Atraumatic Headache in US Emergency Departments
The National Hospital Ambulatory Medical Care Survey (NHAMCS) is conducted annually by the Division of Health Care Statistics of the National Center for Health Statistics (NCHS), Center for Disease Control (CDC). The survey represents a four-stage probability sample of visits to randomly selected non-institutional general hospitals and short-stay (<30 days) hospitals, with the exception of federal, military and VA hospitals, in the USA. The sampling design includes primary selection of units corresponding to geographical regions (eg, counties, townships, metropolitan statistical areas), hospitals within those regions, EDs and outpatient units within those hospitals and patient visits within those EDs and outpatient units.
Hospital staff trained by a field interviewer from the Census Bureau are responsible for collecting data from a random sample of patients during annual recording periods of 4 weeks' duration using standardised patient record forms. Visits are excluded if no health services are rendered. Further information regarding survey methodology is available online. Given that NHAMCS data is publicly accessible and de-identified, this study is non-human subjects research exempt from Institutional Review Board review.
We identified patient cases from the 1998–2008 NHAMCS surveys using the following NCHS-assigned patient 'reason-for-visit' classification codes which reflect the patient's description of his or her problem upon presentation to the ED: headache (1210.0), sinus headache (1410.0), migraine (2365.0) or facial pain (1055.4). Patients with any of the above codes in any of the three reason-for-visit fields were included.
To ensure an accurate representative sample, visits related to trauma or injury were excluded in several ways: by excluding all patient cases for which the survey item asking if the visit was related to injury/poisoning/adverse event was marked 'yes'; by excluding all patients with injury or poisoning-related International Classification of Diseases (ICD-9-CM) diagnosis codes specified (800–999); and by excluding all patients with causes of injury/poisoning/adverse event coded via the Supplementary Classification of External Causes of Injury and Poisoning (E-codes) within the survey. Patient records with absent reason for visit and/or diagnosis codes for all available fields were also excluded. The NHAMCS dataset was used in its entirety as a reference to compare our sample of headache-related ED visits with a representative sample of all ED visits over the same time period.
Absolute numbers of ED visits were approximated using NCHS-assigned sample weights tabulated for each patient visit record, taking into account the four-stage design of the survey, thus providing an unbiased estimate of the annual number of visits. For each case we recorded whether diagnostic CT or MRI was performed. The number of cases in which MRI was ordered was too low to evaluate separately and was also treated as a combined variable in some NHAMCS survey years; consequently, we created a combined CT/MRI variable for all years examined, as has been done in recent studies (eg, by referring to CT/MRI as 'advanced radiology'). Comparing this variable against the study population, ED utilisation of imaging for atraumatic headache was approximated by year. Calculation of the average length of visit is based on data from 2001 onwards because the NHAMCS did not include this variable as a survey item prior to 2001.
We next recorded the frequency with which diagnosis of ICP was associated with use of CT/MRI. ICP was defined from a list of available ICD-9 codes that signify potentially life- or limb-threatening diagnoses made on the basis of neuroimaging. This included subarachnoid haemorrhage, benign or malignant brain neoplasm, stroke (ischaemic, haemorrhagic, embolic, thrombotic), intracranial haemorrhage, cerebrovascular anomaly, thrombosis, cerebral aneurysm, intracranial abscess and vertebral dissection. Trends over time were calculated using the same method as imaging utilisation.
Imaging studies in patients with CNS infection (encephalitis/meningitis) may be normal and the results are frequently non-specific, neither excluding nor definitively establishing a diagnosis. The diagnosis of meningitis is made on the basis of clinical assessment and CSF analysis. The most important role of CT is to identify contraindications to lumbar puncture—that is, occult causes of increased intracranial pressure such as cerebral abscess. For these reasons, the meningitides/encephalitides were excluded from our definition of ICP diagnosed on the basis of imaging.
To identify factors associated with an ED diagnosis of ICP, we examined visits by several demographic variables including age, sex, race and geographical location. We also examined variables related to the patient's visit such as mode of arrival and triage urgency. Some variables (eg, mode of arrival) were not included in the survey data for certain years prior to 2003, so we used years 2003–8 for analysis. Finally, we examined vital signs and several features of presentation by assessing reason-for-visit codes. A total of 18 factors were examined, many of which comprised multiple related reasons for visit in the NHAMCS survey. Adjusted ORs were calculated to identify a subset of variables significantly associated with receiving CT/MRI, defined as an adjusted OR >1.00 with a 95% CI not including 1.00.
Statistical analyses were performed using Statistical Analysis Software (SAS) V.9.1.3 (SAS Institute). Unless specifically noted otherwise, ED visit sample estimates and associated CIs were calculated using relative standard errors (dividing the SE by the estimate itself) of <30% and with at least 30 raw data items in accordance with NCHS recommendations. Adjusted ORs for factors associated with ICP were calculated using logistic regression analysis. The least squares method of linear regression was used for analysis of trends, with p<0.05 deemed statistically significant.
