A few months ago ACEP (the American College of Emergency Physicians) published an update to their 2008 guideline on headache. It's not a comprehensive statement on headache, but focuses mainly on subarachnoid haemorrhage (SAH).

This statement is important, even for doctors outside the USA, because it is one of the first national guidelines to recommend the following 2 evidence-based practices:

Ottawa SAH rule to risk stratify patients with acute severe headache

Negative CT within 6 hours of headache onset to rule out SAH

The paper

Godwin SA, Cherkas DS, Panagos PD, et al. Clinical policy: critical issues in the evaluation and management of adult patients presenting to the emergency department with acute headache. Ann Emerg Med. 2019 Oct;74(4):e41-e74​[1]​

Can we use a risk stratification tool?

Level B recommendation: YES

The Ottawa subarachnoid haemorrhage (SAH) rule is recommended as being very sensitive (approaching 100%) and safe for ruling out SAH. The authors do note, however, that the rule is poorly specific (around 20%), so using it indiscriminately will result in over-imaging.

I think of this rule as the PERC of headaches - great for ruling out the diagnosis, rubbish for ruling it in.

The Ottawa SAH rule

It is only to be used on patients with:1. No neurological deficit, and 2. Headache reached maximum severity within 1 hour of onset

If none of the following features are present, SAH is ruled out:

Mnemonic 1: CT HEADCollapse (witnessed LOC)Thunderclap (pain instantly reaches peak)Hurt neckExertion (onset during)Age >40Decreased neck flexion

Mnemonic 2: ANT LEaFAge >40Neck pain/stiffnessThunderclap onsetLOCExertionFlexion decreased

2. Does a normal CT within 6 hours rule out SAH?

Level B recommendation: YES

The authors seemed to have difficulty finding studies that met their strict methodological grading criteria. Their initial 594 articles were whittled down to the following two...

Perry et al (2011)​[2]​ was a prospective cohort study of 3,132 patients with sudden onset non-traumatic headache. Of the 7.7% who had a SAH, none were missed on early scan. CT head within 6 hours had a sensitivity of 100%.

Dubosh et al (2016)​[3]​ was a systematic review and meta-analysis of 8,907 patients. A total of 13 patients had a SAH that was missed on early scan, but 11 of these were from a single study of 55 patients. (Having most of the outliers in one small study makes me wonder how reliable that particular study's results are.) However, even including this trial the meta-analysis still yields a sensitivity of 98.7% for CT within 6 hours. We reviewed this paper on The BREACH here.

Why 6 hours?

Blood appears hyperdense on CT because of its high protein content. Acute bleeds are denser than surrounding brain tissue and so shine white. Over time blood proteins are degraded and absorbed and the blood becomes less dense, eventually becoming indistinguishable from brain tissue - SAH could be 'missed' if too much time elapses. This process is highly variable, taking hours to weeks, but several studies​[4,5]​ have shown that the sensitivity of CT for spotting acute blood starts to decrease at 6 hours.

What about the UK?

So, yes, it's worth bearing in mind that this is an American document and there is no corresponding recommendation from NICE as yet.

However, the SHED study is due to start data collection in a few months. The aim is to collect observational data on a cohort of 10,000 patients across 100 UK sites. This will then be used to attempt to externally validate both the Ottawa SAH rule and a 6-hour CT only rule out strategy

So a bit of caution is needed while we wait for official guidance - I'd make sure your consultants are on board before discharging patients with a negative 6 hour CT.

More FOAMed on this...

FOAMcast - ACEP clinical policy on headacheJournal Feed - Acute headache - ACEP policy statementSt Emlyns - Let's talk about subarachnoid haemorrhage (SAH)REBEL EM - Does a normal head CT within 6 hours of onset of headache rule out SAH?EM Literature of Note - Is the 6-hour CT for SAH debate over?SGEM #134 - Listen to what the British doctors say about LPs post CT for SAHEmergency Physicians Monthly - LP for subarachnoid hemorrhage: the 700 clubEMbeds - Ottawa SAH rule

References

Godwin SA, Cherkas DS, Panagos PD, Shih RD, Byyny R, Wolf SJ, et al. Clinical Policy: Critical Issues in the Evaluation and Management of Adult Patients Presenting to the Emergency Department With Acute Headache. Annals of Emergency Medicine [Internet] 2019;e41–74. Available from: http://dx.doi.org/10.1016/j.annemergmed.2019.07.009

Perry JJ, Stiell IG, Sivilotti MLA, Bullard MJ, Emond M, Symington C, et al. Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study. BMJ [Internet] 2011;d4277–d4277. Available from: http://dx.doi.org/10.1136/bmj.d4277

Dubosh N, Bellolio M, Rabinstein A, Edlow J. Sensitivity of Early Brain Computed Tomography to Exclude Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Meta-Analysis. Stroke [Internet] 2016;47(3):750–5. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26797666

Edlow JA. Managing Patients With Nontraumatic, Severe, Rapid-Onset Headache. Annals of Emergency Medicine [Internet] 2018;400–8. Available from: http://dx.doi.org/10.1016/j.annemergmed.2017.04.044

Sidman R, Connolly E, Lemke T. Subarachnoid Hemorrhage Diagnosis: Lumbar Puncture Is Still Needed When the Computed Tomography Scan Is Normal. Academic Emergency Medicine [Internet] 1996;827–31. Available from: http://dx.doi.org/10.1111/j.1553-2712.1996.tb03526.x