In a five year multicentre study we derived three decision rules using clinical findings to identify which neurologically intact patients with headache require investigation to rule out arguably the most serious cause of headache, subarachnoid haemorrhage. Arrival by ambulance, age ≥40, complaint of neck stiffness or pain, onset with exertion, vomiting, witnessed loss of consciousness, and raised blood pressure were strongly and reliably associated with subarachnoid haemorrhage. The presence of one or more of these findings in a patient with an acute non-traumatic headache reaching maximum intensity within one hour and that is unlike previous headaches should prompt physicians to consider investigating for subarachnoid haemorrhage.
We have previously shown that while physicians can discriminate between patients with subarachnoid haemorrhage and other forms of headache, they are reluctant to do so solely on their clinical findings.28
Our clinical decision rules, if validated, should provide physicians with evidence to manage more headache patients safely without investigation. Beyond healthcare costs, investigations entail exposure to radiation and perhaps intravenous contrast medium, difficulties in interpreting erythrocyte counts in traumatic lumbar punctures with false positive results, the indirect costs of incidental findings, and the morbidity of a headache after dural puncture. More selective testing can also shorten length of stay in an overcrowded emergency department. We have previously shown that computed tomography of the head adds about three hours to a patient’s length of stay, and performing a lumbar puncture adds another four hours.3
We are not aware of previous studies on clinical decision rules in patients with headache and normal results on neurological examinations. One small prospective study in 137 headache patients, including 23 cases of subarachnoid haemorrhage, concluded, without conducting a multivariate analysis, that there was no single clinical feature that could reliably identify which patients with headache require investigation.29
Other previous studies have attempted to identify risk factors and features suggestive of subarachnoid haemorrhage. A retrospective study of 500 patients with aneurysmal subarachnoid haemorrhage found that risk factors included women aged over 50, men aged under 50, and stressful events.30
Other proposed risk factors include alcohol consumption, smoking, hypertension, and oral contraceptives, but these variables are prevalent among many patients in emergency departments and might not be clinically useful for differentiating subarachnoid haemorrhage from other headaches.31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
Uncommon connective tissue disorders, including Ehlers-Danlos syndrome type IV, autosomal dominant polycystic kidney disease, and Marfan’s syndrome, can lead to subarachnoid haemorrhage but are absent in most patients with subarachnoid haemorrhage.14 46 47 48 49 50 51
Neck stiffness is often cited in reviews of the topic.12 14
Other associations with subarachnoid haemorrhage include acutely raised blood pressure.12
Strengths of study
To develop the clinical decision rules, we followed previously established methodological standards for developing and testing.52 53 54 55
We clearly defined our outcome, subarachnoid haemorrhage, and it was assessed without knowledge of the predictor variables. The predictor variables were prospectively evaluated and documented on standardised data collection forms before head computed tomography or lumbar puncture. We assessed the interobserver reliability of the predictor variables by having an independent second physician repeat the assessment of the patient. Patients were selected without bias and did not differ significantly from missed eligible patients. We were estimating the classification of performances of the clinical decision rules. The large sample size allowed us to achieve narrow confidence intervals for sensitivity. Each of our derived rules contains only four variables, all of which are relatively simple and well defined, which should allow clinicians to easily incorporate the optimal rule into everyday practice.
We set out to derive one or more models that met the methodological standards for development of a clinical decision rule. In particular we sought models that had strong statistical association with subarachnoid haemorrhage and that were clinically sensible and easy to use. We opted to retain the three best performing models for further evaluation, partly because of the high morbidity or mortality associated with a missed diagnosis. In addition, we had some reservation regarding arrival by ambulance as a component of any rule. While this strong predictor presumably captures a measure of severity of headache and seriousness perceived by the patient, it is also probably strongly influenced by the local pre-hospital system and might not be transferrable to regions where people have differing thresholds for calling for an ambulance. Hence, we opted to keep one model without the ambulance variable for additional study.
While we identified a few cases of misdiagnosed subarachnoid haemorrhage, most tests performed yielded negative results. Given the increased costs of medical care and increasing problems of overcrowding in emergency departments, improved diagnostic efficiency is particularly important. Use of these rules could eliminate the need for any tests without missing any cases of subarachnoid haemorrhage for between one in five and one in 10 patients presenting to the emergency department with a headache peaking within an hour.
