Folds of the meningeal layer of the dura mater, the falx cerebri and tentorium
cerebelli, traverse the vertebrate intracranial cavity and thus compartmentalise
the brain. The falx cerebelli, another dural fold, is found in the posterior cranial
fossa and attaches to the inferior aspect of the tentorium cerebelli and to the
internal occipital crest. We present a case of a duplicated falx cerebelli, occipital
venous sinus and internal occipital crest that was detected upon routine dissection
of a male cadaver. Since haemorrhage of a dural venous sinus can be a fatal
complication of posterior cranial fossa surgery, knowledge of venous sinus variations
in this region may prevent unpredictable complications during intracranial
procedures

Khaki, A.A.

Shoja, M.M.

Shokouhi, G.

Tubbs, R.S.

English

Via Medica Journals

Folia Morphol. Vol. 65, No. 2, pp. 171–174Copyright © 2006 Via MedicaISSN 0015–5659www.fm.viamedica.plC A S E   R E P O R T171Address for correspondence: R.S. Tubbs, PhD, Pediatric Neurosurgery, Children’s Hospital, 1600 7th Avenue South ACC 400, Birmingham,Alabama 35233, USA, tel: 205 939 9914, fax: 205 939 9972, e-mail: rstubbs@uab.eduA rare variation of the posterior cranial fossa:duplicated falx cerebelli, occipital venoussinus, and internal occipital crestM.M. Shoja1, 2, R.S. Tubbs1, 2, A.A. Khaki1, 2, G. Shokouhi1, 21Department of Anatomy and Neurosurgery, Tabriz University of Medical Sciences, Tabriz, Iran2Pediatric Neurosurgery Children’s Hospital Birmingham, Alabama, USA[Received 17 February 2006; Revised 21 April 2006; Accepted 21 April 2006]Folds of the meningeal layer of the dura mater, the falx cerebri and tentoriumcerebelli, traverse the vertebrate intracranial cavity and thus compartmentalisethe brain. The falx cerebelli, another dural fold, is found in the posterior cranialfossa and attaches to the inferior aspect of the tentorium cerebelli and to theinternal occipital crest. We present a case of a duplicated falx cerebelli, occipitalvenous sinus and internal occipital crest that was detected upon routine dissec-tion of a male cadaver. Since haemorrhage of a dural venous sinus can be a fatalcomplication of posterior cranial fossa surgery, knowledge of venous sinus vari-ations in this region may prevent unpredictable complications during intracra-nial procedures.Key words: duplication, falx cerebelli, tentorium, anatomicalvariationINTRODUCTIONAnatomical variation is a normal flexibility in themorphology of body structures. Variations general-ly have little effect on body function under normalcircumstances, although these may alter patientmanagement [14]. It has been postulated that clini-cal malpractice is partly attributable to ignorance, inan era of advanced radiology and minimally invasivesurgical procedures, of anatomical variations [14].Indeed, these authors [14] have further stated that“medical progress needs a more accurate knowledgeof the variability of the human morphology to im-prove diagnosis and therapeutic performance”.Folds of the meningeal dura mater traverse andcompartmentalise the intracranial cavity and therebymechanically support various neural elements. Thehuman tentorium cerebelli is a somewhat horizontaldural fold that forms the roof of the posterior cranialfossa and has the largest surface area in relation tobody size in mammals [3]. The tentorium cerebelli iscontinuous with the falx cerebri superiorly and the falxcerebelli inferiorly. The falx cerebelli is a sickle-shapeddural fold that attaches posteroinferiorly to the inter-nal occipital crest. The attached part of the falx cere-belli contains the occipital venous sinus and its anteri-or margin projects between the two cerebellar hemi-spheres [1]. There is usually only one midline-locatedfalx cerebelli, which may be divided inferiorly to createa V-shaped space, the so-called vermian fossa [10]. Thefalx cerebelli is between 2.8 and 4.5 cm in length andis approximately 1–2 mm thick, although thicknessesof up to 13 mm have been described [10, 18]. Thecurrent report describes a single cadaver found to haveduplication of the falx cerebelli and occipital sinus.CASE REPORTDuring the routine dissection of a 49-year-old malecadaver and following the removal of the brain two172Folia