Emerg Radiol DOI 10.1007/s10140-014-1212-6

ORIGINAL ARTICLE

Prevalence of idiopathic cuneate gyrus herniation based on emergency room CTexaminations Catherine Maldjian & Richard Adam

Received: 4 January 2014 / Accepted: 5 March 2014 # Am Soc Emergency Radiol 2014

Abstract Idiopathic brain herniation caused by a focal dural defect can be confused for a mass or post-traumatic herniation. The prevalence of idiopathic brain herniation has never been described. We sought to ascertain the prevalence of idiopathic cuneate gyrus herniation in a general emergency room (ER) population on computed tomography (CT) imaging. The purpose of this study is to elucidate cuneate gyrus herniation and differentiate it from other pathologic conditions such as mass or traumatic herniation and to provide its anatomical prevalence in an ER population. Consecutive emergency room CT scans of the brain were evaluated prospectively for cuneate gyrus herniation over a 1 year period by a neuroradiologist. Of 1,500 brain CT scans evaluated, 11 patients demonstrated idiopathic cuneus gyrus herniation. The prevalence was 0.73 %. CT manifestations are normal brain tissue herniating into the superior cerebellar cistern. Idiopathic brain herniation can be mistaken for a pathologic process. We found the prevalence of one such idiopathic brain herniation, involving the cuneus gyrus, to be exceedingly rare. CT imaging demonstrates normal brain tissue herniating beyond the dural boundary.

defines the prevalence of brain herniation in a specific anatomical location. The purpose of this study is to elucidate cuneate gyrus herniation and differentiate it from other pathologic conditions such as mass or traumatic herniation and to provide its anatomical prevalence.

Keywords Cuneate . Gyrus . CT . Brain . Herniation

There were 1,500 consecutive emergency room (ER) brain CT scans evaluated. Of these, 11 demonstrated idiopathic cuneate gyrus herniation (Fig 1). This consisted of five males and six females. One patient demonstrated bilateral idiopathic herniation (Fig 2). The remaining ten patients demonstrated six left-sided herniations and four right-sided herniations. The prevalence of cuneate gyrus herniation in our ER population was 11/1,500 or 0.73 %. CT images demonstrated herniation of cuneus gyrus into the superior cerebellar cistern in all instances. This portion of the parenchyma extended beyond the normal boundary of the brain and therefore constituted herniation. The brain tissue in this herniation was normal. There was no evidence of mass or cerebral edema or parenchymal abnormalities.

Introduction Idiopathic brain herniation can present as a diagnostic dilemma on imaging and may be misinterpreted as a mass or posttraumatic brain herniation. The true prevalence of idiopathic brain herniation is unknown. This is the first study which C. Maldjian (*) : R. Adam Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, PA, USA e-mail: [email protected]

Materials and methods The Institutional Review Board (IRB) issues the approval. Consecutive computed tomography (CT) scans of the brain referred through the ER were interpreted by one neuroradiologist over the course of a year and the presence of cuneate gyrus herniation was determined prospectively. Images were viewed on a Pacs workstation. Studies with motion artifacts that precluded proper evaluation were excluded.

Results

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Fig. 1 a Unilateral cuneate gyrus herniation (arrow). b Unilateral cuneate gyrus herniation (arrow), a magnified image

Fig. 2 a Example of bilateral cuneate gyrus herniation (arrows). b Example of bilateral cuneate gyrus herniation (arrows), a magnified image

