Understanding PCD Terminology
A Historical Perspective
What Is PCD?
Primary ciliary dyskinesia (PCD) is an umbrella term for a group of rare, genetic disorders that affect the structure and function of motile cilia—tiny, hair-like structures that help move mucus and fluids through various parts of the body. To date, mutations in over 50 different genes have been linked to PCD and disorders of motile cilia. Each gene can carry multiple mutations, any of which may result in PCD. By comparison, cystic fibrosis (CF) is caused by mutations in a single gene—CFTR—which has nearly 2,000 known disease-causing variants. With 50+ genes involved in PCD, the spectrum of possible mutations is vast and still not fully understood.
Understanding “Syndrome” and Naming Conventions
A syndrome refers to a recognizable set of symptoms that tend to occur together. Sometimes this label is applied before the underlying cause is known. Once the cause is discovered, the terminology often shifts to reflect that. For example, what was historically known as Down syndrome is now also referred to as trisomy 21—a reference to its genetic cause (an extra copy of chromosome 21). While both terms are still used, modern medical practice tends to favor names based on biology rather than eponymous ones (those named after people). However, legacy terms such as Kartagener syndrome persist, even though they can lead to misunderstanding.
Kartagener Syndrome: An Early Description of PCD
In the 1930s, Swiss pediatrician Dr. Manes Kartagener described a triad of symptoms—chronic sinusitis, bronchiectasis, and situs inversus totalis (a complete left-right reversal of internal organs). This triad became known as Kartagener syndrome. Because Dr. Kartagener worked primarily with children, he was unaware that infertility was also common among people with this condition—something we now know is linked to the same underlying defect in motile cilia.
From Observation to Mechanism: The Shift in Understanding
In the 1970s, Swedish electron microscopist Dr. Björn Afzelius studied sperm from infertile men and observed that a subset also had bronchiectasis and chronic sinusitis, with about half also exhibiting situs inversus—features of what was then called Kartagener syndrome. He noticed abnormalities in the cilia’s internal structure and hypothesized that all these symptoms could be explained by a single cause: defective motile cilia. Because the technology of the time couldn’t detect subtle motion, the cilia in these samples appeared completely immobile, leading him to coin the term immotile cilia syndrome.
The Modern Term: Primary Ciliary Dyskinesia
By the 1980s, with advances in microscopy and clinical research, scientists discovered that most patients’ cilia were not entirely immobile but moved in disorganized, ineffective ways. In 1983, researchers proposed the term primary ciliary dyskinesia to better capture the wide range of ciliary dysfunction seen in affected individuals. This term—now in use for over 30 years—reflects a clearer, more accurate understanding of the condition and has been globally adopted. Continued use of “immotile cilia syndrome” may indicate outdated knowledge and can hinder accurate awareness.
Situs Abnormalities and PCD
In people with PCD, organ placement can vary because specialized motile cilia play a role in determining left-right body orientation during embryonic development. As a result, individuals with PCD may have:
- Situs solitus: normal organ placement
- Situs inversus totalis: mirror-image organ placement
- Situs ambiguus (heterotaxy): mixed or abnormal organ positioning, sometimes with complex heart or spleen defects
Because these variations occur randomly, even siblings with the same genetic mutation can have different situs arrangements. This randomness underscores why terms like Kartagener syndrome (which refers only to PCD with situs inversus) can be misleading. It gives the impression of a separate disease when it’s actually a subset of PCD with one particular manifestation.
Why Terminology Matters
While Kartagener syndrome is often used as shorthand for PCD with situs inversus, it excludes those with normal or ambiguous organ placement, potentially creating confusion. It’s important to recognize that all three situs patterns can occur within the same genetic diagnosis of PCD.
Furthermore, terms like “immotile” cilia are no longer considered accurate or clinically useful. For instance, some patients with PCD caused by mutations in the DNAH11 gene have hyperactive—but dysfunctional—cilia. Regardless of whether the cilia are immotile, hyperactive, or have abnormal beat patterns, the clinical presentation is largely the same.
The Takeaway
Modern understanding of PCD has moved far beyond early syndrome descriptions. Using current, accurate terminology helps reflect what we now know: PCD is a genetically diverse, cilia-related disorder with a range of symptoms and manifestations. Retiring outdated terms like immotile cilia syndrome or overly specific ones like Kartagener syndrome supports better awareness, diagnosis, and communication within both the medical community and the patient community.