The reported occurrence of ICMAs is 1.2 per cent to 5 per cent of cases. Streptococci and S.aureus account for 50 per cent and 10 per cent of cases, respectively, and are seen with increased frequency among IVDA patients with IE. The distal middle cerebral artery branches are most often involved, especially the bifurcations. ICMAs are multiple in 20 per cent of cases. Mortality rates are similar for multiple or single distal ICMAs. The overall mortality rate among IE patients with ICMAs is 60 per cent. Among those without rupture, the mortality rate is 30 per cent; this approaches 80 per cent if rupture occurs. Clinical presentation is variable Headache, altered sensorium, FND, SAH or IVH. CSF is sterile and contains erythrocytes, leucocytes and elevated protein. Imaging procedures to detect ICMAs are indicated in IE patients with localized or severe headaches, "sterile" meningitis, or focal neurological signs. Contrast-enhanced CT may provide useful initial information. This technique has a 90 per cent to 95 per cent sensitivity for intracerebral bleed and may thus indirectly identify the location of the MA. Magnetic resonance angiography is a promising new technique for the detection of ICMAs, although its sensitivity for aneurysms smaller than 5 mm is inferior to conventional 4-vessel cerebral angiography.
Conventional angiography remains the diagnostic imaging of choice. ICMAs may heal with medical therapy: ICMAs resolved between an initial and follow-up angiogram in 52 per cent of patients treated with effective antibiotic therapy. A decrease in ICMA size was seen in an additional 29 per cent. In 19 per cent of patients, however, the ICMA increased in size by the time of the second angiogram, and a new ICMA was discovered in 10 per cent. Currently, there are no data that precisely identify at risk for imminent rupture, and decisions concerning medical versus surgical therapy must be individualized. It is generally felt that a Single ICMA distal to the first bifurcation of a major artery (e.g., middle cerebral artery) and Multiple ICMAs should be monitored with frequent serial angiograms and excised promptly if the aneurysm enlarges or bleeds.