Assessment of chronic lower limb ischaemia

Introduction

Atherosclerosis is a systemic disease involving the arterial tree throughout the body. The most commonly affected sites are the coronary, carotid, iliac and femoral arteries. When atherosclerosis affects the peripheral circulation it is referred to as peripheral arterial disease (PAD). The pattern of disease is often thought of as affecting three levels – the aorto-iliac, the femoro-popliteal and the tibial segments. Intermittent claudication is typically the first symptom of PAD, most commonly located to the calf muscles, and usually occurs when PAD affects one level – most often iliac or femoropopliteal. More severe disease affecting two or three levels may result in critical limb ischaemia with rest pain, ulceration or gangrene involving the toes or forefoot. The tibial arteries are commonly affected by calcification in patients with diabetes, end-stage renal disease and those of advanced age. This chapter deals with the assessment of patients with chronic lower limb ischaemia and the principles of vascular imaging.

There are several classifications based on the severity of PAD. The simplest is the Fontaine classification (Table 2.1). The Rutherford classification (Table 2.2) is more detailed, useful for reporting standards, but rarely used in clinical practice.

Table 2.1

Fontaine classification of the severity of PAD

Table 2.2

Rutherford classification of the severity of PAD

Grade	Category	Description
0	0	Asymptomatic
I	1	Mild claudication
I	2	Moderate claudication
I	3	Severe claudication
II	4	Ischaemic rest pain
II	5	Minor tissue loss
III	6	Major tissue loss

Intermittent claudication (IC)

IC is a frequent symptom in the elderly, occurring in 14% of men over the age of 65 years, increasing to 21% in those over the age of 85 years (see Chapter 1).

The classic feature of IC is that pain develops on walking in the muscle groups distal to the arterial obstruction, most commonly in the calf, but also in the thigh or buttock, depending on the location of the atherosclerotic lesion. The pain is not felt at rest or when the first few steps are taken, but typically develops progressively on walking and is described as an ache, cramp or tightening in the muscle that usually forces the patient to stop. Occasionally, mild claudication may be felt only while walking uphill or quickly. Pain develops due to ischaemia of muscles and is therefore located to these and not to joints, which is helpful when considering differential diagnoses. Pain from claudication is rapidly relieved by rest and reappears after walking a similar distance.

The differential diagnosis includes osteoarthritis of the hip or knee and lumbar nerve route irritation. Patients with spinal stenosis may have symptoms of IC that are very similar to intermittent claudication due to vascular disease.¹ Whereas patients with vascular claudication get pain relief by just standing still, patients with spinal stenosis need to relieve the pressure in the spinal canal, which they do by sitting or lying down. A history of pain on standing as well as walking therefore suggests neurogenic claudication due to spinal stenosis. Patients with osteoarthritis of the hip with referred pain down the leg and a history similar to claudication usually experience some pain in the buttock or groin when turning their body, even in a sitting or supine position.The pain is not relieved by standing still as the patient needs to lessen the burden on the joint. In addition, pain from osteoarthritis typically begins when walking and gets better after some exercise. Although the diagnosis can usually be established by history and examination alone, minimally invasive investigations, including an exercise test to exclude arterial disease, may be reassuring.

Lumbar nerve route irritation may also cause aching in the calf or down the back of the leg from buttock to ankle. The sensation appears to be very similar to that of claudication, particularly when confined to the calf. Direct enquiry for these symptoms is helpful, but the key feature is again the need to sit or lie to obtain relief. Spinal flexion may release the involved nerve roots, whereas a straight leg raise will often precipitate the pain. When both sciatic nerve irritation or spinal stenosis and PAD coexist in the same patient, it can be extremely difficult to identify which is contributing most to the patient’s symptoms.

