Subarachnoid Hemorrhage and the CT Scan ROBERT DACHS, MD, FAAFP, Ellis Hospital Family Medicine Residency Program, Schenectady, New York MARK A
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AFP Journal Club The Story Behind the Study Subarachnoid Hemorrhage and the CT Scan ROBERT DACHS, MD, FAAFP, Ellis Hospital Family Medicine Residency Program, Schenectady, New York MARK A. GRABER, MD, FACEP, University of Iowa Carver College of Medicine, Iowa City, Iowa ANDREA DARBY-STEWART, MD, Scottsdale Healthcare Family Medicine Residency Program, Scottsdale, Arizona had a lumbar puncture indicating the presence of SAH. Purpose This patient had been experiencing a headache for more In AFP Journal Club, three presenters review an interesting than four days. The remaining 203 patients had negative journal article in a conversational manner. These articles involve “hot topics” that affect family physicians or “bust” findings on CT and on lumbar puncture, resulting in a commonly held medical myths. The presenters give their 99.7 percent sensitivity for CT (95% confidence interval, opinions about the clinical value of the individual study 98.1 to 99.9 percent). The authors conclude that CT is discussed. The opinions reflect the views of the presenters, excellent for identifying SAH, and that follow-up lumbar not those of AFP or the AAFP. puncture is not required for patients with symptoms last- Article ing three days or less. Cortunum S, Sørensen P, Jørgensen J. Determining the sensitivity of computed tomography scanning in early Should we believe this study? detection of subarachnoid hemorrhage. Neurosurgery. Bob: No—retrospective studies are inherently problem- 2010;66(5):900-902. atic. First, they rely on accurate and complete medical record documentation (and we all know how good we are at that). Second, there must be unambiguous and Is CT sensitive enough to rule out SAH? unbiased extraction of the data from the medical records. Bob: Emergency medicine dogma has stated that com- Gilbert and colleagues laid out eight concise steps neces- puted tomography (CT) is not sensitive enough to detect sary for a retrospective review (Table 1).2 Unfortunately, subtle subarachnoid hemorrhage (SAH), and that nega- this study misses many of these standard measures. The tive head CT must be followed by a lumbar puncture. As data abstractors were not identified, other than “two recently as 2008, researchers noted that four-slice CT had experienced members of the neurosurgical staff.” The a 93 percent sensitivity for detecting SAH.1 Because 7 per- study authors did not comment on the abstractors’ train- cent of SAH cases (one in 14) could be missed by CT, the ing or whether they were blinded to the study hypothesis mantra has been “head CT followed by lumbar puncture” or monitored/tested for interrater agreement. How the when SAH is suspected. data were abstracted also was not addressed. Even the This article attempts to make the argument that with basic question of which type of CT machine was used in the development of advanced generation CT machines, the study was not clearly documented. head CT is sensitive enough to detect all cases of However, this study’s bigger problem is the incorpora- SAH, and follow-up lumbar puncture is not required. tion of a type of selection bias known as spectrum bias. Unfortunately, I believe this conclusion is wrong and This phenomenon acknowledges that a diagnostic test’s dangerous. performance changes with different patient populations in different clinical settings. If we look at the results What does this article say? reported from this neurosurgical referral center, 59 per- Bob: This retrospective study was performed in Den- cent of the patients evaluated had an SAH. Clearly, this mark at a neurosurgical unit where the authors reviewed referral population is far different from the population the charts of all patients with suspected SAH who were that arrives in most community hospitals. This study referred to the unit over five years. CT was obtained in does not tell us if CT is sensitive enough in the typical all referred patients. If head CT was read as “normal,” a community hospital setting, where the presentation may lumbar puncture was performed. be less dramatic. CT identified SAH in 295 of the 499 patients evaluated. Andrea: I absolutely agree that you cannot extrapo- Only one patient with negative CT results ultimately late these results to the community hospital setting. ▲ Downloaded from the American Family Physician Web site at www.aafp.org/afp. Copyright © 2011 American Academy of Family Physicians. For the private, noncommercial 632 Americanuse of one Family individual Physician user of the Web site. All other rights reserved.www.aafp.org/afp Contact [email protected] for copyrightVolume questions 84, Number and/or permission 6 ◆ September requests. 