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Angiography using computerised tomography (CTA) should be the imaging technique of choice for patients with isolated third nerve palsy, recommended Scottish neuro-ophthalmologist Robert McFadzean MD at the recent annual meeting of the Canadian Ophthalmological Society. "We've reliably shown that the resolution of the technique is such that we can always pick up an aneurysm causing a third nerve palsy. MRA (magnetic resonance angiography) is just not as reliable in that respect," he told EuroTimes. Catheter angiography, while still considered the gold standard has substantial risks associated with it that just aren't present with CTA. With MRA, the resolution is such that some of the aneurysms that accompany third nerve palsy can be missed, putting it a step below CTA. Missing aneurysms can be deadly, he stressed. "It's been shown in a large series of third nerve palsies that there is an aneurysmal aetiology in up to five percent of patients. And if you work in a neuroscience image institute you'll find that the prevalence is higher," he said. The biggest worry has to do with subarachnoid haemorrhage. Aneurysms that expand and erupt come with a 45% mortality rate. Plus, there is significant morbidity in 50% of survivors. In every patient who presents with isolated third nerve palsy, it is necessary to consider imaging, he said. He emphasised that conventional investigations with catheter angiography aren't exactly the safest approach: "There is a substantial risk of a permanent neurological deficit associated with the investigation. This patient population may be more likely to have vasculopathies increasing the risk for stroke, making catheter angiography a poor choice. There is a consensus among leading US neuro-ophthalmologists that MRI combined with MRA is the initial approach that is indicated in this disorder, possibly followed by cerebral angiography. I believe that is potentially dangerous advice." Several years ago, Scottish researchers performed CTA in a series of 100 consecutive patients with isolated third nerve palsy. If findings were abnormal, researchers then proceeded to perform catheter angiograms. Of patients who were aneurysm free, all went to a spontaneous recovery or else another cause for the third nerve palsy could be demonstrated. CTA showed aneurysms as small as 4.0 mm, he said. The smallest recorded unruptured aneurysm causing a third nerve palsy is 5.0 mm. A study in the medical literature suggested that MRA misses 1.5% of aneurysms that will rupture within eight years."That's unacceptable obviously in a potentially fatal disorder," he said. Generally, the advantages of CTA over MRA include the fact that CTA reveals calcifications, the imaging process is not claustrophobic for the patient, there are fewer artefacts from motion and it can be done an as emergency procedure. MRA has its own advantages in that patients are not exposed to ionising radiation, it can be easily combined with MRI, and no contrast is required, reducing the risk for adverse reactions to contrast agents. Both CTA and MRA are superior to conventional catheter angiography, Dr McFadzean explained. Dr. Martin ten Hove from Queen's University in Ontario , Canada agreed that CTA and MRA are both good modalities for imaging isolated third nerve palsies, but added that picking one over the other depends on how current your technology is. "In Dr. McFadzean's area in Glasgow , the CTA technology used is among the best in the world, unfortunately, not everyone has that generation of CTA. Conventional angiography is still used and remains the gold standard although its use is declining because CTA and MRA technology may soon provide the same diagnostic capability without the risks associated with conventional angiography" he noted. As a group, third nerve palsies are fairly common. Somewhere between 10% to 20% are aneurysmal, emphasising the importance of imaging. Robert McFadzean, MD Martin ten Hove, MD |
