Complex sentence: When Kirsty was 11 weeks old she was diagnosed with neurofibromatosis (NF1), which caused no problems until she was eight years old when she was also diagnosed with scoliosis.
Split into simpler sentences: When Kirsty was 11 weeks old she was diagnosed with neurofibromatosis (NF1). The condition caused her no problems until she was eight years old when she was also diagnosed with scoliosis.