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ORIGINAL ARTICLE
Year : 2016  |  Volume : 64  |  Issue : 5  |  Page : 943-946

Flattened sheet-like fornix forming a “Cobra Hood” deformity: A previously unreported variant of fornix anatomy and its implication for surgical approaches to the third ventricle


1 Department of Anatomy, Postgraduate Institute of Medical Education and Research, Chandigarh, India
2 Seattle Science Foundation, Seattle, Washington, USA
3 Department of Neurosurgery, Postgraduate Institute of Medical Education and Research, Chandigarh, India

Date of Web Publication12-Sep-2016

Correspondence Address:
Tulika Gupta
Department of Anatomy, Postgraduate Institute of Medical Education and Research, Chandigarh - 160 012
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.190289

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 » Abstract 


Background: The fornix is the main efferent tract from the hippocampus and is an important component of the memory pathways. Variations in the anatomy of the fornix are not commonly encountered.
Materials and Methods: The anatomy of the fornix was studied in 30 cadavers of normal adult healthy males who had died in road accidents. The full extent of the hippocampus was prosected up to the tail under the magnoscope.
Results: In 10 of the 30 brains, the crura and the body of fornix were bilaterally broad and flat like a sheet, rather than the usual compact bundle, forming a cobra-like hood over the roof of the third ventricle. The maximum width was approximately 16 mm on the right side (mean: 11.7 mm) and 11 mm on the left (mean: 8.5mm).
Conclusion: Knowledge of this variation will be useful during the transcallosal approach to third ventricle tumors, especially while going subchoroidal, because an unexpected lateral span of the fornix in the surgical corridor can result in inadvertent injury to it, leading to memory defects.


Keywords: Anatomical variation; crura of fornix; surgical approach to third ventricle


How to cite this article:
Gupta T, Sahni D, Tubbs R S, Gupta SK. Flattened sheet-like fornix forming a “Cobra Hood” deformity: A previously unreported variant of fornix anatomy and its implication for surgical approaches to the third ventricle. Neurol India 2016;64:943-6

How to cite this URL:
Gupta T, Sahni D, Tubbs R S, Gupta SK. Flattened sheet-like fornix forming a “Cobra Hood” deformity: A previously unreported variant of fornix anatomy and its implication for surgical approaches to the third ventricle. Neurol India [serial online] 2016 [cited 2019 Dec 14];64:943-6. Available from: http://www.neurologyindia.com/text.asp?2016/64/5/943/190289





 » Introduction Top


The fornix is an important tract carrying bidirectional fibers that connect the hippocampus with subcortical centers. Most of the fibers are efferents from the hippocampus to the mammillary body, anterior thalamic nuclei, and septal areas, with few afferents from the septal nuclei to the hippocampus. These fibers form a thin sheet called the alveus, the fibers of which pass medially and collect just above the medial margin of the hippocampus to form the fimbria hippocampi. The fimbria runs backwards, and at the tail end of the hippocampus continues upwards, forwards, and medially as crus of the fornix. Each crus is a thick discrete fiber bundle, which meets its fellow from the opposite side to form body of the fornix. The crura are closely related to the corpus callosum. The body of the fornix is triangular and formed by these two symmetrical cord-like bundles of fibers. It lies on the roof of the third ventricle, separated from the corpus callosum by the septum pellucidum.

There are very few reports in the literature regarding anatomical variations of the fornix.[1],[2] Here, we report what to the best of our knowledge, is a hitherto unreported variation in the fornix. Knowledge that such a variation occurs has implications in planning surgery for lesions of the third ventricle. Interestingly, this variation was seen in 10 out of 30 brains dissected.


 » Materials and Methods Top


Dissection

Thirty adult formalin-fixed cadaver brains (age range 18–50 years, all males) were subjected to gross and microsurgical dissection [Figure 1] and [Figure 2]. The cause of death in all cases was accidental with no history of neurological, psychological, or musculoskeletal disease. Prosection was carried out under a magnoscope with lens magnification of 2.5× (Vaiseshika magnoscope type 7009. Sr no 11010, Scientific India, 33 IA Ambala). The temporal lobe was dissected and the hippocampus was identified. The full extent of the hippocampus was exposed up to the tail. From the tail, the emerging crus of the fornix was identified and traced further, following it medially and inferiorly to the corpus callosum in the midline, where the two crura join to form the body of the fornix, then divided into the columns of the fornix. Observations were made and measurements were taken using digital Vernier calipers (Mitutoyo, Japan, accurate to 0.02 mm) on specimens having the deformity.
Figure 1: Superior view of the fornix showing the cobra hood deformity

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Figure 2: Depiction of various measurements taken

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One brain with normal fornix was dissected to illustrate the transcallosal approach with delineation of the suprachoroidal as well as subchoroidal corridor [Figure 3].
Figure 3: (a) Inset shows the transcallosal approach. The falx cerebri can be seen. (b) On callosal sectioning, the septum pellucidum and the left fornix can be seen. The choroid plexus can be seen at the fornicial edge. The suprachoroidal approach to the third ventricle is marked with a dotted line and arrow whereas the lower arrow indicates the subchoroidal approach. Foramen of Monro is seen. (c) Coronal section with arrow depicting the transcallosal approach to the third ventricle. The corpus callosum, septum pellucidum with cavum, fornix, and lateral ventricles are visible

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 » Results Top


In 10 of the 30 brains dissected, the crura and body of the fornix were seen as a flat broadsheet rather than the usual compact cord-like appearance (which was seen in the other 20 brains).

