Clubbing : Why it occurs.

Hi everyone !
This is a short post on why clubbing happens.
So it's simple !
It's cause people like to go out and get drunk. 
Just kidding. Here goes.

1. What is clubbing ?
- It's the bulbous enlargement of the terminal digits​ and the nail bed.

2. What are its causes ?
- Symmetrical clubbing can occur due to a host of causes.
- To summarize :
  A. Respiratory
   : Lung  cancer
   : Suppurative lung conditions like   

     Bronchiectasis , lung abscess and Chronic TB.
    : Pulmonary  Fibrosis

   B. Cardiac
   -  Cyanotic heart disease
   - Eisenmenger Syndrome
   - Infective endocarditis

    C. GIT
     -  Inflammatory bowel disease
     -  Cirrhosis - esp Biliary

     D. Endocrine
     -  Thyroid Acropachy
     -   Acromegaly

3. Why does it occur 

So I've spent a lot of time researching theories on how clubbing occurs. And let me tell you in the start itself, they're not clear on why it occurs.
But what makes sense to me , I want to share with you'll! And it was an absolute pain to find something convincing enough. So just stick with me here ;;) 

So the crux of clubbing lies in vasodilation of the digital vessels causing proliferation of the tissue there in.
The most widely accepted theory right now is the megakaryocyte theory.

So in the figure above , the left side in white shows the normal course of a megakaryocyte through the blood. 

In altered  cardiorespiratory conditions , these large platelets either bypass the Pulmonary circulation owing to the shunting produced due to Heart defects or the lung parenchyma itself proves to be less to purify the blood of the platelets.

 This causes these giant platelets to go lodge into the digital circulation causing release of cytokines like Platelet derived growth factor (PDGF) and TGF beta amongst others. These GF's cause vasodilation and in return , nail bed proliferation and collagen deposition.

How do we explain the Clubbing in GI causes ?
IBD - especially Crohn disease seen to have thrombocytosis eventually which may aggravate the PDGF.
In cirrhosis of liver , especially biliary , pulmonary arteriovenous shunting is observed. This could result in the megakaryocyte entrapment as explained.

Another theory suggests inflammation triggers a vagal response causing Vasodilator effects. ( Neurogenic).

Other theories -
Hypoxia induced
Reduced ferritin related
Neurogenic
Humoral - various PG's and other humoral molecules.

The most widely accepted theory is the Megakaryocyte theory.

Hope this satisfied you ! 
Thank you.
Stay awesome. 

~A.P.Burkholderia