Trochlear Groove and Trochlear Cistern: Useful Anatomic Landmarks for Identifying the Tentorial Segment of Cranial Nerve IV on MRI

BACKGROUND AND PURPOSE:The trochlear groove and trochlear cistern are anatomic landmarks closely associated with the tentorial segment of cranial nerve IV. The purposes of this study were to describe the MR imaging appearances of the trochlear groove and trochlear cistern and to test our hypothesis that knowledge of these anatomic landmarks facilitates identification of cranial nerve IV in routine clinical practice.MATERIALS AND METHODS:For this retrospective study, consecutive MR imaging examinations of the sinuses performed in 25 patients (50 sides) at our institution were reviewed. Patient characteristics and study indications were recorded. Three readers performed independent assessments of trochlear groove, cistern, and nerve visibility on coronal images obtained by using a T2-weighted driven equilibrium radiofrequency reset pulse sequence.RESULTS:Interobserver agreement was 78% for visibility of the trochlear groove, 56% for the trochlear cistern, and 68% for cranial nerve IV. Following consensus review, the trochlear groove was present in 44/50 sides (88%), the trochlear cistern was present in 25/50 sides (50%), and cranial nerve IV was identified in 36/50 sides (72%). When the trochlear groove was present, cranial nerve IV was identified in 35/44 sides (80%), in contrast to 1/6 sides (17%) with no groove (P = .0013). When the trochlear cistern was present, cranial nerve IV was identified in 23/25 sides (92%), in contrast to 13/25 sides (52%) with no cistern (P = .0016).CONCLUSIONS:The trochlear groove and trochlear cistern are anatomic landmarks that facilitate identification of cranial nerve IV in routine clinical practice.

Historically, imaging of cranial nerve IV has been difficult.^1,2 These difficulties likely relate to the small size of the nerve and the presence of numerous adjacent blood vessels in the quadrigeminal and ambient cisterns.^3,4 These difficulties are compounded by the fact that MR images are often acquired in the axial plane, nearly parallel to the cisternal course of the nerve.In research settings, high-resolution MR images have reliably demonstrated cranial nerve IV in healthy volunteers; however, long acquisition times ranging from 7 to 26 minutes make these sequences impractical for routine clinical use.^3,5,6 Acknowledging the perceived impracticality of consistently identifying cranial nerve IV in routine practice, more recently published work has proposed using volumetric analyses of the superior oblique muscles as a correlate for the presence or absence of cranial nerve IV in patients with congenital superior oblique palsy.⁷ However, the ability to reliably identify the nerve itself can be important in identifying underlying pathology and can aid in treatment and surgical planning.^3,8Various segments and subsegments have been proposed to describe the course of cranial nerve IV distal to its exit from the dorsal midbrain and proximal to its entrance into the cavernous sinus.^3,4,6,9–11 Some authors consider the tentorial segment to be the distal subdivision of the cisternal segment,^3,6,9,10 and others describe the tentorial segment as distinct from the cisternal segment of cranial nerve IV.^4,11 However, there is generally agreement that the tentorial segment of cranial nerve IV begins when the nerve becomes closely associated with the inferior aspect of the cerebellar tentorium and ends at the entrance of the nerve to the cavernous sinus.Cadaveric dissections have shown that the tentorial segment of cranial nerve IV travels within a shallow groove (Fig 1) inferolateral to the free tentorial edge en route to the cavernous sinus, and this shallow groove has been referred to as the “trochlear groove.”^4,9,10,12 After passing through the trochlear groove but before entering the cavernous sinus, the tentorial segment of cranial nerve IV pierces the tentorium to course a short distance through the posterior petroclinoid fold.^4,13 The space through which cranial nerve IV passes in the posterior petroclinoid fold may contain CSF and has been referred to as the “trochlear cistern” (Fig 2).⁴ These findings are consistent with the common progression of the cranial nerves through a transitional “dural cave segment” in their course from the intracranial to the extracranial compartment.¹⁴Open in a separate windowFig 1.Labeled (A) and unlabeled (F) anterosuperior views of the sella and suprasellar region demonstrate cranial nerve IV passing into the trochlear groove (circle, F). Car.A. indicates carotid artery; Oculom., oculomotor; Post., posterior; Ant., anterior; Cist., cistern; Tuberc., tuberculum; Clin., clinoid; Pet. Apex, petrous apex. Modified with permission from Rhoton AL Jr. The sellar region. Neurosurgery 2002;51(4 suppl):S335–74.¹²Open in a separate windowFig 2.Lateral view of the right cavernous sinus demonstrates cranial nerve IV traveling within the trochlear cistern immediately proximal to entering the cavernous sinus. The trochlear cistern is located inferior to the oculomotor cistern and superior to the Meckel cave. Troch. indicates trochlear; Trig. Gang., trigeminal ganglion. Modified with permission from Rhoton AL Jr. The cavernous sinus, the cavernous venous plexus, and the carotid collar. Neurosurgery 2002;51(4 suppl):S375–410.¹⁶Coronal 2D T2-weighted driven equilibrium radiofrequency reset pulse (coronal T2 DRIVE) is a fluid-sensitive MR sequence commonly performed during sinus MR imaging examinations at our institution. We have observed cranial nerve IV within both the trochlear groove (Fig 3) and the trochlear cistern (Fig 4) on clinical images obtained with this sequence. With respect to the more easily identifiable cranial nerve V, the trochlear groove is typically seen on coronal images at the level of the trigeminal nerve cisternal segment and the trochlear cistern is seen at the level of the Meckel cave.Open in a separate windowFig 3.Labeled (A) and unlabeled (B) coronal T2 DRIVE images demonstrate cranial nerve IV (black arrowhead on the patient''s right) in cross-section traveling along the bilateral trochlear grooves (straight white arrow on the patient''s right). The right tentorial free edge (curved black arrow), right cranial nerve V (V), right parahippocampal gyrus (PHG), belly of the pons (P), and basilar artery (B) are labeled for orientation. The dashed lines on the patient''s left illustrate the measurement of the maximum trochlear groove depth, and the solid lines on the patient''s left illustrate the measurement of the shortest distance between the free tentorial edge and the medial border of the trochlear groove.Open in a separate windowFig 4.Labeled (A) and unlabeled (B) coronal T2 DRIVE images demonstrate cranial nerve IV (white arrow) in cross-section within the right trochlear cistern. The right cranial nerve III (asterisk), right Meckel cave (M), and right parahippocampal gyrus (PHG) are labeled for orientation. The dashed lines illustrate the measurement of the vertical distance between the inferior margin of the trochlear cistern and the superior margin of the Meckel cave.We hypothesized that knowledge of these anatomic landmarks will facilitate identification of the tentorial segment of cranial nerve IV in routine clinical practice.The purposes of this study were to describe the MR imaging appearances of the trochlear groove and trochlear cistern and to test our hypothesis regarding the utility of these landmarks for identifying cranial nerve IV.