Morphol., 2006, Vol. 65, No. 2distinct falces cerebelli were found dividing the in-fratentorial space into three compartments (Fig. 1, 2).The cause of death had been sudden and of an un-known aetiology. Each falx cerebelli (approximate-ly 2.5 × 3.2 cm) demonstrated a distinct base andapex and possessed a distinct venous sinus on eachposteriorly attached border (Fig. 2). These sinuseswere noted to drain into to the left and right trans-verse sinus respectively with no observed connec-tion to the marginal sinus (Fig. 2). Furthermore, af-ter detaching the dura mater from the inner bonysurface of the occipital bone, it was noted that therewere two distinct internal occipital crests arisinginferolaterally to the internal occipital protuberanceand diverging inferiorly near the posterolateral bor-ders of the foramen magnum. The distance betweenthe two falces cerebelli was approximately 3 cm.The brain from this specimen appeared grossly nor-mal with no defect of the vermis. The external sur-face of the occipital squama appeared normal asdid the remaining anatomy of the craniocervicaljunction. The lumina of the superior sagittal andstraight sinuses were found to be normal in theirconfiguration.DISCUSSIONVariations in the form of dura mater are appar-ently rare [17]. Interestingly, Sargon et al. [15] havereported a cadaver with agenesis of the corpus cal-losum and septum pellucidum that was found tohave a five-layered dura mater covering the left ce-rebral hemisphere. Similarly, Loughenbury et al. [11]reported the duplication of the spinal dural sheathfrom C2 to L5. Following a literature search, onlyone other description of a duplicated falx cerebelliwas found and this was the report by Habler andSchlenker [5]. However, their case involved an arach-noid cyst between the two falces cerebelli (notfound in our case) and no mention was made ofthe occipital sinus. These authors [5] regarded thearachnoid cyst found in their case as a cephalic ex-tension of the cisterna magna. Duplicated occipitalsinuses have been reported in the literature. Brow-der et al. [2] have stated that the doubling of oc-cipital sinuses in adults represents the end result ofpreviously existing multi-channelled venous path-ways in the midline suboccipital dura mater of theinfant. However, no mention was made by theseauthors of duplication of the falx cerebelli. The duramater and its partitions associated with therhombencephalon may be observed at approximatelythe 14th gestational week, which is the approximatetime at which the cerebellar vermis appears asa distinct entity [5]. Lang [10] reported that the oc-cipital sinus may be tripled. It is of note that Knott [9]reported that in 44 cadavers he found no trace ofthe occipital sinus in two and a doubled sinus innine cases, one lying on either side of a single in-ternal occipital crest.We theorise that in the present case the dispro-portionate growth of the cerebellar vermis andhemispheres with subsequent formation of twodeep fossae between them resulted in the forma-tion of two distinct dural folds from differentiationand condensation of the intervening mesodermaltissues. Therefore, the attached borders of the du-ral folds to the internal cranial surface created in-creased tension and thus two falces cerebelli. In-deed, Jeffery [8] has shown that the tentoriumFigure 2. Two distinct falces cerebelli dividing the posterior cra-nial fossa into three compartments.Figure 1. Schematic drawing of the duplicated falx cerebelli andoccipital sinus as seen in the present case report.173M.M. Shoja et al., Duplicated falx cerebellicerebelli is rotated inferiorly by disproportionategrowth of the cerebrum versus the cerebellum. Themechanical stress of bony tissue has long beenknown to produce local electrical or electromag-netic currents and that this piezoelectric effect [3]itself enhances osteogenesis via activation of os-teoblasts [19]. Henderson et al. [6] have suggestedthat mechanotransduction of the deeply placeddura mater may regulate the rate of bone deposi-tion in sutures of the cranium.In cases of holoprosencephaly (maldevelopmentof the forebrain) the absence of the interhemispher-ical fissure is associated with the absence of thefalx cerebri [4]. Patients with the Chiari II