The cornerstones in the assessment of chronic lower limb ischaemia are a careful history, palpation of pulses and ankle–brachial pressure index (ABPI) measurement. The important point is to verify that the clinical findings correlate well with the patient’s symptoms. As the patients get older so does the arterial tree. The mere presence of PAD (i.e. reduced ABPI, atherosclerosis visible on ultrasound examination) does not mean that it is the cause of the symptoms. The history should include the duration of symptoms and the mode of onset. Most patients gradually become aware of pain on walking, which is typical in progressive PAD. The patient experiences pain when the arterial supply to the leg muscles is insufficient to meet the metabolic needs during exercise.

The blood supply required by resting muscles is relatively small (130–150 mL/min) and may be increased five- to tenfold during exercise.²

Arterial occlusions can be well tolerated when collaterals can develop, which may provide the same volume flow at rest as in normal individuals.³ Examples include the thigh muscles around the superficial femoral artery at the adductor canal, which is well collateralised by the profundafemoris artery and occlusions of the iliac arteries, where collaterals through the pelvis and buttock may result in normal resting pressures and even a palpable foot pulse (Fig. 2.1). Significant PAD is usually associated with an ABPI of <0.9 (see later), but in such cases an exercise challenge will result in a fall in ankle pressures and disappearance of the distal pulses. If such patients have only limited physical activity they may be asymptomatic.

Figure 2.1 Arteriogram showing well-collateralised occlusion of left superficial femoral artery. The patient had palpable foot pulses and almost normal resting ABPIs. These fell on the left side after exercise.

Critical ischaemia

Ischaemic rest pain, ischaemic ulceration or gangrene of the feet requires urgent investigation and revascularisation in order to avoid limb loss due to progressive tissue necrosis and/or infection. Untreated, the prognosis is poor both for the patient and the leg, and recognition of critical ischaemia is therefore vitally important. There have been several attempts at a consensus for the definition of critical limb ischaemia (CLI).

The European Consensus defines CLI as rest pain for more than 2 weeks, or ulceration/gangrene, and an ankle pressure of <50 mmHg or a toe pressure of <30 mmHg.⁴

However, this definition has been criticised because the pressures required for healing in the presence of ulceration or gangrene may be higher and ankle pressures are often falsely elevated in patients with diabetes (see later).⁵ The Trans-Atlantic Inter-Society Consensus (TASC) II suggests that an ankle pressure of <70 mmHg or a toe pressure of <50 mmHg is more realistic in the presence of ulceration or gangrene.

The precise definition is more relevant for reporting standards than for clinical use and the consensus document recommends that the term critical limb ischaemia should be used for all patients with chronic ischaemic rest pain, ulcers or gangrene attributable to objectively proven arterial occlusive disease.⁶

CLI in its very mild form may start with pain in the forefoot at night sufficient to disturb the patient’s sleep when the patient is in supine position, thereby cancelling the effect of gravity on blood flow to the lower limbs.⁷ When the patient hangs the leg out of the bed or if they stand up and walk, thereby increasing the blood flow to the foot, the pain is relieved. If the patient constantly hangs their feet out of bed at night, or even sleeps sitting in a chair, the limb swells due to dependency oedema. This can lead to a vicious circle with more tissue damage and the key is to use strong painkillers (usually opiates) so that the limb can be kept in the supine position overnight to relieve the oedema prior to arterial reconstruction.

To the experienced eye the diagnosis of CLI seems obvious, but it is easy to miss a small ischaemic lesion on the heel or between the toes. When established necrosis or gangrene is present with absent limb pulses there is no doubt about the diagnosis. The stage when critical ischaemia without necrosis or gangrene is present (Rutherford II 4) is characterised by pallor when the leg is elevated above the heart and by redness when hanging down (Buerger’s or Ratshow’s test positive). The dark red colour is caused by the dilated capillaries of the foot (Fig. 2.2). Normally, only one-third are open at any time but in the state of critical ischaemia autoregulation is paralysed and, as a consequence, all capillaries are open.⁸ It may take a while for pallor on elevation to occur but capillary refill will be abolished immediately.

Figure 2.2 Sunset red foot due to dilated capillaries caused by critical (limb-threatening) ischaemia. Elevation of the foot will result in pallor (Buerger’s or Ratshow’s test positive).

