Hemorrhage isolated to the peri-mesencephalic cistern is seen in about 5% of all patients with subarachnoid hemorrhage (SAH) and is thought to be of venous origin. CT angiogram is an extremely sensitive diagnostic tool and has been shown to reliably identify aneurysms in 10% of patients with peri-mesenchephalic SAH (pSAH). Studies till date have demonstrated that follow up imaging in patients with pSAH does not add clinically significant information, at the risk of increased cost to the patient. The authors performed a retrospective review of patients admitted to their center with pSAH and a meta-analysis of all studies of pSAH to determine the utility of follow up imaging in these patients.
A retrospective review of imaging reports identified 214 patients with pSAH on initial non contrast CT head. Other inclusion criteria included performance of CTA/DSA within 24 hours of admission as well as follow up imaging. Those with history of trauma, diffuse SAH, presence of intraventricular blood were excluded. A total of 18 patients met inclusion criteria and their images, both acute and follow up, were reviewed by neuroradiology and were assessed for a positive diagnostic yield. Additionally a meta-analysis of 40 studies of patients with pSAH was performed, excluding those with less than 5 patients, lack of follow up imaging, increased loss to follow up and CSF xanthocromia without pSAH. Patients from these were classified based on the type of acute and follow up imaging (Conventional angiogram only, CTA only and both conventional angiogram and CTA). Demographic data was analyzed however neurologic deficits and outcomes were not evaluated. Two-way ANOVA analysis of aneurysm detection rate based on initial and follow-up protocols was used to evaluate the effects of each initial and follow-up imaging strategy. The risk of bias within individual studies and across studies was also assessed.
The institutional analysis of 18 patients and the overall meta-analysis of 1,440 patients across 40 studies did not elicit benefit of utilizing follow up imaging. Mean age was 51.9 years and 1,031 of 1440 patients had follow up imaging available. Two-way ANOVA test yielded a p-value of 0.353 for the three initial diagnostic strategies, 0.701 for the three follow-up strategies, and 0.916 for the interaction term of initial and follow-up strategies. There was no statistically significant benefit to performing DSA during the initial evaluation or of any follow-up angiographic studies. Only eight aneurysms in eight patients (0.78%) were detected on any of the follow-up imaging.
This study re-enforces prior observations in this field. Peri-mesencephalic SAH is associated with a low risk of recurrence, vasospasm and hydrocephalus and is often venous in origin. The utility of re-imaging during follow up is low as the negative predictive value of the initial test (CTA or DSA) is high (91-96%). There still remains a possibility of missing small <3mm aneurysms with either test however the low rate of detection with a second imaging study does not justify the cost and risk of subjecting a patient to repeated imaging. These results have clinical implications and like many studies before this one, their application to practice may be difficult as physicians are often compelled to find reasons and search for the etiology of SAH. Regardless, each case has to be handled differently and some may continue to warrant follow up imaging.
