Sirenomelia

Sirenomelia is a rare and severe set of birth defects that causes babies to be born with a single lower limb that some describe as looking like a mermaid’s tail. That’s why it’s sometimes called mermaid syndrome. It can also involve several other birth defects, most of which are severe.

Sirenomelia isn’t a single condition. Instead, it’s an umbrella term for several specific birth defects and conditions that seem to be connected. Many of these defects are severe enough to make sirenomelia fatal at birth or soon after.

In years past, experts thought that sirenomelia was a type of caudal regression syndrome. But newer research shows they aren’t the same thing.

Symptoms and effects of sirenomelia

Babies born with sirenomelia can have any of the following:

• A partially or completely fused lower extremity that some may describe as looking like a mermaid’s tail (this may have bones that are fused together or missing)
• Urinary tract defects that are present at birth (congenital), like having underdeveloped kidneys, missing one or both kidneys, a missing bladder, or a narrowed or missing urethra
• Missing genitalia
• An underdeveloped or missing anus
• Feet that may be formed incorrectly or missing
• Congenital heart defects
• Missing spleen and/or gallbladder
• Underdeveloped lungs

Sirenomelia also usually causes low amniotic fluid during pregnancy or a total lack of it. That’s part of why sirenomelia can affect lung development.

Sirenomelia causes

Experts don’t know why sirenomelia happens. There are a few possible causes and risk factors they strongly suspect. But more research is needed to understand it fully.

Some possible causes and risk factors include:

• Genetics (including inherited and random DNA changes)
• Blood vessels that don’t develop as they should, keeping parts of the lower body from growing properly
• Maternal diabetes
• Environmental factors
• Nonprescription drugs, like cocaine, or prescription skin treatments, like isotretinoin (Absorica®, Accutane®, Amnesteem®, Claravis®, Myorisan®)

One thing to remember is that sirenomelia doesn’t happen because of anything you do before or during pregnancy. You aren’t to blame if your baby has this condition.

How doctors diagnose sirenomelia

The most common way healthcare providers diagnose sirenomelia is during a pregnancy ultrasound scan. If a provider sees signs of sirenomelia on one of these scans, they’ll likely recommend follow-up imaging scans, like an MRI.

How is sirenomelia treated?

There isn’t a treatment for sirenomelia. Most babies born with it can’t survive after birth. That’s usually because they’re born with underdeveloped lungs or they’re missing key organs, like their kidneys.

Instead, treatments focus on treating the effects. Sometimes, surgeries can create a new anus or urethra for a baby born without one. Dialysis may also help babies born without one or both kidneys. But these treatments depend on many factors. Your healthcare provider can tell you more about the treatment options for your baby.

What can I expect if my baby has sirenomelia?

Unfortunately, sirenomelia usually isn’t survivable. The defects it causes are typically so severe that babies don’t survive long after birth.

In very rare cases, babies with very mild forms of sirenomelia may survive into childhood or even early adulthood. But cases can vary widely, so your healthcare provider is the best person to ask about what you can expect.

What is the life expectancy of someone with sirenomelia?

The life expectancy of someone with sirenomelia is much shorter than that of others. That’s because the birth defects that happen with sirenomelia are usually very severe. There are cases where people with sirenomelia have survived into late childhood or even early adulthood. But this is very rare. More research is needed to determine what can be done to extend the life expectancy for people with sirenomelia.