The Answer to Imperfect Computed Tomography Sensitivity for Subarachnoid Hemorrhage: Use Clinical Judgment

to increase awareness of the CT findings in patients withserious causes of headache.

David T. Schwartz, MDDepartment of Emergency MedicineNew York University School of MedicineNew York, NY

doi:10.1016/j.annemergmed.2008.06.471

Funding and support: By Annals policy, all authors are required todisclose any and all commercial, financial, and other relationshipsin any way related to the subject of this article, that might createany potential conflict of interest. The author has stated that nosuch relationships exist. See the Manuscript Submission Agree-ment in this issue for examples of specific conflicts covered by thisstatement.

1. Byyny RL, Mower WR, Shum N, et al. Sensitivity of noncontrastcranial computed tomography for the emergency departmentdiagnosis of subarachnoid hemorrhage. Ann Emerg Med. 2008;51:697-703.

2. van der Wee N, Rinkel GJE, Hasan D, van Gijn J. Detection ofsubarachnoid hemorrhage on early CT: Is lumbar puncture stillneeded after a negative scan? J Neurol Neurosurg Psychiatr. 1995;58:357-359.

3. Perry JJ, Stiell IG, Wells GA, et al. The sensitivity of computedtomography for the diagnosis of subarachnoid hemorrhage in EDpatients with acute headache. Acad Emerg Med. 2004;11:435-436.

4. Schwartz DT. Emergency Radiology: Case Studies. McGraw-Hill,2008, pp. 461-485.

5. Kowalski RG, Claassen J, Kreiter KT, et al. Initial misdiagnosis andoutcome after subarachnoid hemorrhage. JAMA. 2004;291:866-869.

6. Mayer PL, Awad IA, Todor R, et al. Misdiagnosis of symptomaticcerebral aneurysm - Prevalence and correlation with outcome atfour institutions. Stroke. 1996;27:1558-1563.

7. Perry JJ, Stiell IG, Wells GA. Attitudes and judgment of emergencyphysicians in the management of patients with acute headache.Acad Emerg Med. 2005;12:33-37.

The Answer to Imperfect Computed TomographySensitivity for Subarachnoid Hemorrhage: UseClinical Judgment

To the Editor:I read with interest the article by Byyny et al on the

sensitivity of computed tomography (CT) for subarachnoidhemorrhage.1 They concluded that modern CT cannotcompletely exclude subarachnoid hemorrhage and patients withnegative imaging results “. . .must undergo further evaluation toexclude. . .lesions.” These findings reinforce subarachnoidhemorrhage as the diagnostic “perfect storm” where symptomscan present atypically, the gold standard involves a painful,time-consuming test (lumbar puncture), and if missed, mayresult in serious harm. How should we interpret these new
findings? Should these results change our threshold to lumbar
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puncture patients with headache? Step back and think whetherthis study purports to answer these questions; I would assert itdoes not.

When subarachnoid hemorrhage is considered, patients fallinto three categories:

Case 1: Patients who concern us enough to CT and we thinkwe must lumbar puncture if the CT is negative because they arehigh-risk.

Case 2: Patients who concern us enough to CT, but somephysicians (the less risk-averse) may not lumbar puncture if theCT is negative.

Case 3: Patients where other headache causes are likelyclinically (ie, migraine) so we defer testing.

Both Case 1 and 3 seem reasonable. But is Case 2 bad EDpractice if all don’t get a lumbar puncture? Maybe—but maybenot. If you consider a calculation of the risk, it may be actuallyrational to defer lumbar puncture in some. In fact, a policy ofnot providing the gold standard test to everyone in whom adiagnosis is considered mirrors how we approach other high-risk, low-probability decisions. Few of the commonly used EDtests for pulmonary embolism or acute coronary syndrome are100% sensitive. Yet we use many of these tests (such as D-dimer) as diagnostic endpoints. Furthermore, both pulmonaryembolism and acute coronary syndrome are more common thanthe often-sought, rarely found “CT negative, lumbar puncturepositive” subarachnoid hemorrhage which I have only seentwice in my years of practice. In both instances, patients fell into“Case 1”.

Let’s run the numbers. If there is a 2% pretest probability ofsubarachnoid hemorrhage, we apply 91% sensitivity and 100%specificity. That yields a negative predictive value of 99.8%. Thenumber needed to test is 500. In other words, you must lumbarpuncture 500 patients to find one occult subarachnoidhemorrhage. Of those, 150 (30%) may develop debilitatingpost-dural headaches; fewer may require blood patches. So islumbar puncturing everyone in Case 2 rational or are wesubjecting patients to unnecessary, painful tests?

The authors point to decision rules for the answer. However,a reasonable attempt to create a decision rule for subarachnoidhemorrhage resulted in one with limited utility because itreported “arrived by ambulance” as a factor.2 Until we have datathat either objectively classifies low-risk patients or demonstratesthat a liberal approach misses more occult subarachnoidhemorrhage than a conservative “lumbar puncture all”approach, I believe the answer is to use your best clinicaljudgment.

