Left-Sided Strokes Are More Often Recognized Than Right-Sided Strokes
The Rotterdam Study
Background and Purpose—Left-sided strokes are reported to be more common than right-sided strokes, but it is unknown whether they occur more often or are simply recognized more easily by clinicians. In a large unselected community-dwelling population, we examined the frequency of clinical left- and right-sided strokes and transient ischemic attacks (TIAs) and compared it with the frequency of left- and right-sided infarcts on MRI.
Methods—This study was conducted within the population-based Rotterdam Study. Between 1990 and 2012, 13 894 participants were followed up for first-ever stroke and TIA. MRI scans were performed within a random subgroup of 5081 persons and were rated for the presence of supratentorial cortical and lacunar infarcts. We compared frequencies of left- and right-sided strokes, TIAs, or MRI infarcts using binomial and Fisher exact tests.
Results—After a mean follow-up of 9.6 (±6.0) years, 1252 patients had a stroke, of which 704 were ischemic, and 799 participants had a TIA. Within the subgroup with MRI, we identified 673 infarcts. Ischemic strokes were more frequently left-sided (57.7%; 95% confidence interval, 53.7–61.6) than right-sided, similar to TIAs (57.8% left-sided; 53.4–62.3). In contrast, we found no left-right difference in distribution of infarcts on MRI (51.9% left-sided; 48.1–55.6).
Conclusions—Clinical ischemic strokes and TIAs are more frequently left-sided than right-sided, whereas this difference is not present for infarcts on MRI. This suggests that left-sided strokes and TIAs are more easily recognized. Consequently, there should be more attention for symptoms of right-sided strokes and TIAs.
Several hospital-based studies have reported that left-sided strokes are more frequent than right-sided strokes.1–3 A predilection for the left side may be explained by characteristics of the atherosclerotic plaque in the left carotid artery or by anatomy.3 The finding that isolated aphasia is a typical presentation of cardioembolic stroke or transient ischemic attack (TIA) also suggests that cardiac thrombi may preferably affect the left hemisphere.4 Another hypothesis is that the strokes in hospitals are a selection of strokes with symptoms that are better recognized or perceived as more severe. Left-sided strokes might be referred more frequently because they lead to clear symptoms, such as aphasia, whereas right-sided strokes may lead to less explicit symptoms, such as hemineglect or spatial disorientation.1,5
Previous MRI studies also suggested that right-sided strokes are more often unnoticed because they found more right-sided silent infarcts in patients with carotid stenosis and atrial fibrillation.6,7 An important advantage of MRI studies is that these not only detect clinical strokes but also clinically silent infarcts, thereby providing a better estimate of the true distribution of left- and right-sided infarcts. To our knowledge, no study has compared the distribution of clinical strokes with that of MRI-defined cerebral infarcts within an unselected community-dwelling population. This can distinguish between an actual higher frequency of left-sided infarcts versus a higher frequency of clinically recognized strokes.
In a population-based study, we investigated the frequency of left- and right-sided ischemic strokes and TIAs and compared this with the frequency of left- and right-sided cerebral infarcts on MRI.
Methods are available in the online-only Data Supplement.
Setting and Study Population
This study was embedded within the prospective population-based Rotterdam Study8 and Rotterdam Scan Study.9 From 1990 to 2012, we continuously followed up 13 894 participants (mean age, 65.5±10.3 years; 59.3% women) for occurrence of stroke and TIA and between 2005 and 2011 examined 5081 random participants (mean age, 64.2±11.1 years; 55.0% women) for the presence of cerebral infarcts using MRI. The Rotterdam Study has been approved by the medical ethics committee according to the Population Study Act Rotterdam Study, and written informed consent was obtained.
