Define clubbing. Describe its grades, pathogenesis and causes, and outline the clinical approach.

Definition

Clubbing is bulbous enlargement of the terminal phalanges due to increased soft tissue and connective tissue proliferation in the nail bed, with loss of the normal Lovibond angle (the angle between the nail plate and the proximal nail fold, normally < 160°; in clubbing it becomes ≥ 180°).

Clinical Signs & Grading

• Schamroth's window sign: when the dorsal surfaces of opposing terminal phalanges are apposed, the normal diamond-shaped gap is obliterated — the earliest reliable bedside sign.
• Fluctuation / nail-bed sponginess: the nail bed feels boggy on rocking the nail.
• Increased curvature: in both longitudinal and transverse planes ("parrot-beak", "drumstick").

Pathogenesis

Normally, megakaryocytes and large platelet clumps are fragmented in the pulmonary capillary bed. In diseases with right-to-left shunting or with pulmonary vascular disruption, these clumps bypass the lung and impact in the distal digital vasculature, releasing platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). These promote vascular hyperplasia, oedema, fibroblast proliferation and new vessel formation in the nail bed. VEGF is induced by hypoxia, which explains clubbing in chronic hypoxic states.

Causes

Cardiac: cyanotic congenital heart disease (Fallot's tetralogy, Eisenmenger syndrome), infective endocarditis, atrial myxoma.

Endocrine: thyroid acropachy in Graves' disease — the only endocrine cause.

Special Patterns

• Unilateral clubbing: aneurysm of aorta/subclavian artery, Pancoast tumour, axillary artery lesion, hemiplegia.
• Differential clubbing (toes clubbed, fingers spared): patent ductus arteriosus with reversed shunt (Eisenmenger's), where desaturated blood reaches the lower limbs only.
• Pseudoclubbing: hyperparathyroidism and chronic renal failure cause subperiosteal resorption of terminal phalanges — the Lovibond angle is preserved.

Clinical Approach

Confirm with Schamroth's sign, grade it, then look for the underlying cause: examine the chest for bronchiectasis and consolidation, the heart for murmurs and cyanosis, the abdomen for hepatosplenomegaly, and the joints for HPOA. Investigate with chest X-ray, CT thorax, echocardiography and, when indicated, bronchoscopy.