Abstract
Objective
Chronic subdural hematomas (CSDH) are more common on the left hemisphere than on the right. We verified this left predilection of CSDH and tried to explain the reason for this discrepancy.
Methods
We investigated the laterality of CSDH in 182 patients who were treated from January 2005 to December 2009. We examined the symmetry of the cranium and the location of the lesion.
Results
CSDH was more common on the left-side. The cranium was symmetric in 63 patients, asymmetric in 119 patients. The asymmetric crania were flat on the right-side in 77 patients, on the left-side in 42 patients. The density of the CSDHs was hypodense in 29 patients, isodense 132 patients, and the others in 21 patients. Bilateral hematomas were more common in the hypodense group. In the right flat crania, the hematoma was more commonly located on the opposite side of the flat side. While in the left flat crania, the hematoma was more common on the same side.
Conclusion
CSDHs occurred more frequently on the left side. The anatomical asymmetry of the cranium influences the left predilection of CSDH.
Keywords: Chronic subdural hematoma, Computed tomography, Craniocerebral trauma, Diagnosis, Laterality
INTRODUCTION
Chronic subdural hematoma (CSDH) may occur on either side or both sides of the cranial space. However, actual distribution of CSDH is not equal in both sides. CSDHs are more commonly found on the left than on the right in many reports2,5,8,16,18,21). There was no statistically significant difference in terms of laterality in the acute or subacute subdural hematomas16).
There are reports indicating that the laterality of the CSDH depends on the cranial morphology2,12). The brain is asymmetric in structure and function6,10). Like the brain, the cranium is frequently asymmetrical. The posterior region of the left hemisphere is more often larger than the right3). If it is true, the cranium is more often flat on the right side. The posture of the cranium tends to tilt to the flat side, and then the CSDH should theoretically be more often located on the left side. We evaluated the laterality of CSDH and tried to find the explanation for this discrepancy.
MATERIALS AND METHODS
From January 2005 to December 2009, we managed 182 patients with CSDH. We made the diagnosis by computed tomography (CT) in 162 patients. In 20 patients, the diagnosis of CSDH was made by the magnetic resonance imaging (MRI). We examined the symmetry of the cranium and the location of the lesion. The asymmetry of the cranium is checked by a simple method. We identified the flat side using three lines passing the midline and both sides of the cranium (Fig. 1). The side of the smaller angle is the flat side. For the purpose of this study, we defined the cranium asymmetrical, when the difference of the angles was bigger than 2 degrees.
Fig. 1.
The asymmetry of the cranium is checked by three lines passing the midline and both sides of the cranium. The side of the smaller angle is the flat side.
The laterality of the hematoma was divided into either the opposite or the same. It was classified as the opposite when the hematoma was on the opposite of the flat side. It was classified as the same when the hematoma was on the same side. In bilateral CSDHs, we classified the laterality according to the site of the larger hematoma.
According to the density (Hounsfield unit : HU), the hematomas were classified into hypodense (less than 20 HU), isodense (from 20 HU to 50 HU), and the other groups. The other included one calcified CSDH and 20 patients in whom the first diagnosis was made by MRI only.
Statistical significance was tested using the chi-square test. It was considered significant, when p < 0.05.
RESULTS
Symmetry of the cranium and laterality of the hematoma
The cranium was asymmetric in 119 (65.4%) patients. The asymmetric crania were flat on the right side in 77 patients and on the left side in 42 patients (Table 1). CSDH was more common on the left side. Bilateral CSDHs were slightly more common in the symmetric crania; however, this difference was not statistically significant.
Table 1.
Symmetry of the cranium and laterality of the hematoma
p = 0.847 by the chi-square test
Relation to the hematoma density and laterality
The density of the CSDHs was hypodense in 29 patients (15.9%), isodense 132 patients (72.5%), and the others in 21 patients (11.5%) (Table 2). Bilateral hematomas were more common in the hypodense group. Except the others group, this difference was statistically significant (p = 0.013).
Table 2.
Density and laterality of the hematoma
p = 0.013 by the chi-square test except others
Relation to the cranial morphology and laterality in asymmetric crania
In the right flat crania, the hematoma was more commonly located on the opposite side of the flat side. While in the left flat crania, the hematoma was more common on the same side (Table 3). This difference based on the cranial morphology was statistically significant (p = 0.008). In 41 patients with the same side hematomas, the CSDH was originated from the acute subdural hematoma (ASDH) on the same side in three cases. However, in the remaining 38 patients, we could not find any corresponding reasons of the same side.
Table 3.
