What is gangrene?
Gangrene is the localised death of body tissue. Dry gangrene is due to prolonged ischaemia (infarction) or inadequate oxygenation or lack of blood flow. Ischaemia affecting proximal blood vessels usually affects the lower limbs. Ischaemia of the peripheries may cause gangrene of fingers and toes.
Peripheral gangrene due to ischaemia
What are the early signs of ischaemia?
Proximal severe acute ischaemia presents as a pale, paralysed pulseless limb. This is a surgical emergency because it may progress to extensive gangrene if the obstruction is not rapidly removed.
Distal peripheral vascular obstruction presents as blue, grey or purple patches, blisters or ulcers at the ends of the digits. See blue toe syndrome.
Ischaemia in other sites may also result in localised necrosis of the skin and deeper tissues.
What causes ischaemia?
Ischaemia is due to an acute or chronic interruption in blood supply and is often due to a combination of both.
Acute ischaemia may be due to:
- Acute arterial thrombosis
- Vascular trauma
- Extreme cold injury (frostbite)
- Infection-related disseminated intravascular coagulopathy or purpura fulminans.
Emboli are particles that flow through the bloodstream until they lodge at an arterial junction or narrowing. Emboli may be composed of:
- Cholesterol dislodged from the heart (80%) or a large artery (aorta, iliac vessel)
- Septic embolus (infectious particles)
- Fat dislodged during an operation
- Foreign objects.
Large emboli lodge at the femoral artery bifurcation (43%), the iliac arteries (18%), the aorta (15%), and the popliteal arteries (15%). Small ones are deposited in the fingers and toes, palms and soles.
Thrombosis is a blood clot arising within a blood vessel. Predisposing factors include:
- Hypotension (low blood pressure)
- Low cardiac output
- Aneurysm (a bulge in the wall of a weakened artery)
- Aortic dissection (a split in the wall of the aorta)
- Bypass graft (a surgical procedure)
- Atherosclerotic narrowing of the artery.
Whatever the primary cause of ischaemia, thrombosis due to sluggish blood flow can make it worse.
Chronic ischaemia may be due to:
- Peripheral arterial disease
- Venous insufficiency.
Peripheral arterial disease
Chronic peripheral arterial disease is most often due to atherosclerosis. This is a build-up of cholesterol, fibrin and other proteins within the arterial wall, causing the vessel to narrow and eventually become completely blocked. This process is also called arteriosclerosis obliterans.
Risk factors for peripheral arterial disease include:
- Hypertension (high blood pressure)
- Hyperlipidaemia (high blood fats)
- Cardiovascular disease including myocardial infarction, atrial fibrillation, valvular disease
- Cerebrovascular disease including stroke, transient ischaemic attack
- Autoimmune diseases: vasculitis, arthritis, coagulopathy
- Malignancy (cancer).
Peripheral arterial disease may also be due to:
- Connective tissue disease, including systemic sclerosis
- Buerger disease (thromboangiitis obliterans).
The veins may fail to clear blood from the tissues because of valve dysfunction (varicose veins) and obstruction from deep venous thrombosis (DVT) or thrombophlebitis. High pressure within the veins (venous hypertension) results in movement of proteins and fibrin into the soft tissue. This leads to fibrosis, dysfunctional capillary formation and fat necrosis (lipodermatosclerosis). When severe, these processes may cause ischaemia and gangrene.
Clinical features of vascular obstruction
Signs of peripheral vascular disease depend on which tissues are ischaemic and its severity. A patient may present with:
- Claudication (intermittent leg pain)
- Rest pain
- Shiny, thin, dry, pale, cool skin
- Brittle nails
- More advanced disease may be revealed by livedo reticularis (fishnet appearance)
- Poor healing and ulceration (most often leg ulcers)
- Gangrene: an area of grey or black necrotic (dead) tissue.
Investigations in a patient with ischaemia or dry gangrene
When a patient presents with peripheral ischaemia or gangrene of unknown cause, a thorough physical examination is undertaken to evaluate the vascular system including the heart and the peripheral pulses. An electrocardiogram (ECG) assesses cardiac function. Blood pressure is measured by Doppler ultrasound of both lower limbs and both upper limbs to calculate the Ankle Brachial Index (ABI).
- ABI 1–1.4 is normal when the pressure in the lower limbs is the same or greater than the upper limbs.
- ABI 0.5–0.8 indicates mild to moderate arterial disease.
- ABI >0.5 indicates severe arterial disease.
Imaging to evaluate peripheral vascular disease may include:
- Vascular ultrasound – to locate the site(s) of obstruction in arteries and veins.
- Angiography – a scan using a dye to identifying the type, location and extent of vascular obstruction in the affected limb.
- Chest Xray.
- Magnetic resonance (MRI) – which can replace traditional angiography.
- Computed tomography (CT) – this can reveal calcification and with contrast (dye) can reveal vascular obstruction.
- Echocardiogram – ultrasound of the heart valves to identify a source of emboli.
Blood tests assess blood count, kidney function, electrolytes, lipid profile, coagulation status and inflammatory markers such as D dimer and C reactive protein.
What is the treatment for ischaemic gangrene?
Treatment of gangrene varies depending on location and cause but is centred around radical surgical debridement +/- amputation. Surgical procedures may also include:
- Removal of embolus or thrombus
- Balloon catheterisation or stent
- Arterial or venous bypass surgery
- Hyperbaric oxygen treatment.
Medical treatment may include:
- Antiplatelet agents, including aspirin, clopidogrel, cilostazol, and pentoxifylline
- Lipid-lowering agents such as statins
- Treatment of underlying conditions such as diabetes
- Cessation of medications that cause vasoconstriction
- Cessation of smoking
- Avoidance of exposure of the affected area to the cold.
What is the prognosis for gangrene?
Prognosis of ischaemic gangrene depends on the extent of disease, the underlying cause and the timing of appropriate treatment.
There will be scarring, and there may be some reduction in function, especially if significant debridement or amputation has been necessary. Recurrence of ischaemia is likely due to medical comorbidity, especially diabetes.