Rare causes of ischaemia

The vast majority of cases of chronic lower limb ischaemia are caused by atherosclerotic PAD, but some rare conditions exist that tend to affect a younger age group. A good history of IC in a young patient should be taken seriously, as some of these conditions may progress rapidly to CLI. Resting ABPIs should be measured in all patients with leg pain on exercise, especially if foot pulses are absent. Those with a good history of IC and palpable foot pulses or normal ABPI should also undergo an exercise test and post-exercise ABPIs. Persistent sciatic artery is so rare that in my 30 years I may have seen one case.

Persistent sciatic artery

In this congenital anomaly, the embryonic axial limb artery, the sciatic artery, does not obliterate and remains continuous with the popliteal artery, providing the major blood supply to the lower limb. The anomaly is bilateral in 22% of cases and is commonly associated with failure of the iliofemoral vessels to develop properly. The presenting symptoms include pain and a pulsatile mass in the buttock due to aneurysmal degeneration of the artery as it emerges from the sciatic foramen. Thrombosis or distal embolism may lead to acute ischaemia.⁹ Although pedal pulses may be present, the femoral pulse will be reduced or absent if the iliofemoral vessels are hypoplastic (Cowie’s sign) and IC will result if neither system has developed properly (Fig. 2.3). Symptomatic patients can be treated by combined bypass grafting and endovascular exclusion of the aneurysm.⁹ Asymptomatic patients should be monitored for aneurysm development.

Figure 2.3 Persistent bilateral sciatic arteries arising from the common iliac arteries with dilatation and intimal irregularity of the left sciatic artery at the level of the acetabulum.

Cystic adventitial disease (CAD)

CAD, caused by a cystic abnormality of the adventitia of the artery, most commonly affects the popliteal artery and may present with IC. The contents resemble that of a ganglion and the cysts may be connected to the synovium of the knee joint. IC may be severe and of rapid onset. The condition should be particularly suspected in young patients without significant risk factors for PAD. Pedal pulses sometimes disappear on knee flexion. Arteriography may show an unusually smooth ‘hourglass’ or eccentric stenosis (Fig. 2.4). Ultrasound scanning will demonstrate the cystic abnormality, as will computed tomography (CT) or magnetic resonance (MR) scanning. Angioplasty is ineffective. The affected segment of artery requires resection and repair with an interposition vein graft via a posterior approach.¹⁰

Figure 2.4 Smooth ‘hourglass’ stenosis of popliteal artery due to cystic adventitial disease. A similar appearance may be seen in popliteal entrapment, especially during active plantar flexion.

Popliteal artery entrapment

The condition is not common but should be suspected when a young patient, particularly an athlete, complains of classic critical ischaemia. It can be anatomical or functional; in the anatomical variant, the artery courses around the medial head of the gastrocnemius muscle rather than between the two heads or, more rarely, passes deep to the popliteus muscle. Compression of the artery occurs during flexion, resulting in IC. Aneurysmal degeneration and/or thrombosis may develop. Two-thirds of cases are bilateral and the popliteal vein is involved in 10%.¹¹ Examination may reveal reduction or obliteration of pedal pulses during active plantar flexion. Duplex scanning or arteriography using this manoeuvre may also demonstrate kinking or compression of the popliteal artery. CT or MR scanning can also demonstrate the anatomical abnormality. Symptomatic patients should be treated by the division of the medial head of gastrocnemius and/or reconstruction of the popliteal artery. Surgery may be indicated for an asymptomatic contralateral limb whenever anatomical entrapment is detected.¹² In the functional variant of the condition, an anatomically normally positioned artery is compressed against a hypertrophied gastrocnemius or the soleal muscle ring. Unlike the anatomical variant, functional entrapment should only be treated when symptomatic.