Forty eight enrolled patients had serious conditions other than subarachnoid haemorrhage. For these patients, it was apparent from the documentation that physicians were concerned about the possibility of other pathology before they obtained results of imaging or lumbar puncture. We did not explicitly ask physicians if they were investigating for other conditions on the study forms or exclude patients in whom subarachnoid haemorrhage was not the most probable serious diagnostic consideration. Given the heterogeneity of headache, it would be impractical to generate one clinical decision rule for all causes. Generally speaking, most other causes of serious headache would have other clinical clues (such as fever and transient or persistent neurological deficits) to help guide investigations for other significant pathology.
Our inclusion criteria allowed for patients with non-thunderclap headaches to be enrolled by specifying an onset to peak intensity of up to one hour. While this could have diluted the acuity, we note that the reported time to peak headache intensity was up to several minutes in our patients with subarachnoid haemorrhage. In addition, we excluded patients with a history of three or more similar headaches (same intensity and character) in the past over a time frame of over six months. This was intended to eliminate patients with chronic recurrent headaches, in whom our rules should not be applied. We recognise that physicians might overlook exclusion criteria when applying any decision rule and emphasise that such extrapolation is not evidence based. Another potential limitation was the lack of an established standard definition of a positive subarachnoid haemorrhage. We believe that our composite outcome is the best definition available at this time. In addition, we derived three potential clinical decision rules rather than choosing just the best performing rule, mainly on the basis that arrival by ambulance might not be useful without validation in regions with different cultural or business models for ambulance services.
As many as a third of eligible patients might not have been enrolled in this study. This is almost certainly overstating the magnitude of this potential limitation as we coded patients as being “missed” if we could not definitely determine that they did not meet the eligibility criteria. Because of poor recording, often we could not determine how rapidly the headache peaked. Hence, by default, these patients were deemed missed, even though many would probably not have peaked within one hour. This conclusion is reinforced by the lower investigation rates and lower rates of subarachnoid haemorrhage in these “missed” patients.
Finally, the proposed clinical rules need to be validated before being incorporated fully in clinical practice, as essential component of the development of any clinical decision rule. While we did carry out bootstrapping analysis for internal validation, the rules require independent validation before they can be implemented.
While the rules should not yet be used to explicitly rule out subarachnoid haemorrhage, they certainly can be considered to help to identify high risk patients. Hence, patients with any one or more of the seven findings (age ≥40, witnessed loss of consciousness, complaint of neck pain or stiffness, onset with exertion, arrival by ambulance, vomiting, diastolic blood ≥100 mm Hg or systolic blood ≥160 mm Hg) should be considered carefully for subarachnoid haemorrhage and undergo rapid and thorough investigation to rule out this life threatening condition.
The three proposed rules are being prospectively and explicitly evaluated in an ongoing study to determine their accuracy for subarachnoid haemorrhage, their interobserver agreement for interpretation, and their potential impact on investigation. Once this prospective evaluation has been completed, the optimal rule can be chosen for implementation into clinical practice. The best performing rule will allow clinicians to be more selective in determining which patients require investigation. This will improve care of patients by directing testing for those at high risk who might otherwise not be adequately investigated. At the same time, the rule should result in no computed tomography or lumbar puncture for patients at low risk, thus decreasing their morbidity from unnecessary tests.
What is already known on this topic
- Subarachnoid haemorrhage can occur in emergency patients presenting with sudden severe headache, even if they are neurologically intact at initial presentation
- Lumbar puncture is traditionally carried out to exclude this possibility, even if results of computed tomography are negative
- Most sudden headaches are benign and self limiting; universal investigation is inefficient and involves unnecessary exposure to radiation and post-lumbar puncture headache
What this study adds
- Three similar but distinct decision rules for subarachnoid haemorrhage have perfect sensitivity and are based on readily available pragmatic and consistent predictor variables
- Use of any one of these rules can reduce unnecessary investigation without missing any cases of subarachnoid haemorrhage