Repture of an abdominal aortic aneurysm remains one of the most dangerous events in vascular surgery, the mortality being close to 100% if the condition is not treated. The diagnosis may, however, be difficult as many other life threatening conditions produce a similar clinical picture. Any delay in operating on these patients has a serious adverse effect on their outcome; many die before they reach the operating theatre, so the mortality in the community remains over 90%. The perioperative mortality has remained constant at 30-50% despite advances in anaesthetic and surgical techniques during the past two decades.
In contrast, mortality after elective repair of an aneurysm that has not ruptured is usually between 2% and 5%. Even operations on selected patients over the age of 80 have a mortality of only about 10%. The most important way of improving the poor prognosis of patients with ruptured abdominal aortic aneurysms is to diagnose the aneurysm before it ruptures. Careful assessment by surgeons and cardiologists can then be made before operation is contemplated.
The incidence of abdominal aortic aneurysm is increasing in the United Kingdom; it is more common among men than women, the ratio being about 5:1. Although the natural course is unknown, aneurysms expand at a rate of about 0.5 cm a year and this rate increases as the aneurysm enlarges. The risk of rupture increases proportionately with the diameter, but even aneurysms <4 cm have a slight risk of rupture. Less than half the patients with symptomatic aneurysms will survive a year, and asymptomatic aneurysms also carry a high risk. The average time between diagnosis and rupture is 16 months.
Ruptured abdominal aortic aneurysms are responsible for 1-1.5% of all deaths in men over the age of 65 in the Western World, with an incidence of 25-30/100 000 population.
Screening has recently been advocated to reduce the high incidence of ruptured aneurysms. The group at risk comprises men over 65, among whom there is a 2% prevalence of abdominal aneurysms .4 cm in diameter. Furthermore, a single ultrasound scan that shows an entirely normal aorta in a 60 year old man does not need to be repeated. Aneurysms <4 cm in diameter can usually be treated conservatively and followed up with six monthly ultrasound scans. Whether to operate on asymptomatic aneurysms <5 cm in diameter is controversial, but the risk of rupture was shown to be small in two recent population studies; the operation is indicated only if the aneurysms expand or start to cause symptoms.
A screening programme for men over 60 would probably be cost effective because:
* The prevalence of aortic aneurysm in this group is 20%
* Ultrasonography is a simple, cheap, and accurate test
* The mortality associated with elective surgery is between 2% and 5%–and 90% if the aneurysm ruptures
* The life expectancy after successful aortic replacement is similar to that of age matched peers, the five year survival being more than 60%.
Between 30% and 60% of abdominal aortic aneurysms are asymptomatic, and the commonest presenting symptom in the rest is low back pain caused by erosion of vertebral bodies. Erosion into the gastrointestinal tract (usually the third part of the duodenum) to form a primary aortoenteric fistula is rare, but carries a high mortality. Erosion into the vena cava is also rare and also carries a high mortality unless the diagnosis is made before operation. Distended superficial veins (which may be pulsatile), ankle oedema, cardiac failure, and raised central venous pressure in a patient with shock, together with a machinery murmur on abdominal auscultation, may alert the clinician to the diagnosis.
Pressure on peripheral nerves may cause pain in the groin or thigh and compression of adjacent veins may cause ankle oedema. The aneurysmal sac may fill with thrombus and clot, and debris may embolise distally to the legs, causing intermittent claudication or rest pain of sudden onset. Smaller emboli may occlude arterioles to the skin, causing small punctate areas of necrosis in the feet. Rarely occlusion of the mesenteric vessels may cause lef sided ischaemic colitis that presents with diarrhoea, which may be bloody. Organisms may multiply in the artheroma or thrombus within the lumen of the aneurysm and the patient may complain of non-specific symptoms such as weight loss, anorexia, and malaise.
Occasionally, dense retroperitoneal fibrosis associated with an inflammatory aneurysm may occlude the ureters, causing hydronephrosis. Inflammatory aneurysms also cause severe backache that is associated with generalised malaise and anorexia. A high erythrocyte sedimentation rate is suggestive of this and computed tomography can usually confirm the diagnosis.
A pulsatile, expansile, abdominal mass is palpable in 80-90% of patients with abdominal aortic aneurysms. In thin patients smaller swellings (4-5 cm in diameter) may easily be felt. The diagnosis may be difficult in obsese patients, however, and the size of the aneurysm is usually overestimated. An aneurysm that is tender on palpation may indicate impending rupture, and urgent referral to a vascular surgeon is mandatory.
The diagnosis is confirmed by ultrasound scanning, but inaccuracies of up to 8 mm must be expected. In addition, the neck of the aneurysm may also be incorrectly diagnosed as being above the renal arteries if an elongated tortuous aorta is twisted forwards.
Computed tomography of the abdomen provides more accurate antomical information, but is more expensive. Arteriography is essential in a patient though to have a thoracoabdominal aneurysm, abnormal renal function, or symptoms of distal occlusive disease; if the aneurysm is uncomplicated, however, it is probably unnecessary.
It is important that the patient is warned before the operation of the small but pertinent risks associated with repair of an abdominal aortic aneurysm. The mortality is 2-5%, and a rare complication is paraplegia. A more usual problem is retrograde ejaculation, but true impotence is uncommon. Sigmoid ischaemia resulting in bloody diarrhea or perforation may occur in 1-2% of patients; leg ischaemia may also arise from embolisation of the pultaceous debris in the aneurysmal sac.
The advances in anaesthetic and perioperative care during the past decade have made a great impact on results, but the operation has also been greatly simplified. In 60% of patients it is possible to insert a short Dacron tube rather than the more extensive aortobi-iliac or aortobifemoral grafts. Dissection of vessels can then be kept to a minimum and the graft laid inside the sac without disturbing surrounding structures.
Although prophylatic antibiotics are given at the time of operation, a small perentage of grafts still become infected. Patients may present with malaise, anorexia, weight loss, and fever. A gammacamera scan after injection of radiolabelled white cells may confirm the diagnosis, and computed tomography may show gas surrounding the grat. A secondary infective aortoenteric fistula may present with haematemesis and malaena and should be suspected in a patient who has had an aneurysm repaired and who presents with such symptoms. Usually the blood loss into the gastrointestinal tract is intermittent, giving time for the diagnosis to be confirmed. Treatment is by removal of the graft together with revascularisation of the legs with bilateral axillofemoral grafts.
Aortic aneurysms that include the origins of the visceral vessels used to be considered inoperable. Following recent encouraging reports from the United States, however, surgeons in some centres in the United Kingdom are repairing thoracoabdominal aneurysms, albeit with a higher mortality than after the repair of infrarenal aneurysms. In contrast to infrarenal aneurysms, thoracoabdominal aneurysms tend to be symptomatic. Most patients present with pain, although they may have other symptoms including dysphagia, dyspnoea, hoarse voice, and chronic cough. Careful preoperative assessment f cardiac, respiratory, and renal function is necessary and the risks of death, and visceral and spinal cord ischaemia, should be explaned to the patient. Given the poor prognosis associated with these aneurysms, however, with only a quarter of the patients suriving two years, the results of surgery seem to be justified by survivial rates of 60% at two years.
Many aortic aneurysms are asymptomatic, so some form of screening is necessary to diagnose them. The most cost effective method is the routine abdominal examination of patients in the general practitioner’s surgery, and two thirds of aneurysms >4 cm will be diagnosed by careful clinical examination. There are few absolute contraindications to operation. In particular, age is no bar.