This is a list of major and frequently observed neurological disorders (e.g., Alzheimer's disease), symptoms (e.g., back pain), signs (e.g., aphasia) and syndromes (e.g., Aicardi syndrome). There is disagreement over the definitions and criteria used to delineate various disorders and whether some of these conditions should be classified as mental disorders or in other ways. Some disorders are also in the list of mental disorders.