What is Digital Clubbing, Really?

In nursing school, I learned that digital clubbing was a key sign of COPD and I accepted this as gospel and moved on with my life. I used this information to answer exam questions on the topic and I always included assessing for it in my care plans for patients with COPD.

But, like all things in nursing, there’s so much more to this story. 

What is digital clubbing?

Digital clubbing refers to the rounding and bulging of the fingertips and fingernails. Though it can occur in the toes as well, we generally assess for digital clubbing at the fingertips. 

Digital clubbing can be primary or secondary. Primary or idiopathic digital clubbing can be hereditary and is also associated with hypertrophic osteoarthropathy, a condition characterized by the proliferation of osseous tissue, soft tissue and skin in the distal extremities.

Secondary digital clubbing is considered a symptom of underlying disease and is associated with a wide variety of conditions, such as: 

• Pulmonary disease – including interstitial lung disease, interstitial lymphocytic pneumonia, pulmonary fibrosis, bronchiectasis, cystic fibrosis, empyema, sarcoidosis, tuberculosis, and lung abscesses
• Cardiovascular disease – including infective endocarditis, cyanotic congenital heart disease, and any condition that causes right-to-left shunting
• Cancer –  including lung cancer, liver cancer, GI tumors, esophageal cancer, mesothelioma, thyroid cancers, and lymphoma
• Gastrointestinal disease – including cirrhosis, inflammatory bowel disease, roundworm infection (ascariasis), celiac disease, peptic ulceration of the esophagus, and hepatopulmonary syndrome
• Dermatologic disease – including Volavsek syndrome and Fischer syndrome
• Other conditions – included thyroid acropachy, sickle cell disease, hyperparathyroidism and pregnancy
• Overuse of certain medications – including prostaglandins, laxatives, and interferon alfa-2A

Notice what’s not on this list? COPD. The evidence shows that if a patient with COPD has digital clubbing, underlying pulmonary conditions (or even other non-pulmonary conditions) must be ruled out.

How is digital clubbing evaluated?

Digital clubbing is often evaluated by observing for a positive Schamroth sign, which is done through visual inspection of the fingertips and fingernails. Ask the patient to put their fingers together with the first knuckles and fingernails touching. The fingertips will be pointing down, almost as though they are making a heart shape with their hands. 

In a healthy individual without clubbing, there is a diamond shaped window at the base of the nail beds. If there is no diamond shaped window, the fingers are clubbed and the physician will perform further assessments to verify clubbing and identify the underlying cause. As a nurse, this is the assessment you’ll most likely perform when observing for digital clubbing.  

Digital clubbing may be further evaluated through measurements which can look at two different aspects, the profile angle and the hyponychial angle. This is not likely to be something done at the bedside, but for your reference, a normal profile angle is less than 176 degrees and a normal hyponychial angle is 192 degrees.

Jajic, Zrinka & Jajic, Ivo & Nemčić, Tomislav. (2001)

Yet another way to evaluate digital clubbing is with the digital index. The index is a summation of the phalangeal depth of all ten fingers. An index of 10.2 or higher is indicative of digital clubbing. 

How is digital clubbing categorized?

Digital clubbing can be categorized based on progressive changes to the fingertips and nail bed.

• Grade 1: There is some fluctuation and softening noticeable in the nail bed
• Grade 2: The angle between the nail bed and the proximal nail fold is greater than 160 degrees
• Grade 3: There is accentuated convexity of the nail bed
• Grade 4: There is a noticeable clubbed appearance to the fingertip
• Grade 5: The nail and nearby skin appear shiny and longitudinal striations are present

The pathophysiology of digital clubbing

The pathophysiology of digital clubbing is actually much more complex than “chronic hypoxia” which is what many nurses learn. In fact, the specific pathophysiology is actually unknown. Here is a bit of what we do know. 

