Rest pain and critical ischaemia

When PAD worsens, there is insufficient blood to supply the tissues of the leg even at rest and this often presents as continuous pain in the most distal part, namely the forefoot. Nerve ischaemia, manifesting as nocturnal rest pain and a form of neuritis, often precedes the onset of rest pain. It occurs at night when perfusion of the extremities is reduced. Patients often gain relief by hanging their foot over the side of the bed or walking for a few steps. This increases cardiac output, improving perfusion of the lower extremities and providing partial relief of ischaemic neuritis.

Critical ischaemia occurs when the blood supply falls to a critical level at which the viability of distal tissues or the limb is at risk. Critical ischaemia is defined in the European Consensus Document as 'persistently recurring rest pain requiring regular analgesia for more than 2 weeks, or ulceration or gangrene of the foot and toes in combination with an ankle systolic pressure less than 50 mmHg'. Patients with diabetes and severe PAD may not, however, experience rest or night pain due to peripheral neuropathy and loss of sensation. A much higher proportion of these patients will require a surgical procedure such as an amputation. The prognosis for these patients is poor: 50% of those with critical ischaemia will die of atherosclerotic disease within 5 years (Cheng etal., 2000).

Gangrene

Gangrene is tissue death with the appearances of pallor and mottling, progressing to purple and then the characteristic black appearance due to haemoglobin breakdown forming iron sulphide. Traditionally, it can be classified either as dry or wet gangrene. Dry gangrene is the result of a chronic decrease in blood supply leaving a dry, wrinkled appearance. Wet gangrene usually occurs in diabetics, those patients in whom the arterial supply is suddenly occluded or when there is mixed venous and arterial disease. Infection is also usually present.

Ischaemic ulcers are typically painful, and are located at pressure areas such as the heel and in between toes. These areas should always be examined in a routine leg examination. Ulceration in the leg is, however, most commonly due to venous disease and only about 10% are purely secondary to arterial insufficiency. Venous ulcers tend to be located in the gaiter region and have signs of venous hypertension.

Clinical signs

The limb is often cold and pale, and hair loss from the medial aspect of the leg is a characteristic sign. There is poor capillary refill and impaired venous filling of superficial veins, which may even empty to form venous guttering if the blood supply is severely impaired. When critically ischaemic legs are elevated to an angle of 30°, they will turn pale. Buerger's test will also be positive: elevate the leg and then in the dependent position the microvasculature becomes dilated with blood rushing into the foot to give the appearance of hyperaemia. Evidence of trophic changes such as ulceration on pressure points or gangrene at the extremities are common and may be compounded by the presence of neuropathy and infection. The ankle and foot will likely be oedematous due to the ischaemia and a neuropathy affecting sensation or motor function may be present.

Treatment

Critical ischaemia is a much higher priority for vascular surgical assessment. These patients should be seen swiftly by a vascular specialist for further investigations, e.g. duplex and/or angiography, and for urgent treatment. The preferred option is angioplasty or surgical bypass grafting with either autologous saphenous vein or an artificial graft. Angioplasty is ideal if the stenotic lesions are short and proximal, with surgical bypass as the next solution if angioplasty fails. Critical ischaemia threatens the viability of the limb and therefore some attempt at improving blood flow surgically is justified.

Sympathectomy no longer has a role for providing symptomatic control for patients with critical ischaemia. Sclerosed arteries in diabetic patients have very little capacity to dilate after a sympathectomy. Prostacyclin infusions, however, may prolong the survival of critically ischaemic legs (Marchesi etal., 2003).

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