Lesson of the week: Playing the odds in clinical decision making: lessons from berry aneurysms undetected by magnetic resonance angiography

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Abstract
The probability of a disease following a diagnostic test is critically reliant on prior clinical probability The purpose of this report is twofold—to report the potential for magnetic resonance angiography to miss sizeable intracranial aneurysms and to highlight the value of simple, quantitative clinical reasoning when interpreting the results of diagnostic tests. Subarachnoid haemorrhage accounts for a quarter of all cerebrovascular deaths, and over a third of those who survive have major neurological deficits. 1 2 Intracranial aneurysms, the commonest cause of subarachnoid haemorrhage, may present with rupture, mass effect, or, rarely, with emboli phenomena in large aneurysms. The typical presentation of rupture is headache of instantaneous onset that remains continuous and is often associated with nausea, vomiting, meningism, or loss of consciousness. About a third of patients with aneurysmal subarachnoid haemorrhage will re-bleed, and this is a major cause of poor outcome. 3 4 The risk of re-bleeding peaks on the first day and then declines.5 Most studies therefore support the need for surgery soon after rupture, and delay in diagnosis or misdiagnosis as migraine or meningitis can have catastrophic consequences. 6 7 Unruptured intracranial aneurysms causing mass effect may present as pain or neurological deficit depending on the site and size of the aneurysm. Such aneurysms are often large or giant,8 and, as most intracranial aneurysms occur at the junction of the internal carotid and posterior communicating artery, the commonest clinical sign is oculomotor palsy. Unruptured intracranial aneurysms causing mass effect are at high risk of subsequent rupture, estimated at 6% a year.9 The optimal method for detecting intracranial aneurysms is intra-arterial digital subtraction angiography. This procedure carries an associated morbidity of transient or permanent neurological disability (of 1% and 0.5% respectively).10 This associated morbidity and increasing access to magnetic resonance imaging has led …