In the early stages, changes to the nail bed are a result of increased interstitial edema. As the malformation continues, the size of the distal digit increases as a result of increased vasculature and changes to the connective tissue. There is a general consensus that in most types of clubbing, distal digital vasodilation is present. This vasodilation leads to increased blood flow in the fingertips (and toes). What causes this vasodilation remains unclear. It has been proposed that vasodilation may be due to hypoxia, a circulating or local vasodilator, a neural mechanism or a combination of these and yet-to-be discovered factors. 

Let’s look very briefly at each of these. 

The hypoxia theory suggests that individuals with pulmonary disease and cyanotic heart conditions have higher levels of circulating vasodilators. But since many conditions known to cause hypoxia do not involve clubbing,  and since hypoxia is absent in many (if not most) cases of digital clubbing, it cannot be supported as a significant cause of the condition. 

The presence of circulating vasodilators in digital clubbing is supported by evidence related to cyanotic congenital heart disease. Because these patients have a right-to-left shunt, these vasodilators (which are normally inactivated when they flow through the lungs) do not get inactivated and flow into systemic circulation. Studies and experience show that when the shunting situation is corrected, the clubbing subsides. Some vasodilators that are proposed to be responsible include bradykinin and prostaglandins, among others. 

The vagal system has also been proposed as a possible player in the development of digital clubbing. In patients with disease conditions associated with organs that are vagally innervated, digital clubbing incidence is increased. This is further supported by the fact that digital clubbing has been reported to improve after a vagotomy is performed. However, researchers are not yet fully convinced this is a strong factor for digital clubbing since it doesn’t explain why non-vagally innervated organs can also be associated with the condition.

Now things are getting interesting

Another interesting and well supported theory stems from work done by researchers Dickinson and Martin.

This theory stems from the idea that a key location for platelet biogenesis is actually in the lungs. Researchers found that there are lower numbers of platelets in venous blood when compared to arterial blood. This led to further examination and the proposal that platelets are created in the lungs when megakaryocytes (cells that produce platelets) are physically broken down in pulmonary capillaries. 

In a person with normal cardiopulmonary circulation, megakaryocytes are too large to fit through the smallest pulmonary vessels. As such, they are broken down into smaller components and platelets are produced. However, in an individual with abnormal cardiopulmonary function (such as a right-to-left shunt) these larger cells are able to enter systemic circulation and eventually lodge in the fingers and toes.

As Rosenberg states in a 2017 study, these clumps of megakaryocytes at the periphery can release high concentrations of platelet derived growth factor (PDGF) into the fingertips. Platelet derived growth factor leads to increased permeability, increased vascularity, and connective tissue changes that cause the malformation in digital clubbing.

But, it’s important to note that not everyone with digital clubbing has a right to left shunt or abnormal cardiopulmonary vasculature. And while follow up studies have supported this theory as a cause of digital clubbing, it does not explain the pathophysiology of clubbing in other conditions such as inflammatory bowel disease or cirrhosis.

In his article published in the journal Circulation, Dr. Rutherford discusses other possible pathophysiological explanations including abnormal capillary growth patterns, local hypoxia, platelet activation, and increased plasma levels of vascular endothelial growth factor (VEGF) which can be triggered by inflammatory states and cancer. 

In short, the pathophysiology of digital clubbing and the conditions it is associated with it go far beyond what most nursing students are taught. 

How many people are affected by digital clubbing?

While primary digital clubbing is rare, secondary clubbing is more common and studies show it is present in approximately:

• 29% of patients with lung cancer, with most cases occurring in non-small cell carcinoma
• 38% of patients with Crohn’s disease
• 15% of patients with ulcerative colitis
• Up to 33% of patients with pulmonary tuberculosis

Again, notice what’s not on this list? COPD. To reiterate, clubbing is considered to be an unusual finding in COPD and its presence should be a sign to the physician to look for an underlying cause (especially lung cancer.)

I hope this look at digital clubbing shines some light on a complex topic that, until now,  you may have thought was pretty easily understood.  Now, when you have a patient with digital clubbing, you’ll know they could have a wide variety of underlying disorders.

Take this topic on the go by tuning in to episode 287 of the Straight A Nursing podcast. Listen from any podcast platform, or straight from the website here.

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