Predictors of the Subarachnoid Hemorrhage of a Negative CT Scan
To the Editor:
A good-quality head CT scan will reveal subarachnoid hemorrhage (SAH) in 93% of cases within 24 hours and in >98% of cases within 12 hours after the onset of symptoms.1,2 Because of rapid clearance of blood, delayed head CT scanning may be normal despite a suggestive history.3 However, the recently published guidelines by the American Heart Association do not involve the predictors of the SAH on a negative CT scan.4 This study sought to determine the predictors of the SAH on a negative CT scan.
An inception cohort consisted of 367 patients with SAH admitted to the hospital between August 1995 and December 2008. Third-generation CT scanners were used in all patients. The diagnosis of SAH was established on the basis of admission CT scans or by xanthochromia of the cerebrospinal fluid if the CT scan was negative. The patients with a negative CT scan were divided into an emergency CT scan-negative group (0 to 3 days of onset) and a nonemergency CT scan-negative group (4 to 14 days of onset) and 32 patients were randomized who had SAH according to a positive CT scan (control group) and compared individually. Multiple logistic regression was used to identify predictors of SAH of a negative CT scan. Spearman rank correlation was used to analyze the delayed the time of head CT scan in relation to the score of the CT rating scale.
Twenty-three of the 367 patients who had SAH on a negative CT scan were diagnosed by lumbar puncture. Of those, 3.3% (12 of 367) patients were in the emergency CT scan-negative group and 3.0% (11 of 327) patients were in the nonemergency CT scan-negative group. Baseline characteristics of the patients are given in the Table. A multiple logistic regression showed that low Hunt Hess grade (OR, 0.141; 95% CI, 0.031 to 0.636; P=0.011) and normal diastolic blood pressure (OR, 0.917; 95% CI, 0.852 to 0.986; P=0.020) were independent predictors of SAH on a negative emergency CT scan. Delaying the time of CT scan (OR, 3.701; 95% CI, 1.419 to 9.650; P=0.007) was an independent predictor of SAH in the nonemergency CT scan-negative group. Furthermore, there was a significant negative correlation between the time of the head CT scan in the nonemergency CT scan-negative group and the SAH CT rating scale score (r=−0.677, P=0.000).
Despite improvements in diagnostic imaging, CT may be less sensitive in patients with SAH presenting with so-called “minor leaks” or normal neurological status.2,5 Therefore, lumbar puncture should be performed in any patient with suspected SAH and negative or equivocal results on CT scan.6 SAH on a negative CT scan may be predicted by a low Hunt Hess grade, normal diastolic blood pressure, and delayed time of the CT scan on admission. The most likely explanation of this finding is that some patients have only a small amount of blood on an initial CT and in sporadic patients, the extravasation might be too small to be detected.2 The delayed time of the head CT scan was negatively related to the CT rating scale scores. The most logical explanation for this relationship is that a small amount of blood or “minor leaks” may disappear too fast to be detected on head CT if the patient presents a delayed time for a head CT scan.
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