Methods
The National Hospital Ambulatory Medical Care Survey (NHAMCS) is conducted annually by the Division of Health Care Statistics of the National Center for Health Statistics (NCHS), Center for Disease Control (CDC). The survey represents a four-stage probability sample of visits to randomly selected non-institutional general hospitals and short-stay (<30 days) hospitals, with the exception of federal, military and VA hospitals, in the USA. The sampling design includes primary selection of units corresponding to geographical regions (eg, counties, townships, metropolitan statistical areas), hospitals within those regions, EDs and outpatient units within those hospitals and patient visits within those EDs and outpatient units.
Hospital staff trained by a field interviewer from the Census Bureau are responsible for collecting data from a random sample of patients during annual recording periods of 4 weeks' duration using standardised patient record forms. Visits are excluded if no health services are rendered. Further information regarding survey methodology is available online. Given that NHAMCS data is publicly accessible and de-identified, this study is non-human subjects research exempt from Institutional Review Board review.
We identified patient cases from the 1998–2008 NHAMCS surveys using the following NCHS-assigned patient 'reason-for-visit' classification codes which reflect the patient's description of his or her problem upon presentation to the ED: headache (1210.0), sinus headache (1410.0), migraine (2365.0) or facial pain (1055.4). Patients with any of the above codes in any of the three reason-for-visit fields were included.
To ensure an accurate representative sample, visits related to trauma or injury were excluded in several ways: by excluding all patient cases for which the survey item asking if the visit was related to injury/poisoning/adverse event was marked 'yes'; by excluding all patients with injury or poisoning-related International Classification of Diseases (ICD-9-CM) diagnosis codes specified (800–999); and by excluding all patients with causes of injury/poisoning/adverse event coded via the Supplementary Classification of External Causes of Injury and Poisoning (E-codes) within the survey. Patient records with absent reason for visit and/or diagnosis codes for all available fields were also excluded. The NHAMCS dataset was used in its entirety as a reference to compare our sample of headache-related ED visits with a representative sample of all ED visits over the same time period.
Absolute numbers of ED visits were approximated using NCHS-assigned sample weights tabulated for each patient visit record, taking into account the four-stage design of the survey, thus providing an unbiased estimate of the annual number of visits. For each case we recorded whether diagnostic CT or MRI was performed. The number of cases in which MRI was ordered was too low to evaluate separately and was also treated as a combined variable in some NHAMCS survey years; consequently, we created a combined CT/MRI variable for all years examined, as has been done in recent studies (eg, by referring to CT/MRI as 'advanced radiology'). Comparing this variable against the study population, ED utilisation of imaging for atraumatic headache was approximated by year. Calculation of the average length of visit is based on data from 2001 onwards because the NHAMCS did not include this variable as a survey item prior to 2001.
We next recorded the frequency with which diagnosis of ICP was associated with use of CT/MRI. ICP was defined from a list of available ICD-9 codes that signify potentially life- or limb-threatening diagnoses made on the basis of neuroimaging. This included subarachnoid haemorrhage, benign or malignant brain neoplasm, stroke (ischaemic, haemorrhagic, embolic, thrombotic), intracranial haemorrhage, cerebrovascular anomaly, thrombosis, cerebral aneurysm, intracranial abscess and vertebral dissection. Trends over time were calculated using the same method as imaging utilisation.
Imaging studies in patients with CNS infection (encephalitis/meningitis) may be normal and the results are frequently non-specific, neither excluding nor definitively establishing a diagnosis. The diagnosis of meningitis is made on the basis of clinical assessment and CSF analysis. The most important role of CT is to identify contraindications to lumbar puncture—that is, occult causes of increased intracranial pressure such as cerebral abscess. For these reasons, the meningitides/encephalitides were excluded from our definition of ICP diagnosed on the basis of imaging.
To identify factors associated with an ED diagnosis of ICP, we examined visits by several demographic variables including age, sex, race and geographical location. We also examined variables related to the patient's visit such as mode of arrival and triage urgency. Some variables (eg, mode of arrival) were not included in the survey data for certain years prior to 2003, so we used years 2003–8 for analysis. Finally, we examined vital signs and several features of presentation by assessing reason-for-visit codes. A total of 18 factors were examined, many of which comprised multiple related reasons for visit in the NHAMCS survey. Adjusted ORs were calculated to identify a subset of variables significantly associated with receiving CT/MRI, defined as an adjusted OR >1.00 with a 95% CI not including 1.00.
Statistical analyses were performed using Statistical Analysis Software (SAS) V.9.1.3 (SAS Institute). Unless specifically noted otherwise, ED visit sample estimates and associated CIs were calculated using relative standard errors (dividing the SE by the estimate itself) of <30% and with at least 30 raw data items in accordance with NCHS recommendations. Adjusted ORs for factors associated with ICP were calculated using logistic regression analysis. The least squares method of linear regression was used for analysis of trends, with p<0.05 deemed statistically significant.
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