Subarachnoid Haemorrhage

Ottawa SAH Clinical Decision Rule14

The Ottawa Subarachnoid Haemorrhage (SAH) Clinical Decision Rule is for use in ruling out SAH as cause for acute headache in adults.

Patients who meet any positive criteria cannot have SAH ruled out by the rule and the authors state that patients require further investigation if one or more finding is present. Those who do not meet any of the criteria can exclude SAH as a diagnosis and consider an alternative cause for their presentation. The rule comprises 6 components:

  • Age ≥40
  • Neck pain or stiffness
  • Witnessed loss of consciousness
  • Onset during exertion
  • Thunderclap headache (pain peaking within 1 second)
  • Limited neck flexion on examination

Of note, this rule cannot be used in those with:

  • Any new neurological deficits
  • Papilloedema
  • History of prior aneurysm, SAH, brain neoplasm, hydrocephalus
  • Three or more similar headaches in the preceding six months
  • Reassessment of the same headache already investigated by CT and LP
  • Those who present >14 days since headache onset

The initial study was composited from three previous clinical decision rules.15 The initial study demonstrated a 100% sensitivity (95% confidence interval [CI] 97.2%–100%) with a specificity of 15.3% (95% CI 13.8% –16.9%).14 It should be noted that not all patients underwent full work-up (81.8% had CT imaging and 40.6% had lumbar puncture), and producing a rule in this way may result in a decision rule that is over-fitted to the data.

A later study performed at the same sites to validate the study found a sensitivity of 100% (95% CI 94.6%–100%) and a specificity of 13.6% (95% CI 13.1%–15.8%).16 It should be noted that this study was underpowered and did not meet its recruitment target and missed a high number of eligible cases (33.8%), with a high number of SAH in the non-recruited population.

A later before/after implementation study at these same centres enrolled 3672 patients (1743 before and 1929 after implementation) to validate the 6-hour rule and Ottawa SAH rule.17 The sensitivity in this study was 100% (95% CI 98.1% – 100%) with a specificity of 12.7% (95% CI 11.7% – 13.9%). Its utility as a rule to determine who should receive CT head imaging was limited; the proportion of patients undergoing CT head imaging before (88.0%) and after (87.5%) the rule was essentially unchanged (p=0.643).

Most validation of CDRs should be performed at an external site in order to improve external validity; each of these validation studies was performed at the same sites with a team well-versed in use of the rule. Furthermore, these studies used neuroradiologists and were based in neurosurgical centres. There has only been one external validation study for the Ottawa SAH rule, which was a retrospective study of 454 patients. This found a sensitivity for detecting SAH of 100%, but with a much wider 95% CI – 62.9%–100%.18

TERN’s SHED Study is a prospective observational study which will look to externally validate the Ottawa CDR in the UK population (amongst other aims!) The study will start recruiting in October 2021.19