Jesse M. Pines, MD, MBA, MSCEDepartment of Emergency MedicineCenter for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of MedicinePhiladelphia, PA


Annals of Emergency Medicine 161

Correspondence


1. Byyny RL, Mower WR, Shum N, et al. Sensitivity of noncontrastcranial computed tomography for the emergency departmentdiagnosis of subarachnoid hemorrhage. Ann Emerg Med. 2008;51:697-703.

2. Perry JJ, Stiell IG, Wells GA, et al. Clinical decision rule to safelyrule out subarachnoid hemorrhage in acute headache patients inthe emergency department. Acad Emerg Med. 2007; 13:S9.

In reply:We thank Dr. Schwartz for his comments about our article.

We agree that emergency physicians should use a Bayesianapproach to risk assessment; thereby including the patient’spretest probability of disease and the test characteristics todetermine the patient’s posttest disease probability. This posttestprobability allows physicians to make a risk-benefit assessmentof further testing and/or therapy and to have a well-informedconversation with their patients.

There are, however, several problems with these suggestions.We have no scale to assess a patient’s pretest probability ofdisease. Patients with “thunder-clap” headache have anapproximately 12% risk of having a subarachnoid hemorrhageas a cause of their headache.1 However, we do not know howother historical or physical exam findings interact to change thisprobability. Kowalski et al found that we still miss a large ofnumber of patients who are later admitted to neurosurgicalintensive care units with subarachnoid hemorrhage and thatthese patients do poorly when compared to those who arecorrectly identified on their first encounter.2

Missed subarachnoid hemorrhage can result in neurologicdevastation or death. The important risks of lumbar punctureare false positive results and the morbidity of subsequentcerebral angiography. The use of CT angiography lessens theserisks.

Their presumption of the pretest probability of disease of 2%appears to be artificially low given that the literature suggests a1% rate of subarachnoid hemorrhage among all headachepatients presenting to the emergency department (ED).3 If weevaluated 25% of all ED patients presenting with headache,which we believe would be a high percentage, this would resultin a 4% risk of subarachnoid hemorrhage. Using the suggestedBayesian analysis, one would miss 1 in 250 patients withsubarachnoid hemorrhage by not doing the lumbar puncture.Possibly some physicians and patients would find this anacceptably low risk; we would not.

We agree with Dr. Pines that the accuracy of the CTinterpretation is paramount when determining the sensitivity ofCT scan in detecting spontaneous subarachnoid hemorrhage.
We could have had a senior neuroradiologist review all of the
162 Annals of Emergency Medicine

cranial CTs; however, this would have reduced thegeneralizability of our results. What would the sensitivity resultsmean to a community emergency physician relying on a night-hawk read? In our study CTs were interpreted by communityradiologists, radiology residents and academic radiology faculty.

In Perry’s cited study,4 the sensitivity of head CT was 100%(95% confidence interval [CI] 94%-100%) but with asensitivity analysis they were 98% (95% CI 91%-100%). Theconfidence intervals in our study overlap with these and,therefore, are not significantly different. Additionally, Perry had60 patients lost to follow up. It is possible that some of thepatients were lost to followup because they died fromsubarachnoid hemorrhage.

We agree with the comments concerning false positivelumbar puncture results as noted above. However, until we havea validated decision-rule for accurately predicting a patient’spretest probability of subarachnoid hemorrhage, which giventhe prevalence of disease would require a multicenterprospective derivation and then subsequent validation, wecontinue to perform and recommend lumbar punctures onpatients with suspected subarachnoid hemorrhage with anegative non-contrast cranial CT.

Richard L. Byyny, MD, MSDepartment of Emergency MedicineDenver Health Medical CenterDenver, Colorado

Larry J. Baraff, MDWilliam R. Mower, MD, PhDUCLA Emergency Medicine CenterDavid Geffen School of Medicine at UCLALos Angeles, CA



1. Morgenstern LB, Luna-Gonzales H, Huber JC, Jr, et al. Worstheadache and subarachnoid hemorrhage: prospective, moderncomputed tomography and spinal fluid analysis. Ann Emerg Med.1998;32:297-304.

2. Kowalski RG, Claassen J, Kreiter KT, et al. Initial misdiagnosis andoutcome after subarachnoid hemorrhage. JAMA. 2004;291:866-869.

3. Edlow JA, Caplan LR. Avoiding pitfalls in the diagnosis ofsubarachnoid hemorrage. N Engl J Med. 2000;342:29-36.

4. Perry JJ, Stiell IG, Wells GA, et al. Clinical decisions rule to safetyrule out subarachnoid hemorrhage in acute headache patients in
the emergency department. Acad Emerg Med. 2007;13:S9.
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The Answer to Imperfect Computed Tomography Sensitivity for Subarachnoid Hemorrhage: Use Clinical Judgment