Study to Compare Gabapentin to Tramadol in Children With Chronic Neuropathic or Mixed Pain

Inclusion Criteria: 1. Male or female, aged 3 months to less than 18 years at screening. 2. Informed consent by parent(s) and/or legal guardian according to each country legal requirement. 3. Assent by the patient, where applicable, according to each country legal requirement. 4. Subjects that meet the diagnostic criteria for neuropathic or mixed pain. 5. Subjects that present with chronic pain defined as the recurrent or continuous pain persisting more than 3 months. Duration of pain will be determined from the date of the first pain experienced. 6. Subjects that present with at least moderate pain as defined by average pain intensity of ≥4/10 as assessed during a 3-day screening period 7. Stable underlying disease condition and treatment. 8. In presence of malignant diseases, subjects in clinical remission and/or no expected changes in their therapeutic protocol during participation to the present study. Exclusion Criteria: 1. Pain duration of more than 5 years. 2. Current use of gabapentin or tramadol. 3. History of failure to respond to adequate treatment by gabapentin or tramadol/opioids for neuropathic pain. 4. History of epileptic condition except febrile seizure disorder. 5. Subjects with sleeping apnoea syndrome of any origin or subjects with history of severe respiratory impairment. 6. Subjects with diagnosis of sickle cell disease. 7. Subjects that present significant cognitive impairment. 8. Subjects that present current, controlled or uncontrolled, co-morbid psychiatric diagnosis that can impair pain diagnosis and assessment such as severe depressive conditions or psychosis. 9. Subjects with history of suicidal ideation or behaviour. 10. History of substance abuse in particular opioids. 11. Subjects under prohibited concomitant medication 12. Subjects in need for corticosteroid oral treatment or corticosteroid infiltrations to treat pain caused by infiltration or compression of neural structures, e.g. peripheral nerves or spinal cord. 13. Subjects born prematurely at ≤ 36 weeks gestational age, if recruited during the first year of age. 14. Subjects with a body mass index (BMI) for age and gender of \< 5th percentile or \> 95th percentile. 15. Subjects with glomerular filtration rate \< 90 mL/min/1.73 m2 (Schwarz equation). 16. Subjects with significant hepatic impairment or with Aspartate Transaminase (AST) or Alanine Transaminase (ALT) enzymes 3 times the upper limit of the age-specific reference range. 17. Subjects with known allergy, hyper…