Central Pain Study for ABX-1431

Inclusion Criteria: * Patients with chronic central pain, present for at least 3 months due to one of the four following diagnostic groups: NMOSD, LETM, MS, or TM. * Patient has a diagnosis of NMOSD with anti-AQP4 IgG or without AQP4-IgG as defined by the International Panel for NMO Diagnosis at least 3 months prior to the screening visit (IPND 2015) * Patients with idiopathic, LETM, comprising an intramedullary MRI lesion or continuous segments of spinal cord atrophy with a clinical history consistent with acute myelitis, and with chronic pain syndrome consistent with the lesion neuroanatomy may be enrolled. These LETM patients must also demonstrate evidence of acute, inflammatory myelopathy in the past (e.g. clinically acute symptomatic myelitis plus inflammatory CSF analysis (e.g. elevated WBC and IgG index) or imaging consistent with inflammatory myelopathy). LETM must be diagnosed at least 3 months prior to the screening visit. * Patient with MS defined by McDonald criteria \[1\] or Poser criteria \[2\] at least 3 months prior to screening visit. * Patients with post-infectious, autoimmune or idiopathic TM diagnosed by a neurologist at least 6 months prior to the screening visit. * Patient's chronic pain must be neuropathic in nature, and anatomically plausible based on underlying neuropathology. If patient has an additional source of pain (e.g., vascular ischemic pain, transient spasm associated pain, headache, chronic lower back pain or concomitant osteoarthr…