Complex sentence: It's unusual for macrostomia to occur on its own and it is included as a symptom for many diseases including craniofacial microsomia and could potentially have multiple causes; its origin is not yet fully understood.
Split into simpler sentences: It's unusual for macrostomia to occur on its own and it is included as a symptom for many diseases including craniofacial microsomia. Macrostomia could potentially have multiple causes; its origin is not yet fully understood.