Discussion Mechanical displacement of normal brain is the definition of brain herniation. This is typically imaged by spiral CT in the acute setting [1]. Its causes include trauma, ischemia, infection, or neoplasia. In the acute ER presentation post-trauma, subfalcine herniation constitutes the most common type of brain herniation [2]. Idiopathic brain herniation is distinctly unusual, but needs to be differentiated from pathological herniation when present. Only two cases exist in the literature. In one case, the patient exhibited trigeminal neuralgia and the magnetic resonance imaging (MRI) findings were incorrectly interpreted as a mass lesion [3]. The herniation in that case involved the parahippocampal gyrus extending into the superior cerebellar cistern. On intraoperative biopsy, normal brain tissue was revealed. In another case, the herniation involved the cuneate gyrus and was correctly established on MRI which showed normal brain tissue in an abnormal location [4]. The patient in that report presented with chronic headaches, a nonlocalizing sign potentiallyrelated to intracranial processes [4]. The MRI demonstrated a focal area of soft tissue signal intensity confluent with the right cuneus gyrus and extending into the superior cerebellar cistern. The anomalous right cuneus gyrus was isointense to the adjacent gyri on

both pre- and post-contrast studies, thereby excluding a mass. A focal dural defect was noted on post-contrast coronal imaging study. Based on MR findings, the finding was interpreted as non-neoplastic in origin and diagnosed as herniation of the right cuneus gyrus through the medial tentorium. Differentiation of this variant from a mass precluded unnecessary biopsy [4]. The current study seeks to determine the prevalence of this anomaly or normal variant in an ER population and describe CT findings that would differentiate it from other pathologic conditions such as mass or traumatic herniation. We showed that the prevalence of this variant in 1,500 ER patients based on CT examination is 11/ 1,500 or 0.73 %. Therefore, this is a very rare occurrence. CT images demonstrated herniation of normal brain tissue into the superior cerebellar cistern in all instances. The brain tissue in this herniation was normal. There was no evidence of mass or cerebral edema or parenchymal abnormalities that could have caused this finding in any of the patients. Posteriorly the cuneate gyrus contains projections from the fovea and anteriorly it contains projections from the peripheral visual field. We found that the anterior portion is the area consistently herniated into an anomalous location . One

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explanation for preservation of perceptual visual field is that patients adapt by using the foveal visual field [4]. Alternatively, herniation of cuneus gyrus may not disturb normal axonal pathways, but merely displaces them anteriorly [4]. Formal visual field testing could determine which of these explanations is correct [4]. Although visual-evoked fields (VEF) or functional MR imaging (fMRI) may provide interesting data pertaining to neurological sequelae of gyral herniation, currently there is no such data and the incidence of this finding is so rare that it may be difficult to obtain this data [4]. The diagnosis of gyral herniation, however, can be readily established with conventional CT. It has been suggested that an error occurring during meningeal embryogenesis is responsible for this anomaly [4]. Early in development a layer of meninges separates the telencephalon and diencephalon . During development, as the telencephalon displaces the walls of the diencephalon, the interceding meningeal layers regress and thalamic tissue becomes contiguous with the floor of the cerebrum [5]. The median part of the tentorium involutes as the head of the embryo changes from vertical to oblique orientation, leaving only the lateral portions of the tentorium [6]. The forces which regulate dural regression may locally impact dura at the tentorial reflection and incite a discrete dural defect which could give rise to a small focal herniation of brain parenchyma [4]. This theory has been invoked to explain interdigitating brain tissue through a falx cerebri defect in Chiari II malformation [7]. In conclusion, gyral herniation has been previously reported and mistaken for a mass in one report where biopsy was performed [3]. We report CT findings similar to the first MRI description of cuneate gyrus herniation, demonstrating gyral herniation into the superior cerebellar cistern [4]. This anomaly is likely congenital or developmental, since there is no concomitant mass or mass effect in any of the cases. The

prevalence of this finding has not been previously described and, based on our data, is rare with no gender predilection. Of the ten unilateral cases, six were left-sided and four were rightsided; therefore, there is no clear predilection for laterality. The CT appearance is that of normal brain tissue herniating beyond the dural boundaries. Knowledge of this variant may preclude unnecessary interventions. Acknowledgement The data for this investigation was obtained from Bronx Lebanon Hospital Center. Ethical standards This study complies with the current laws of the country in which it was performed. Conflict of interest The authors declare that they have no conflict of interest.

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