How do you differentiate a pressure deep tissue injury from ischemic changes on the lateral foot?

Differentiating Deep Tissue Pressure Injury from Ischemic Changes on the Lateral Foot

To distinguish a pressure deep tissue injury (DTPI) from ischemic changes on the lateral foot, immediately assess for a history of prolonged immobilization or "time down" (suggesting DTPI) versus symptoms of peripheral artery disease such as claudication, rest pain, or absent pulses (suggesting ischemia), then perform ankle-brachial index (ABI) testing—an ABI ≤0.90 confirms arterial insufficiency, while normal perfusion studies with localized purple discoloration over a bony prominence indicates DTPI. 1, 2

Clinical History: The Critical First Step

Pressure injury history:

• Document any period of immobilization, including "time down" at scene of injury, prolonged surgical procedures, or inability to reposition 2
• DTPIs most commonly appear over the coccyx, sacrum, buttocks, and heels—the lateral foot is an atypical location for classic pressure injury 3
• Patients with DTPI are typically older with lower body mass index 3

Ischemic history:

• Ask specifically about exertional leg symptoms: aching, burning, cramping in the calf or foot that resolves within 10 minutes of rest (claudication) 1
• Inquire about rest pain in the foot, particularly at night, relieved by dependency 1
• Document cardiovascular risk factors: age ≥65 years, diabetes, smoking, known atherosclerotic disease elsewhere 1

Physical Examination Findings

DTPI characteristics:

• Purple or maroon discoloration with defined borders and surrounding erythema in light-skinned patients 2
• Persistent hyperpigmentation rather than blanching in dark-skinned patients 2
• Skin may be intact initially but rapidly deteriorates despite appropriate interventions 3
• The area is typically localized directly over a bony prominence 3
• Surrounding tissue perfusion is normal (pulses present, capillary refill normal) 2

Ischemic changes characteristics:

• Pallor with extended capillary refill time (>2 seconds) after finger pressure 1
• Absent or diminished dorsalis pedis and posterior tibial pulses 1, 4
• Foot may be cool to touch (poikilothermia) 1
• Distribution follows vascular territories (angiosomes) rather than pressure points 5
• May involve multiple toes or entire forefoot, not just areas over bone 5

Objective Vascular Testing: The Definitive Differentiator

Mandatory initial test:

• Perform resting ABI with Doppler waveforms immediately 1
• ABI ≤0.90 confirms peripheral artery disease and ischemic etiology 1
• ABI 0.91-0.99 is borderline and warrants further testing 1
• ABI 1.00-1.40 is normal and effectively excludes arterial insufficiency as primary cause 1

When ABI is unreliable (>1.40 suggesting noncompressible vessels):

• Measure toe-brachial index (TBI) with waveforms—this is essential in diabetic patients with medial arterial calcification 1
• TBI <0.70 indicates PAD; toe pressure <30 mmHg indicates severe ischemia incompatible with healing 1, 4
• Obtain transcutaneous oxygen pressure (TcPO₂) or skin perfusion pressure (SPP) 1
• TcPO₂ >30 mmHg or SPP >40 mmHg predicts wound healing capacity 1

Algorithmic Approach

1. Immediate assessment: Check bilateral pedal pulses and capillary refill 1, 4

   • If pulses absent or diminished → proceed directly to ABI testing 1
   • If pulses normal → consider DTPI but still perform ABI to exclude subclinical PAD 1

2. Perform ABI testing 1

   • ABI ≤0.90 → Diagnosis: Ischemic ulcer → urgent vascular surgery consultation 1
   • ABI >1.40 → obtain TBI 1
   • ABI 1.00-1.40 with normal pulses → Diagnosis: DTPI 1, 2

3. If ischemia confirmed (ABI ≤0.90 or TBI <0.70):

   • Obtain duplex ultrasound, CTA, or MRA to define anatomy for revascularization 1
   • Classify as chronic limb-threatening ischemia (CLTI) if rest pain or tissue loss present 1
   • Immediate vascular intervention required to prevent amputation 1

4. If perfusion normal (ABI 1.00-1.40, TBI ≥0.70):

   • Diagnosis: DTPI 3, 2
   • Implement aggressive pressure redistribution and offloading 3
   • Monitor closely as DTPI rapidly deteriorates to stage 4 injury in many cases 3

Critical Pitfalls to Avoid

Do not rely on skin appearance alone: Both conditions can present with purple discoloration, but ischemia typically involves broader tissue distribution following vascular territories, while DTPI is localized over bony prominences 2, 5

Do not skip vascular testing in diabetic patients: Diabetes causes both neuropathy (masking pain from pressure) and arterial disease, making clinical examination unreliable 1, 4

Do not assume lateral foot location excludes pressure injury: While uncommon, DTPIs can occur at any pressure point, and the lateral malleolus is a recognized site 3, 2

Do not delay vascular consultation if ABI abnormal: Ischemic wounds without revascularization have 20-25% primary amputation rates 1

When Both Conditions Coexist

Patients with PAD who experience prolonged immobilization can develop both ischemic tissue damage and pressure injury simultaneously 6. In this scenario:

• The ABI will be abnormal (≤0.90), confirming arterial insufficiency 1
• Revascularization must precede or accompany pressure relief interventions 1, 5
• Without restoring arterial perfusion, pressure redistribution alone will not achieve healing 5