Understanding Your Diagnosis: Segmental Progressive Overgrowth

Receiving a diagnosis of Segmental Progressive Overgrowth with Fibroadipose Hyperplasia can feel overwhelming, especially after you may have spent months or years noticing that parts of your or your child’s body are growing at different rates ^[1]. It is common for patients and parents to feel isolated during this time, particularly if local doctors have not seen this specific condition before ^[2]. Understanding the biology of the diagnosis can help you navigate this new landscape and realize that this is a rare, complex condition that requires specialized care.

What is PROS and Fibroadipose Hyperplasia?

This condition is part of a larger group of disorders known as PIK3CA-Related Overgrowth Spectrum, or PROS ^[1]. This is an “umbrella” term for several different clinical phenotypes that all share the same genetic root cause: a change in the PIK3CA gene ^[3].

The term Fibroadipose Overgrowth (FAO) (sometimes called Fibroadipose Hyperplasia) describes the specific way the tissue is growing ^[4].

• Fibro-: Refers to fibrous tissue, which is the tough, supportive connective tissue in the body.
• -adipose: Refers to fat tissue.
• Hyperplasia/Overgrowth: Means an increase in the number of cells or volume, leading to the overgrowth of an organ or tissue.

In affected areas, the body’s “growth switch” stays in the “on” position, causing fat and fibrous tissue to grow more than it should ^[3]. This overgrowth is often segmental, meaning it only affects specific parts of the body—like one leg, a foot, or one side of the torso—rather than the whole body ^[1].

Why This Is Not Your Fault

One of the most important things for families to know is that you did not cause this condition, and you could not have prevented it. Segmental Progressive Overgrowth is caused by a postzygotic somatic mosaic mutation ^[1].

To understand this, it helps to break down those terms:

• Postzygotic: This means the genetic change happened after the egg was fertilized ^[5]. It was not present in the mother’s egg or the father’s sperm.
• Somatic: The change occurred in a regular body cell, not typically in the reproductive cells.
• Mosaic: Just like a mosaic piece of art is made of different colored tiles, the body has two different types of cells. Some cells have the PIK3CA mutation, and others do not ^[5].

Because this change happens randomly during early development in the womb, it is usually not “inherited” in the traditional way ^[5]. It is a “spelling error” that occurred as cells were dividing. This means that siblings are generally not at increased risk, though individualized genetic counseling is always recommended to discuss specific reproductive risks rather than making absolute claims ^[5].

The Challenge of Rarity

You may find that your local specialists are unfamiliar with PROS. This is because the condition is very rare; while exact numbers are hard to pin down, one estimate suggested it occurs in approximately 1 out of every 22,000 births ^[6].

There are several reasons why statistics for this condition are difficult to determine:

1. A Wide Spectrum: Because the mutation can happen at different times during development, it can look very different from one person to the next. Some have very mild, localized overgrowth, while others have more extensive involvement ^[1].
2. Diagnostic Hurdles: The mutation is mosaic, meaning it might not show up in a standard blood test ^[7]. However, the absence of a blood variant does not negate a clinical diagnosis; to find the mutation, doctors often have to test a sample of the actual affected tissue ^[5].
3. Recent Discovery: The genetic link between these different overgrowth patterns was only discovered relatively recently, so many older cases may have been misdiagnosed or remained undiagnosed ^[8].

What to Expect Over Time

The “progressive” part of the name means that the overgrowth typically continues over time ^[9]. In one specific cohort study of patients with PROS, over 80% showed some degree of progression over time, with an average growth rate of about 7% per year in the affected areas ^[10]. However, this statistic reflects that particular group and cannot be used as a personal prognosis; growth can fluctuate, slow, or stabilize.

Because of this, care is focused on monitoring the overgrowth and managing any complications, such as limb-length differences, heaviness in the affected limb, or pain ^[11][12]. Management is a long-term process. It usually involves a team of specialists—including geneticists, orthopedic surgeons, and vascular anomaly experts—who work together to create a plan tailored to your specific growth pattern ^[2]. While the journey can be complex, understanding the biological “why” is the first step in advocating for the best possible individualized care.