In our study of clinically-defined TIA patients, we found that 40% of TIA events were evaluated by MRI/DWI, and that 15% of these had ischemic lesions identified on DWI. The frequency of MRI/DWI utilization in our study region was much higher than reported by Edlow et al., who found that MRI was performed in fewer than 5% of TIA cases in their study of the National Hospital Ambulatory Medical Care Survey from 1992–2001.14
Our MRI utilization rates were also much higher than the 3% reported in a Canadian study.15
Despite our relatively high rates of MRI utilization, performing MRI/DWI on all cases of TIA would more than double the current number of MRIs performed for TIAs in our community. Using the Medicare reimbursement rate of $535.42 for the technical fees (i.e., no physician billing included) of an MRI/MRA with contrast,18
performing an additional 478 MRI/DWI imaging studies within our community in 2005 would have resulted in about $250,000 of additional costs. Extrapolating the findings in our study to the United States, with an estimated 240,000 TIAs per year,19
may thus result in a minimum of $70 million increase in annual costs for the evaluation of TIA.
Our 15% positive DWI rate is less than half the 39% aggregate positive DWI rate reported in a recent meta-analysis of 19 studies.10, 11
The largest of the studies involved 300 TIA patients seen non-urgently at a TIA/minor stroke referral clinic. The positive DWI rate in that study was 16%. Within that study, the likelihood of a positive DWI decreased nonlinearly with longer time from symptom onset.3
The median time from symptom onset to MRI in the study was 17 days (interquartile range=10 to 23). Over ninety percent of patients in our study had MRI performed within 2 days of symptom onset (). Thus, we do not believe delays in performing MRI/DWI contributed to our relatively low positive DWI rate. Further, multiple studies with DWI performed greater than 24 hours from onset have reported positive rates of 30%–67%.4–6
The inclusion of these single center and imaging registry studies in the meta-analysis may have skewed the overall rates of positive DWI higher than may be the case within the general population.
We found that patients evaluated with MRI/DWI were younger, were less likely to have traditional stroke risk factors such as a previous stroke or TIA, pre-existing diabetes or atrial fibrillation. They were also less likely to have a pre-existing disability as defined by modified Rankin. However, patients in whom DWI was positive were older and more likely to have atrial fibrillation (). Thus, our reported rate of positive DWI may be lower than is truly the case. Our findings regarding the association of older age and atrial fibrillation with a positive DWI are consistent with some publications but in contrast to others.6, 20
It is unknown whether performing MRI/DWI on all TIA patients in our community would double the positive DWI rate and place our positive rates at the level reported in the meta-analysis.10, 11
The fewer imaging studies in older, higher risk patients may represent a more aggressive approach to evaluation of younger patients without a clear TIA etiology, while older patients with atrial fibrillation or other presumed etiology may be less likely to be evaluated with MRI. On the other hand, it may be that older patients may have pacemakers or other contraindications that preclude performance of MRI.
We did not find any associations between symptom type or duration and the likelihood of a positive DWI. This is in contrast to other reports that suggest that motor deficits and duration of symptoms were associated with a positive DWI.4, 20–22
The discrepancy of our findings with these reports may relate to differences in the populations studied. Two studies were conducted at a single center in Spain,21, 22
another at a center in France,20
and a single center study of 42 patients in the United States.4
By comparison, our large study of a biracial patient population comprised 16 hospitals in our region.
Our study had several limitations. First, cases were identified retrospectively from inspection of medical records, and there is always a risk of incomplete case ascertainment. However, prospective monitoring of all ED patients is not feasible. Also, since we limited our study to ED TIA patients and MRI/DWI was not performed on all cases of TIA, our 15% positive DWI rate may represent a biased estimate of the “true” DWI positive rate among clinically-defined TIA patients. However, no estimates of MRI/DWI use to date have included all presenting TIA cases. Further, the large number of patients evaluated by DWI in our study is comparable to the largest study previously published on the subject. Another limitation of our study is that we included only cases that were adjudicated by stroke physicians (after review of all available data) as true TIA. In clinical practice, the decision to perform an MRI after admission is often based on the early differential diagnosis, in contrast to our reliance on the ultimate discharge diagnosis to identify cases. Thus, our methodology would have excluded cases that met a clinical definition of a TIA but in whom MRI revealed a different discharge diagnosis. Lastly, the findings of practice patterns in our community may not be generalizable to other communities.
Performing MRI/DWI on all clinically-defined TIA patients in our community would reveal more cases of actual infarction. However, given current imaging practice patterns, marked increases in the use of MRI would be needed to evaluate every TIA patient using the preferred imaging modality as per the current national guideline. Future studies should assess the cost-benefit of any such increases in the use of MRI/DWI for TIA, whether longer term outcomes are improved by increased rates of neuroimaging, and whether MRI/DWI is warranted for all TIA patients.