Assessment of Clinical Stroke and TIA
Strokes and TIAs were identified from medical records and confirmed by a neurologist. Assessment of the hemispheric side was based on clinical symptoms and computed tomographic imaging/MRI described in these medical records. Ischemic strokes were identified based on neuroimaging reports.10
Assessment of Infarcts on MRI
Infarcts were rated on fluid-attenuated inversion recovery, proton density-weighted, and T1-weighted sequences. Lacunar infarcts were defined as focal lesions ≥3 mm and <15 mm in size with the same signal characteristics as cerebrospinal fluid on all sequences and with a hyperintense rim on the fluid-attenuated inversion recovery sequence.9
Frequencies of left- and right-sided supratentorial strokes, TIAs and MRI infarcts were compared with the expected frequency of 50% using a binomial test. Fisher exact tests were used to compare the distribution of clinical strokes and TIAs with the distribution of MRI infarcts. Analyses were performed using IBM SPSS Statistics version 21.0 (IBM Corp, Armonk, NY).
During 9.6±6.0 years of follow-up, 1252 participants had a clinical stroke and 799 a TIA (mean age, 78.7±9.2 years; 61.6% women). A total of 588 ischemic strokes and 465 TIAs occurred supratentorially (Figure 1). A total of 673 supratentorial infarcts were identified on MRI. Ischemic strokes occurred more often left- than right-sided (57.7% left-sided; 95% confidence interval, 53.7–61.6), similar to TIAs (57.8% left-sided; 53.4–62.3). We did not find a significant left–right difference for cerebral infarcts on MRI (51.9% left-sided; 48.1–55.6; Figure 2). Importantly, direct comparison revealed that clinical ischemic stroke and TIA were significantly more frequently left-sided than infarcts on MRI (P=0.02). Figure 2 and Figure I in the online-only Data Supplement also show the left–right distribution for various subtypes of clinical strokes and MRI infarcts (eg, lacunar versus cortical; symptomatic versus silent MRI infarct).
In this population-based study, we found that clinical ischemic strokes and TIAs occur more often left-sided than right-sided, whereas we did not find such a difference for cerebral infarcts on MRI.
Previous studies found a similar higher incidence of clinical left-sided strokes and TIAs, but did not compare their results with infarcts on MRI, which includes clinically silent infarcts. These studies, therefore, did not help to distinguish between a true higher incidence of left-sided stroke because of a predilection of infarcts for the left side versus a difference in recognition of left- and right-sided strokes.1–3 In our study, clinical ischemic strokes and TIAs were more frequently left-sided than right-sided. This was different from the distribution of infarcts on MRI. This suggests that left-sided strokes are recognized better or perceived as more severe, whereas right-sided strokes are missed. This might be the consequence of complex right-sided symptoms, such as hemineglect and spatial disorientation. Furthermore, patients might not present themselves to the hospital because of anosognosia. However, this is speculative because we do not routinely examine our participants to detect symptoms that might have been missed.
To determine the effect of missing right-sided strokes, additional studies should examine differences in long-term disability between left- and right-sided strokes.
Strengths of this study are the thorough collection of strokes and the availability of MRI in a subgroup of the study population. A limitation is that we could not compare frequencies of clinical strokes and TIAs with frequencies of MRI infarcts in exactly the same population because of small overlap. Also, participants with a severe stroke probably did not visit the study center for MRI scanning because of their physical limitations. This might have led to some selection bias in the MRI population. Furthermore, because of the lack of clinical neuroimaging after stroke, not all strokes could be classified into ischemic or hemorrhagic and remained unspecified. Many of those unspecified strokes were not referred to hospital, partly because they were perceived as less severe. Left-sided strokes were not more frequent in this group, which supports our and other results that left-sided strokes are perceived as more severe and referred to a hospital more often. A final consideration is that the distribution of left-sided MRI infarcts was not exactly 50 to 50, especially for cortical MRI infarcts. It is therefore possible that there also is a small, but true predilection for left-sided strokes, which remained obscured in our study because of insufficient power.
In conclusion, our findings suggest that the higher frequency in clinical left-sided strokes and TIAs compared with right-sided events is in a large part because of a better recognition of those. Consequently, there should be more attention for symptoms of right-sided strokes and TIAs.
Sources of Funding
This study was supported by Netherlands Heart Foundation (Nederlandse Hartstichting) 2012T008 and Erasmus University Medical Center (Erasmus MC) Fellowship 2013.
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.114.007385/-/DC1.
- Received September 8, 2014.
- Revision received November 6, 2014.
- Accepted November 12, 2014.
- © 2014 American Heart Association, Inc.
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