Her left leg is much bigger than her right due to gene mutation

Medical Mysteries is a series that spotlights rare diseases or unusual conditions.

- Judith Tan

Even before Cassie Chow was born, her mother's pregnancy scans had already shown she had an enlarged left leg.

Right after her birth, she was taken to the neonatal intensive care unit for observation due to the abnormal difference in limb size. While all her vital signs and organs were in order, she also had large port-wine stains - dark reddish birthmarks - all over her body.

"The paediatric doctor was not sure what she had," her mother Lynnette Tay, 38, told The Sunday Times, but it appeared to be a rare condition.

"As with rare diseases, there is very limited awareness and management available. We had to do an MRI scan (when she was) about two weeks old. The doctors had to make multiple assumptions (and) still could not diagnose her accurately," added Ms Tay, a civil servant.

Cassie was eventually referred to Associate Professor Mark Koh, who heads the department of dermatology at KK Women's and Children's Hospital (KKH), by an oncologist from the National University Hospital where she was born.

It was not until she turned one in 2021 that she was officially diagnosed with PIK3CA-Related Overgrowth Spectrum (Pros), which is a group of conditions caused by a gene mutation that affects one in about 20,000 live births.

"We had to do biopsies to confirm that Cassie has Pros," Ms Tay said.

Dr Koh told ST the condition leads to "overgrowth or malformations of various tissues, including blood vessels, lymph vessels, soft tissue, muscle and bone. This can, in turn, lead to various consequences, including pain, dysfunction, infections and abnormal blood clotting".