15, 2011 Journal Club be managed with intravenous fluids, intravenous caf- Table 1. Strategies to Improve Accuracy and feine, or a blood patch.3 Are you willing to risk a patient’s Minimize Inconsistencies in Medical Chart life because of the fear of a postprocedure headache? Review Another strategy that has been recently advocated, but one I cannot support, is to follow the negative head Training CT with CT angiography.4 First, you are talking about a Train chart abstractors to perform their jobs. Describe the lot of radiation and the risk of contrast media–induced qualifications and training of the chart abstractors. Ideally, train abstractors before the study starts, using a set of reactions with this strategy. Second, this recommenda- “practice” medical charts. tion is based on a mathematical model and has never Case selection been tested. Third, we know that 2 percent of a typical Use explicit protocols and describe the criteria for case selection middle-aged population harbors asymptomatic intra- or exclusion. cranial aneurysms.5 If one were to follow this proposed Definition of variables strategy of negative head CT followed by CT angiogra- Define important variables precisely. phy and an aneurysm is discovered, a lumbar puncture Abstraction forms would have to be performed anyway to determine Use standardized abstraction forms to guide data collection. whether it is symptomatic. If the aneurysm is not bleed- Ensure uniform handling of data that are conflicting, ambiguous, missing, or unknown. ing and was found incidentally, you have another prob- Meetings lem—what are you going to do with an asymptomatic Hold periodic meetings with chart abstractors and study aneurysm? The overwhelming number of these small coordinators to resolve disputes and review coding rules. aneurysms (smaller than 5 mm) will never rupture, Monitoring but will likely result in significant patient anxiety and Monitor the performance of the chart abstractors. neurosurgical consultation. If the neurosurgeon elects to Blinding intervene, you can expect a 2 percent mortality rate and Blind chart reviewers to the etiologic relation being studied or 11 percent morbidity.5 the hypotheses being tested. If groups of patients are to be And a last note: when all the patients in this study compared, the abstractor should be blinded to the patient’s group assignment. with negative head CT underwent lumbar puncture, Testing of interrater agreement four of the remaining 204 patients were diagnosed with A second reviewer should reabstract a sample of charts, blinded viral meningitis. The take-home message here is that the to the information obtained by the first correlation reviewer. lumbar puncture gives you the answers you need in a Report a kappa statistic, intraclass coefficient, or other measure patient with new, unexplained headache. of agreement to assess interrater reliability of the data. Andrea: When deciding whether to proceed with the Adapted with permission from Gilbert EH, Lowenstein SR, Kozol- head CT followed by lumbar puncture in a patient with McLain J, et al. Chart reviews in emergency medicine research: where a headache, you may ask yourself if you should even are the methods? Ann Emerg Med. 1996;27(3):306. consider SAH in this patient’s differential diagnosis. A recent study prospectively evaluated 1,999 patients with sudden severe headache peaking within one hour And when we talk about the level of sensitivity we are of onset. Twelve findings on history and three findings willing to accept, we must remember what disease pro- on physical examination were associated with SAH cess we are talking about. For example, if a test is 93 per- (Table 2).6 The authors suggest that if patients do not cent sensitive for a self-limiting disease like influenza or have any of these characteristics, the chance that the pharyngitis, we would all find that acceptable. However, patient with a headache has an SAH is nil. A prospective 93 percent sensitivity for identifying a life-threatening study to assess the reliability of this suggestion is cur- problem, such as SAH or myocardial infarction, is not rently ongoing. acceptable. Because SAH often presents with a herald Bob: The article Andrea is referring to brings home bleed before a large, fatal rupture, this is the point when two points. One, the authors performed their study in the physician can truly save a patient’s life—each bleed six Canadian hospitals, and of the 1,999 patients with has approximately a 50 percent mortality rate. How- sudden severe headache, only 130 (6.5 percent) had SAH. ever, a herald bleed is often of small volume and is not This spectrum of patients better reflects what we see in detected by CT. community hospital settings. Two, of the 130 patients Mark: I am not sure why there is a movement to try with SAH, the hemorrhage was identified on CT in 121 to avoid the lumbar puncture. Yes, there is transient patients and on lumbar puncture in nine patients. The discomfort, and a small percentage of patients develop bottom line is that in this clinical setting, CT is only postdural puncture headaches, but these headaches can 93 percent sensitive for detecting SAH.6 September 15, 2011 ◆ Volume 84, Number 6 www.aafp.org/afp American Family Physician 635 Journal Club Table 2.