Objectives: To determine the sensitivity of third-generation CT scanners for diagnosed nontraumatic subarachnoid hemorrhage (SAH) and to assess the impact of symptom duration on sensitivity.Methods: A retrospective chart review was performed in a university-affiliated tertiary care hospital with an annual ED volume of >100,000 patients. The target population was all patients who presented to the ED from January 1991 to September 1994 with symptoms suggestive of SAH and who had a final diagnosis of nontraumatic SAH based on either a positive CT scan or positive spinal fluid analysis. Patients referred from outside facilities were included if they had a CT done at the study site. All C T scans were done using thirdgeneration scanners. Official CT scan reports were used to categorize scans as positive or negative.Results: There were 140 patients identified with SAH, with a mean age of 56 years (range 10-88). The sensitivity of CT in the diagnosis of nontraumatic SAH when performed at or before 12 hours of symptom duration was 100% (80/80), and 81.7% (49/60) after 12 hours of symptom duration (95% CI 95-100% and 69.5-90.4%, respectively; p c 0.0001). Eleven of the 140 patients had a negative CT and positive spinal fluid analysis, yielding an overall sensitivity of 92.1% (129/140).Conclusion: The sensitivity of third-generation CT scans for SAH decreases with time from the onset of symptoms. In this sample population, CT was able to detect all patients scanned 512 hours after symptom onset. Although the study demonstrated good sensitivity of CT scan reports for SAH when the scan was performed after 5 1 2 hours of symptom onset, additional real-time experience is needed to better define the potential risk of a missed SAH should this population not receive the customary lumbar puncture examination in the setting of a negative CT scan.Key words: subarachnoid hemorrhage; intracranial bleed; computed tomography; CT scan; neurology.
In a previous paper a method for simulating the electric potentials on the surface of the brain was introduced. This method consisted of the construction of a layer of radially oriented current dipoles in a conducting sphere that simulated the head so that the voltages generated by the layer would take the values measured on the surface of the medium (the scalp). The harmonic potential function for this layer was then evaluated in the interior of the medium in an attempt to approximate the potentials that would be generated by the actual neural sources but which could not be observed without recourse to invasive recording techniques. This method, the cortical imaging technique (CIT), has been previously tested by applying it to artificially generated data where the "cortical surface" potentials were known and could be compared with CIT-generated potentials. In this paper the method is tested by applying it to the scalp-recorded potentials evoked by right median nerve stimulation, where direct cortical recordings are available for comparison, and to the scalp-recorded epileptiform discharges from two patients where the spike foci were well defined. The effects of varying the "noise ratio," an input parameter in CIT which allows one to account for noise in scalp-recorded data, is discussed.
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