Fibromuscular dysplasia (FMD)

Although FMD usually affects the renal and carotid arteries, it can affect the proximal upper and lower limb arteries in young people, causing IC. The external iliac artery appears the commonest site of involvement and arteriography may demonstrate a classic beaded appearance (Fig. 2.5). Patients with iliac FMD should be screened for renal involvement. Symptomatic stenoses usually respond well to angioplasty.¹³

Figure 2.5 Fibromuscular dysplasia affecting the left external iliac artery of a 12-year-old boy.

Buerger’s disease

Buerger’s disease (thromboangiitis obliterans) is a systemic vasculitis that affects medium-sized arteries and veins. It should be considered in any heavy smoking young male claudicant, especially if they are of Middle or Far Eastern origin.¹⁴ Vasospastic symptoms and superficial thrombophlebitis commonly occur and patients may progress rapidly to, or present with, CLI.

The crural vessels are usually severely affected, with patent arteries to the knee joint and typical ‘corkscrew’ collaterals in the calf.¹⁵

The pathophysiology and management of this condition and other causes of vasculitis such as Takayasu’s are covered in Chapter 12.

History and examination

History

As all patients with PAD are at risk of myocardial infarction or stroke, a full history for other cardiovascular diseases and risk factors is essential (see Chapter 1).

In addition to previous myocardial infarction, stroke, coronary artery bypass grafting and arterial surgery, direct enquiry should include symptoms suggestive of angina or transient cerebral ischaemia. Although approximately 30% of patients with symptoms of limb ischaemia will have a history of myocardial infarction, it is not uncommon that angina or transient ischaemic attacks are diagnosed for the first time. The investigation and treatment of these symptoms usually have a higher priority than PAD (see Chapter 3). The enquiry for cardiovascular risk factors is equally important and obviously covers smoking, diabetes mellitus, hypertension, dyslipidaemia, and a detailed drug and family history. Upper extremity exertional pain, postprandial abdominal pain and erectile dysfunction in men are other important potential clues to the presence of significant occlusive atherosclerotic disease in other vascular territories.

Smoking may also cause chronic obstructive pulmonary disease. This and any cardiac disease will limit exercise capacity, limiting the benefit of any revascularisation, and may render the patient unfit for any open vascular reconstruction. Enquiry should be made, therefore, of any symptoms that may suggest that the patient may be unfit for intervention.

Examination

Signs of atherosclerosis and its risk factors in any part of the cardiovascular system should be elicited as well as examination findings to establish the site and severity of PAD. Examination should be systematic and include the following: measurement of blood pressure in both arms and the inter-arm difference; palpation of all peripheral and central pulses; pulse rate, rhythm and heart sounds; examination of the hands and abdominal aorta; careful inspection of the feet, particularly of skin integrity, colour and temperature. In addition, skin changes and calf hair loss may also be suggestive of chronic lower limb ischaemia.¹⁶

The body mass index (BMI) should be calculated by measuring height and weight.¹⁷ This is a good estimate of obesity, a predictor of life expectancy and anaesthetic risk, but also adds to the patient’s walking incapacity. In some cases, weight loss will reduce the patient’s symptoms substantially.

Palpation of pulses is subjective, and is influenced by the sensitivity of the fingers, the experience of the examiner, the obesity of the patient and the warmth of the room. Any patient with symptoms from the lower limb that could be of ischaemic origin should have ABPI assessed. The femoral artery should always be palpable, whether it is pulsatile or occluded, if it is examined properly, unless the patient is very obese (Fig. 2.6). If it is occluded it can often be palpated as a hard cord due to atherosclerosis. If the pulse feels weak it is often because of proximal obstruction causing reduced femoral artery blood pressure. The popliteal pulse is more difficult to palpate, particularly in a well-muscled or obese subject, but should always be felt to exclude an aneurysm (Fig. 2.7). Palpation of the foot pulses should always be described. It may be difficult when the foot is swollen or the room is cold. Presence or absence of foot pulses on physical examination is a ‘weak’ and subjective sign of PAD and should always be supplemented by objective measurements (ABPI). The absence of a single foot pulse may have little clinical significance and, although it should be recorded, it is not an indication for more detailed investigation.