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|Year : 2016 | Volume
| Issue : 5 | Page : 947-949
Anatomic variations of the fornix and its clinical implications
Chandrashekhar E Deopujari, Chandan B Mohanty
Department of Neurosurgery, Bombay Hospital Institute of Medical Sciences, 12, New Marine Lines, Mumbai, Maharashtra, India
|Date of Web Publication||12-Sep-2016|
Chandrashekhar E Deopujari
Department of Neurosurgery, Bombay Hospital Institute of Medical Sciences, 12, New Marine Lines, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Deopujari CE, Mohanty CB. Anatomic variations of the fornix and its clinical implications. Neurol India 2016;64:947-9
Fornix means an “arch” in Latin. It is an important white matter tract, carrying majority of the efferents from the hippocampus and containing five times more axons than the optic tract. It is a part of the well-known “Papez circuit.” Surgical transgression of the fornix causing memory loss is a well-known phenomenon.
Transchoroidal approaches have an inherent risk of forniceal or vascular injury as they involve opening of the choroidal fissure and moving the fornix to the opposite side. The transchoroidal approach consists of a subchoroidal and a suprachoroidal approach. The subchoroidal approach involves opening the taenia thalami and, hence, may injure the thalamostriate vein in the process. The suprachoroidal approach, on the other hand, involves opening the taenia fornicis, which does not put any important vascular structure at risk. Lesions located in the middle and posterior third ventricle are better accessed by these approaches. Usually the opening made utilizing these approaches should not be more than 1.5cm from the foramen of Monro.
The transcallosal interforniceal approach is useful for lesions located in the anterior and middle of the third ventricle and hypothalamus. This approach involves opening of the midline forniceal raphe from the foramen of Monro up to 2 centimeters posteriorly, providing surgical access from the transforaminal route as well as the interforniceal route. However, a significant risk of bilateral forniceal injury exists. The interforniceal approach may be facilitated by the presence of cavum septum pellucidum. This approach has been well utilized in children less than 12 years of age with hypothalamic hamartomas, because of the existence of potential cavum space, which obliterates later in life.,
Anatomical variations in the fornix are an uncommonly reported phenomena. Longitudinal striae (fornix longus) are supracallosal white matter fibres which are located below the indusium griseum. These striae contain embryological rudiments of fibres from the hippocampus and fornix. The congenital absence of fornix is seen in holoporencephaly. Interestingly, more mean and axial diffusivity was noted on Diffusion Tensor Imaging (DTI) in the fornix of overweight individuals in comparison to normal individuals. This was probably secondary to the habitual, reward related behavior between the hippocampus, ventral striatum (via fornix) and prefrontal region.
In the study of 30 cadaver brains by Gupta et al., the authors have noted the presence of crura and body of the fornix as a flat sheet in 10 specimens with normal anterior pillars (cobra-hood deformity). The mean width on the right side was 11.7 mm, and the mean width on the left side was 8.5 mm. Thus, this lateral extension of the fornix may make it more susceptible to injury during the suprachoroidal approach. At the same time, a cobra-hood deformity may also make the fornix more susceptible to injury during an interforniceal approach.
The surprisingly high incidence of forniceal variation (33%) in this report will make it mandatory to detect this variation preoperatively in every case. The only way to detect this variation preoperatively is to perform a magnetic resonance tractography utilizing diffusion tensor imaging (DTI) to look for the anatomy and lateral extent of the fornix whenever such an approach is planned. However, permanent memory deficits are rare in a suprachoroidal approach where unilateral forniceal damage invariably occurs. Bilateral forniceal damage with an interforniceal approach in the presence of cobra hood deformity may be secondary to the difficulty in finding the true midline in these cases rather than the structural anomaly itself.
The transchoroidal and interforniceal approaches have been performed mainly in specialized centers. It has also not been used widely due to unfamiliarity with the approach, long working angles and the presence of important structures in the surgical corridor. In addition, the use of magnetic resonance tractography is restricted to centers where radiologists are well versed with this technique. In our surgical experience of 248 intraventricular tumors over the last 15 years, only 5 patients have been operated by the interforniceal approach [Figure 1] and the suprachoroidal approach have been used in 8 other cases [Figure 2]. Endoscopy was utilized in 72 of these patients [Figure 3]. Practically, the wide-spread use of endoscopic transnasal and other endoscopic skull base and intraventricular surgical trajectories have reduced the need for the transchoroidal approach in general. The use of the transforaminal route with the help of an endoscope has facilitated removal of even large third ventricular tumors using natural anatomical corridors, further reducing the need for transchoroidal approaches [Figure 4].
|Figure 1: (a) MRI of a patient with a colloid cyst, where the cyst was not presenting at the foramen of Monro in earlier surgery. The recurrent cyst was excised by the interforniceal approach. (b) Postoperative MRI at 3 months shows complete cyst excision with preserved anatomy of the fornix. Post-operative memory disturbance recovered over a period of 1 year|
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|Figure 2: : MRI of an atypical colloid cyst located in the middle of the third ventricle (a and b). Suprachoroidal approach was employed to reach the roof of the third ventricle and achieve a complete resection of the colloid cyst, depicted by the postoperative MRI (c and d)|
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|Figure 3: : MRI of a hypothalamic hamartoma [arrows] (a and b). Endoscopic assisted transforaminal rather than interforniceal approach was utilized to disconnect the hamartoma. Intraoperative endoscopic picture (c) shows the disconnected hamartoma and CT scan shows the intact fornix (d). Arrow shows the site of disconnection. Interforniceal approach is frequently used for disconnection of a hypothalamic hamartoma|
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|Figure 4: : MRI of a large ependymoma occupying the whole of the third ventricle (a and b). Transforaminal approach was employed to achieve complete resection of the ependymoma depicted by postoperative MRI (c and d)|
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Thus, in addition to these factors, if the anatomic variation described by Gupta and coworkers is indeed as common as claimed by them, then it will lead to a further decline in the use of the transchoroidal and interforniceal approaches. Therefore, the results from this study will have to be validated by a large radiological study utilizing magnetic resonance tractography. Interestingly, in a small pilot study of 9 healthy individuals, magnetic resonance tractography did not reveal any variations in the fornix. Similarly, another magnetic resonance tractography study of 36 healthy individuals did not show any difference between the right and the left side of the precommisural fornix. Magnetic resonance tractography studies, however, may shed light on the existence of any further variations in the direction of the white matter fibres present within the cobra hood deformity of the fornix, like the presence of decussation or transverse fibres within the deformity rather than just flattening.
The study of embryological basis of the cobra hood deformity of fornix may be another interesting area of research.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]