To the naked eye, it resembled the hood of a cobra, and therefore, we chose to label it a “cobra hood” deformity of the fornix [Figure 1]. In all cases, the sheet-like arrangement was present bilaterally, even though the sheets were of unequal dimensions on the right and left sides. Each sheet-like crus was triangular in shape with its apex at or near the level of the anterior commissure and its base at the tail of the hippocampus. Hence, the sheet-like arrangement encompassed the crura and body of the fornix whereas the columns of the fornix were spared and appeared normal. The bilateral sheets together appeared as a larger triangle with the hippocampal commissure seen as a small triangle on the midline at its base, at the level of the splenium of the corpus callosum.

The mean anteroposterior (AP) length of the fornix sheet, measured from the apex (at the level of the anterior commissure) to the base, was 30.6 mm (31.5 mm on the right side and 30.5 mm on the left). The width of each sheet was measured at three points on each side, namely, the anterior end (mean), the level of the midpoint of the AP length, and the point of the maximum width. The maximum width was approximately 16 mm on the right side (mean: 11.7 mm) and 11 mm on the left (mean: 8.5mm) [Table 1] and [Figure 2]. The mean thickness of the fornix sheet at the lateral edge was 1.62 mm (1.62–2.21 mm). There was no difference in the thickness between the right and left sides in any of the cases.
Table 1: Morphometric data of the fornix sheet

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 » Discussion Top


A broad-sheet-like deformity of the fornix covering the roof of the third ventricle like a hood has not been reported previously. This cannot be a preservation artifact because all the other fiber bundles in the brains were normal. Moreover, the fornix in many other brains, similarly preserved for the same length of time, had the form of a discrete bundle.

Variations in the anatomy of the fornix are not commonly reported. Most of the literature on abnormalities of the fornix relates to the pathophysiology of schizophrenia.[3],[4],[5] The fornix abnormalities may be caused by the degeneration of the fiber tract. Atrophy of the fornix has been reported in most patients with temporal lobe seizures secondary to mesial temporal sclerosis. In epileptic patients with unilateral hippocampal sclerosis, there is volume loss in the ipsilateral fornix.[6] Defects such as thin or deficient crura or body are also seen in some patients with myelomeningocele or  Chiari malformation More Details.[7]

Interestingly, no neurological diseases were reported in any of the cadavers and the deaths were caused by unrelated accidents. This assumes significance and suggests that this type of fornix abnormality could be present in the normal population. At present, it is not a routine practice to perform a detailed imaging study of the fornix when one is planning a third ventricular surgery. From a surgical point of view, the knowledge that the fornix anatomy may be variable assumes importance. Lesions in the third ventricle are commonly operated by the interhemispheric transcallosal approach [Figure 3]. After callosal section, entry into the third ventricle may be via an interfornicial route or by the supra- or subchoroidal corridor. In the normal situations, the interfornicial approach involves working between the two columns of fornix, which are closely applied together. In patients with a cobra-hood type of fornix, the fornix forms the roof of the third ventricle as a single broad flattened band. In such situations, the interfornicial route may cause significant fornix damage with postoperative neurological deficits such as memory disturbances, aphasia, or some impairment in consciousness. In patients in whom this type of deformity can be determined preoperatively, a transventricular subchoroidal or suprachoroidal route, rather than an interforniceal approach may be a safer alternative to access the third ventricle.

It, therefore, may be wise and prudent, prior to surgery, to perform special imaging sequences to study the disposition of the fiber tracts of the fornix in relation to the roof of the third ventricle in patients with third ventricular lesions.

It is difficult to estimate the prevalence of fornix abnormalities in the general population. This can only be done in large autopsy series or by dedicated magnetic resonance imaging of healthy volunteers.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in the study or the findings reported in this paper.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest

 
 » References Top

1.
Bergman RA, Thompson SA, Afifi AK, Saadeh FA. Compendium of anatomoic variations: Text, atlas and world literature. Baltimore: Urban and Schwarzenberg; 1988.  Back to cited text no. 1
    
2.
Postans M, Hodgetts CJ, Mundy ME, Jones DK, Lawrence AD, Graham KS. Inter individual variation in fornix microstructure and macrostructure is related to visual discrimination accuracy for scenes but not faces. J Neurosci 2014; 3:12121-6.  Back to cited text no. 2
    
3.
Zahajszky J, Dickey CC, McCarley RW, Fischer IA, Nestor P, Kikinis R, et al. A quantitative MR measure of the fornix in schizophrenia. Schizophrenia Res 2001;47:87-97.  Back to cited text no. 3
    
4.
Chance SA, Highley JR, Esiri MM, Crow TJ. Fiber content of the fornix in schizophrenia: Lack of evidence for a primary limbic encephalopathy. Am J Psychiatry 1999;156:1720-4.  Back to cited text no. 4
[PUBMED]    
5.
Davies DC, Wardell AM, Woolsey R, James AC. Enlargement of the fornix in early onset schizophrenia: A quantitative MRI study. Neurosci Lett 2001;301:163-6.  Back to cited text no. 5
[PUBMED]    
6.
Baldwin GN, Tsuruda JS, Maravilla KR, Hamill GS, Hayes CE. The fornix in patients with seizures caused by unilateral hippocampal sclerosis: Detection of unilateral volume loss on MR images. AJR Am J Roentgenol 1994;162:1185-9.  Back to cited text no. 6
[PUBMED]    
7.
Vachha B, Adams RC, Rollins NK. Limbic tract anomalies in pediatric myelomeningocele and Chiari II malformation: Anatomic correlations with memory and learning-Initial investigation. Radiology 2006;240:194-202.  Back to cited text no. 7
[PUBMED]    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
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