Cranial morphology and laterality of hematoma in asymmetric crania
p = 0.008 by the chi-square test
The origin of CSDH and laterality of the hematoma
The origin of CSDH was identified as the ASDH by the serial CT scans in 12 patients. The crania were asymmetric in eight of them. Among them, the hematoma was on the opposite side in four, on the same side in two, and bilateral in two patients. In four symmetric crania, the hematoma was on the left in three and right in one patient. There was no corresponding laterality in CSDHs resulting from ASDHs.
In 24 patients, the origin of CSDH was identified as the subdural hygroma (SDG) by the serial CT scan. The crania were asymmetric in 17 of them. Among them, the hematoma was on the opposite side in 11, on the same side in five, and bilateral in one patient. In 7 symmetric crania, the hematoma was on the right in one, left in three, and bilateral in three patients. CSDHs from SDGs tend to locate on the opposite side.
DISCUSSION
CSDH was more common on the left side in this study as CSDHs occurred more frequently on the left side2,5,8,16,18,21). Even in strokes, the left hemispheric events was more common than the right4). MacFarlane et al.16) suggested that the right-sided lesions were likely to be documented less frequently as a result of the underdiagnosis of these lesions rather than a true increase in left-sided lesions.
The cranium was more commonly asymmetric in this study. The asymmetric crania were more commonly flat on the right side than on the left side. The human brain is asymmetric in structure and function10). Morphological left-right asymmetry appears to be the rule, rather than the exception in biological systems6). Structural asymmetries in the human brain and observations of macroscopic structural asymmetries in areas known to be functionally asymmetric have proliferated in the past two decades6). The main and most consistent observations include the right frontal and left occipital petalia; marked indentations of the inner table of the skull resulting from the greater protrusion of the adjacent cerebral lobes3,6). The left occipital pole is frequently wider and protrudes further posterior than the right3,7,9,11). If the posterior region of the left hemisphere is more often larger than the right3), the posture of the cranium tends to tilt to the flat side. Then the left side of the cranium would be the top, when a man lies on his back. CSDHs frequently originate from SDGs13,15,20). SDGs usually locate on the top of the head14). CSDHs originated from SDGs will locate on the left side. Since CSDH frequently originate from SDG, the laterality of CSDH is influenced by the gravity and cranial posture. The anatomical asymmetry of the cranium is the cause of the left prevalence of CSDH.
The method measuring cranial asymmetry is quite simple. The midline is a line passing both crista gali and internal occipital protuberance. From a point on the midline, we draw two lines passing the most prominent part of skull on both sides. If the distance from the point to the external occipital protuberance is too much long, the difference of the angles would be smaller. However, we could measure the overt asymmetry by this simple method. Minute asymmetry may not cause head tilting.
We identified that the hematoma was more commonly located on the opposite side to the flat side in the right flat crania. However, when the cranium was flat on the left side, the hematoma was more commonly on the same side in this study. Although a few ASDH may occur at the same side, it is not easy to explain these exceptional cases. If strokes were more common in the left hemisphere than the right4), atrophic changes of the brain is more severe in the left hemisphere. Theoretically, if the atrophic change of the brain progresses, reduction of the cerebral mass in the left hemisphere exceeds the right side, which may create intracranial pressure difference. If the pressure of the left hemisphere is lower than the right, SDGs will locate on the left side, since SDGs usually occur at the least pressure in the cranium as a lesion of ex vacuo. Premorbid condition producing a space of the least pressure is most important for the development or recurrence of CSDH. The only factor associated with recurrence of CSDH was the re-expansion rate at one week after the surgery18). An upright posture soon after burr-hole surgery was associated with a significantly increased incidence of CSDH recurrence1,17,19). A bilateral SDG becomes a unilateral CSDH by pressure difference produced by either dynamics between expansion and resorption or tilting of the cranium13).
The density of the CSDHs was most often isodense in this study. The age of the isodense hematoma is older than the hypodense hematoma12). The density of the CSDH originated from SDGs would be hypodense at first. That is the reason why bilateral hematomas were more common in the hypodense group in this study. Even the hyperdense ASDHs usually become hypodense within 3 weeks12). Then, it becomes isodense by the repeated microhemorrhage, which is the mechanism of enlargement of CSDH12). The laterality of CSDH is dependent on the initial site of ASDH and pressure difference produced by either dynamics between expansion and resorption or tilting of the cranium.
CONCLUSION
In our study, the CSDHs occurred more frequently on the left side. The anatomical asymmetry of the cranium is considered to influence the left prevalence of CSDH.
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