dysport) associated with rehabilitation treatment on pain control in patients with a type of back pain called primary myofascial pain syndrome.

Inclusion Criteria:

• Patients with primary myofascial pain syndrome of cervical and dorsal localization, less than two years of length, who must have been diagnosed with the evidence of "trigger points" in which conventional treatment with oral analgesia, rehabilitation and local anesthesia has not been clinically successful for more than 15 days.
• Punctuation of 4cm or more in a 10cm visual analog scale.
• Previous positive response (self-limited reduction -less than 15 days- of pain) after an anaesthetic infiltration in the trigger point.

Exclusion Criteria:

• Patients diagnosed with fibromyalgia or with a spread pain.
• Patients having received previously botulinum toxin.
• Patients having received anesthetic injections at the trigger points within the month before the visit.
• Patients having received corticosteroids injections at the trigger points within three months before the selection visit.