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arteries. As with other aneurysms, there are patterns of hematoma on the CT scan that point to a PICA aneurysm. Intraventricular hemorrhage and hydrocephalus are seen in 93% of cases, and posterior fossa SAH - with or without supraten-torial SAH - is seen in 95% of cases.

Whilst there are some advocates of repeat angiography in all angiographically negative SAH, others suggest that it is not necessary in technically good, carefully evaluated angio-grams. A pragmatic approach to this problem is to perform MRI in negative cases; this can reveal abnormalities in 14% of patients. MRA can be performed at the same time. Repeat angiogra-phy is reserved for cases where severe spasm, large focal hematoma or brain edema could have obscured the aneurysm or vascular malformation, particularly if the pattern of blood on the CT scan points to a particular site. In patients with repeated SAH and negative investigations, it is worth considering a spinal cause of the SAH as this has been reported with spinal tumors or vascular malformations.

If multiple aneurysms are detected, it is important to decide which has bled. There may be clues on the CT scan such as surrounding hematoma giving rise to a filling defect or localized SAH. The larger aneurysm, the irregular lobulated configuration, and localized spasm may point towards the aneurysm that has bled. If distinction is not possible, then multiple aneurysms will need protecting at the same operation.

Other indications for angiography include: assessment of aneurysms (e.g.fusiform, dissecting, mycotic, giant) that have not presented with SAH; assessment of AVMs and other vascular lesions, such as carotico-cavernous fistula or dural fistula; demonstration of tumor vascular-ity, particularly if pre-operative embolization is being considered; and investigation of various cerebrovascular disorders, such as vasculitis.

The final indication is to diagnose accurately the degree of stenosis of cervico-cranial atherosclerotic disease. Initial screening and investigation of this should be non-invasive with a combination of ultrasound, MRA and CTA, dependent on local expertise and availability. Angiography should be reserved for patients where the non-invasive investigations indicate stenosis on the borderline between surgical or conservative treatment, as complications of angiography are undoubtedly higher in athero sclerotic patients. Consideration should be given to non-selective aortic arch angiography, which may well produce adequate diagnostic information, particularly in conjunction with the findings of other imaging modalities.

Myelography

Myelography is still used to assess the contents of the thecal sac and any abnormal or extrinsic impressions. It is now reserved for patients who are unable to undergo MRI because of a non-compatible implant, such as a pacemaker, or where metallic spinal instrumentation causes severe local artifact, making MRI of the region uninterpretable.

Typically, a total of 3 g of non-ionic iodinated contrast medium is introduced by lumbar puncture before various radiographic projections are taken. Further information is often obtained by CT, particularly to delineate the extent of a spinal block or provide more detail of the exit foramen.

Myelography is unpleasant for the patient, with minor side-effects (eg. headache) occurring frequently. Small needle size (25 g) and pencil shape (Sprotte needle) significantly reduce these complications. More serious complications of chemical meningitis, seizures and neurological deficit are now very rare with modern non-ionic contrast media.

It is doubtful if myelography is ever indicated when MRI can be used. Myelography has been a sensitive method of determining the presence of a spinal dural arteriovenous fistula (AVF), and a technically adequate normal myelogram is said to exclude an AVF and make spinal angiography unnecessary. Advances in MRI, notably linear-array surface coils and the use of gadolinium, have replaced myelography in this condition. The prominent draining vein of the AVF is enhanced with contrast on Tl-weighted images and is seen as low signal on spin-echo T2 weighting. It remains true that if myelogra-phy or MRI raises the suspicion of a spinal AVF, then formal spinal angiography becomes necessary. This is a complex and time-consuming procedure that requires selective catheteriza-tion of all prospective feeding vessels to the spinal cord and canal.

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