malfor-mation (hindbrain herniation through the foramenmagnum, seen almost exclusively in patients bornwith a myelomeningocele) may not have a falx cer-ebelli or internal occipital crest [16]. This may bedue to a crowded posterior cranial fossa (with inhi-bition of the development of these dural folds) inthese patients with vermian hindbrain herniationand a small posterior cranial fossa [16]. This notionwould support the notion that the development ofthe brain, either cerebrum or cerebellum, is a pre-cedent event in the formation of dural folds.Michaud et al. [13] have reported a case of agene-sis of the vermis in which the falx cerebelli was no-tably small.Variations of the dural partitions (such as thefalx cerebelli) are rarely reported in the literature.Anatomical variations of the dural venous sinusesin the region of the confluence of the sinuses maypresent problems with diagnostic and operativeprocedures [12]. Hollinshead [7] has stated thatthe occipital sinus is always a potential source ofdifficulty in posterior approaches to the posteriorcranial fossa because of its variability. The clini-cian who operates intracranially or interprets ra-diological imaging should be aware of the ana-tomical variations found within the posterior cra-nial fossa.REFERENCES1. Boskovic M, Josifov J (1964) On the morphogenesisand content of the falx cerebelli. Verh Anat Ges, 58:292–298.2. Browder J, Kaplan HA, Krieger AJ (1975) Venouslakes in the suboccipital dura mater and flax cere-belli of infants: surgical significance. Surg Neurol,4: 53–55.3. Chakkalakal DA, Johnson MW (1981) Electrical pro-perties of compact bone. Clin Orthop, 161: 133–145.4. Chang LH (2003) Alobar holoprosencephaly: report oftwo cases with unusual findings. Chang Gung Med J,26: 700–706.5. Habler W, Schlenker M (1982) Double falx cerebelli:case report. Acta Neurochir, 62: 265–269.6. Henderson JH, Longaker MT, Carter DR (2004) Suturalbone deposition rate and strain magnitude during cra-nial development. Bone, 34: 271–280.7. Hollinshead WH (1982) Anatomy for surgeons. The headand neck. 3rd Ed. JB Lippincott, Philadelphia, p. 74.8. Jeffery N (2002) Differential regional brain growth androtation of the prenatal human tentorium cerebelli.J Anat, 135: 144.9. Knott JF (1881) On the cerebral sinuses and their vari-ation. J Anat Physiol, 16: 27–42.10. Lang J (1991) Clinical anatomy of the posterior cranialfossa and its foramina. Thieme, New York, p. 6.11. Loughenbury PR, Wadhwani S, Soames RW (2005)Unusual variant of the spinal dural sheath: a casereport. Abstract presented at the 4th Joint Meetingof the American Association of Clinical Anatomistsand the British Association of Clinical Anatomists,New York.12. Magden AO (1991) Triple straight sinus — report of2 cases. Anat Anz, 173: 17–22.13. Michaud J, Mizrahi EM, Urich H (1982) Agenesis ofthe vermis with fusion of the cerebellar hemishperes,septo-optic dysplasia and associated anomalies. Re-port of a case. Acta Neuropathol, 56: 161–166.14. Sańudo J, Vázquez R, Puerta J (2003) Meaning andclinical interest of the anatomical variations in the 21stcentury. Eur J Anat, 7: 1–3.15. Sargon MF, Brohi Özeksi P, Tonak AK, Cumhur M (2002)Agenesis of the corpus callosum and septum pelluci-dum together with a multiple layered duramater. Neu-roanatomy, 1: 2–4.16. Tubbs RS, Dockery SE, Salter G, Elton S, Blount JP,Grabb PA, Oakes WJ (2002) Absence of the falx cerebelliin a Chiari II malformation. Clin Anat, 15: 193–195.17. Tubbs RS, Kelly DR, Lott R, Salter EG, Oakes WJ (2005)Complete ossification of the human falx cerebri. ClinAnat, 19: 147–150.18. Williams PL, Bannister LH, Berry MM, Collins P, Dyson M,Dussek JE, Ferguson MWJ (1995) Gray’s anatomy.38th Ed. Churchill Livingstone, New York, pp. 256–257.19. Yonemori K, Matsunaga S, Ishidou S, Yoshida H (1996)Early effects of electrical stimulation on osteogenesis.Bone, 19: 173–180.

A rare variation of the posterior cranial fossa: duplicated falx cerebelli, occipital venous sinus, and internal occipital crest

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A rare variation of the posterior cranial fossa: duplicated falx cerebelli, occipital venous sinus, and internal occipital crest

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