What causes brown discoloration of feet in diabetic patients?

Brown Discoloration of Feet in Diabetic Patients

Brown discoloration of the feet in diabetic patients is most commonly caused by hemosiderin deposition from subcutaneous hemorrhage secondary to repetitive mechanical stress on neuropathic feet, or by dependent rubor indicating severe peripheral ischemia.

Primary Causes of Brown Discoloration

Subcutaneous Hemorrhage and Hemosiderin Deposition

• Loss of protective sensation from diabetic peripheral neuropathy leads to abnormal biomechanical loading, causing callus formation and subsequent subcutaneous hemorrhage that appears as brown discoloration 1, 2
• The pathophysiological sequence involves: neuropathy → insensitive foot → abnormal walking pattern → high pressure areas → callus formation → increased abnormal loading → subcutaneous hemorrhage 1, 2
• Patients continue walking on the insensitive foot, perpetuating the cycle of trauma and hemorrhage that manifests as brownish discoloration 1

Peripheral Arterial Disease (PAD) Manifestations

• Dependent rubor (reddish-brown discoloration when the foot is dependent) is a cardinal sign of severe peripheral ischemia in diabetic patients 1
• This discoloration occurs alongside pallor when the foot is elevated, representing critical vascular compromise 1
• PAD is present in up to 50% of patients with diabetic foot complications and causes tissue hypoxia that can manifest as skin color changes 2

Clinical Assessment Approach

Physical Examination Findings

• Visual inspection should specifically assess for dependent rubor, pallor on elevation, absence of hair growth, and dystrophic toenails—all signs of peripheral ischemia 1
• Examine for areas of callus formation with underlying brownish discoloration suggesting subcutaneous hemorrhage 1
• Palpate pedal pulses and assess for warmth, as brown discoloration with warmth may indicate infection, while cool brown areas suggest ischemia 1

Vascular Assessment

• Measure ankle-brachial index (ABI): normal is >0.9, <0.5 indicates severely impaired circulation, and >1.3 suggests calcified vessels from medial arterial sclerosis 1
• An ABI <0.5 or ankle pressure <50 mmHg is indicative of severely impaired circulation that can cause dependent rubor and tissue discoloration 1
• Consider additional testing including skin perfusion pressure (≥40 mmHg), toe pressure (≥30 mmHg), or transcutaneous oxygen pressure (TcPO2 ≥25 mmHg) 1

Underlying Pathophysiology

Neuropathic Mechanisms

• Motor neuropathy contributes to foot deformities creating areas of high pressure vulnerable to repetitive trauma 2
• Autonomic neuropathy reduces sweating, leading to dry skin prone to cracking and secondary changes 2
• Sensory neuropathy allows minor trauma to go unnoticed, with continued ambulation causing progressive tissue damage 2

Vascular Mechanisms

• Diabetic peripheral vascular disease typically affects the popliteal artery or vessels of the lower leg more distally than in non-diabetics 1
• Calcification of the media layer of vessels is a typical hallmark of diabetic peripheral vascular disease 1
• The combination of neuropathy and ischemia (neuro-ischemic foot) may present with brown discoloration but minimal symptoms despite severe vascular compromise 1

Critical Clinical Pitfalls

• Do not assume brown discoloration is benign pigmentation—it often represents either subcutaneous hemorrhage from repetitive stress or dependent rubor from critical ischemia 1
• Brown discoloration with absent pedal pulses and dependent rubor requires urgent vascular imaging and consideration for revascularization 1
• Patients with peripheral neuropathy may have atypical or absent symptoms despite severe ischemia causing tissue discoloration 1
• An ABI >1.3 does not rule out vascular disease—it indicates poorly compressible vessels from arterial wall calcification, requiring alternative vascular assessment 1