Clinical Question: Does a normal head CT scan within 6 hours of headache onset rule out subarachnoid haemorrhage?
Title: Sensitivity of Early Brain Computed Tomography to Exclude Aneurysmal Subarachnoid Haemorrhage. A Systematic Review and Meta Analysis.
Authors: Nicole M. Dubosh et al.
Published: Stroke, March 2016.
I know not very recent! However it is big on social media and has not made it into established or ad hoc clinical practice locally.
Summary: This was a systematic review and meta-analysis to determine the accuracy of a CT only strategy of investigating suspected SAH with 3rd generation CT performed within 6hrs of headache onset.
3rd generation scanners relate to those >64slices (PRUH has these)
Original articles were selected to answer this question excluding trauma, patients <15years age, older scanners and CT >6hrs of headache onset.
The completed study selection process yielded 5 articles from an original search strategy yield of 882.
One of the studies was prospective, the others retrospective.
Four of the studies were diagnostic accuracy studies, the other a case-control study.
Studies were assessed for quality and bias risk and findings of heterogeneity between studies was noted. The gold standard test for SAH (CT positive or LP positive) was applied unevenly across all the studies.
Estimations were made on 2 of the studies as they only reported on negative CT results.
Several of the studies had different interpretations of what a positive or negative result was so the authors produced pooled results of all the 5 studies based on a worst case scenario.
Pooled figures were Sensitivity was 0.987, Specificity of 0.999 and Negative LR of 0.010.
3 studies with 2 x 2 data were used for a sensitivity analysis to ensure robustness of the main findings.
These gave a pooled incidence of SAH of 19.1%. Sensitivity was 0.986, Specificity of 0.996. The incidence of missed SAH in this population was 0.87 per 1000.
The Authors conclude that a negative 3rd generation CT brain scan performed within 6 hours of sudden headache onset is sufficient to rule out SAH in adult patients with a normal GCS who are neurologically intact.
Strengths, Weaknesses and Clinical Relevance
This study outlined clearly the aims with a focused question and specific population.
The search for articles appears to have been comprehensive enough (882 articles). Selection of the articles was robust also with 2 independent investigators screening the article abstracts to select articles for full review (40). Full text review then yielded the final 5 articles. Interobserver agreement was good at 87.5%.
As with all systematic reviews the crucial element is the strength of the individual papers that are in the analysis. There is considerable heterogeneity of the studies used, and the majority are retrospective. 2 of the papers did not report on positive CT numbers so 2×2 tables to discern accuracy had to be drawn from estimations from the Perry et al paper. The authors did use “worst case scenario” with one of the papers (Marks) where the 11 missed cases and still came up with an excellent sensitivity and Negative LR.
With only 5 papers in this review, I felt it prudent to read through them all. A brief summary is below –
Perry et al, 2011 –
The only prospective study in the meta-analysis. Multicentre cohort study based in Canadian ED with 3132 pt’s enrolled. Patients were presenting with acute headache maximal <1hr and CT timing was <6hrs from onset of headache. Scans reported by neuro or general radiologist.
Not every patient got the gold standard test of LP (49.4%). However, follow up was robust – 6 months telephone or if lost (2%) neurosurgical centre/coroners enquiries across whole Ontario state.
CT within 6hrs yielded SAH incidence of 12.7% and 100% for all of Sens, Spec, PPV, NPV.
Backes et al, 2012 –
Retrospective single centre study in Netherlands looking at allcomers with suspected SAH. 137 pt’s had CT scan within 6hrs. All scans read by a neuroradiologist. Of the scans <6hrs, 50% of those were positive although 11% of the positives were perimesencephalic bleeds (venous bleeds with no intervention needed).
All negative CT patients underwent LP. On those CT negative, 1 patient had positive bleed due to a cervical AVM but this patient had neck pain/stiffness with no headache. Excluding the 2 patients with no headache yielded 100% again for all of Sens, Spec, PPV, NPV.
Mark et al, 2013 –
USA Retrospective case-control study with 2 goals looking at CT <6hrs as well as a headache decision rule. Aimed to enrol 70pts but only enrolled 55 cases. Enrollment was based on confirmed SAH, not acute headache. 11 out of 55 patients with a negative CT within 6hrs had a supposed SAH. However, the LP RBC counts and xanthochromia differ in half of the 11 cases. In addition, significant data was missing and sudden onset headache made up only 75% of the cases.
Stewart et al, 2014 –
UK based single centre (teaching hospital) retrospective study looking at accuracy of CT within 12hrs of onset in cohort investigated for SAH. Subset of 65 patients with onset <6hrs looked at. Sens was 100% in this cohort. Authors did not recommend the strategy as cohort small.
Blok et al 2015 –
A multicentre study in non-academic hospitals (i.e. like PRUH). Retrospective review of CT negative patients but also including those CT reports later changed to positive after LP (2nd look and amendment). 760 pts total and 52 had positive LP. One CT amended. Of 52 LP positives, 8 had aneurysms on further scans but rupture deemed unlikely. 43 no aneurysm or scan not done (follow up at least 22months). NPV 99.9% stated.
As you can see the papers have significant differences between them. Most of the papers to tend to agree on the sensitivity of CT undertaken within 6hrs of onset. Only the Perry paper was prospective and one could argue that the whole meta-analysis is carried by this paper with its design and large data set.
The Mark et al study does stand out as having significant false negatives although this paper had issues with study design and interpretation of the gold standard.
The pooled incidence sounds very high but it is worth noting that those early presenters would be expected to have a higher incidence due the severity of their symptoms.
So, should we expect better data before we come to a definitive conclusion? A large, multicentre diagnostic study with all patients undergoing the gold standard of LP after negative CT is very unlikely to happen. An LP is painful with significant complications of post LP headache and false positive results leading to downstream testing and even treatment.
This review is probably as good as the evidence base is going to get I feel.
So what would I do now?
I would definitely consider a rule out after negative CT within 6hrs of thunderclap headache onset. The patient would have to be well appearing patient with no neuro signs and with SAH being the only likely diagnosis. Anyone still clutching their head, vomiting, altered GCS etc. I wouldn’t. Discharging the well patient after a clean CT scan seems reasonable and better for the patient and an already stressed emergency bed base. No rule or pathway has a 0% miss rate. However <1 in 1000 is arguably better than other EM testing strategies for high risk presentations such as PE, chest pain etc.
However, for those of you that are juniors the following really needs to be stressed –
- Please do not do this without senior discussion and ideally face to face review.
- This is only for suspected aneurysmal SAH. There are other sinister causes of headache such as venous thrombosis, meningitis etc. If the differential is wider than SAH alone do not use this guidance.
- This is not yet widespread practice and is not endorsed by any local or national guideline or any College/Society. As such this is at the fringes of what is deemed acceptable.
- If you do decide LP is not necessary, make sure you discuss/share the decision with the patient.
So, what to the rest of you think?