Subtonsillar and vallecular triangles as gateways to dorsal brainstem and fourth ventricle lesions: descriptive and quantitative analysis of microsurgical anatomy.

The subtonsillar triangle (ST) and the vallecular triangle (VT) are used for accessing brainstem lesions, particularly in the medulla. The microsurgical applications of these triangles were analyzed through descriptive and quantitative methods. Five formalin-fixed latex-injected cadaveric heads were examined to identify the ST and VT. Three additional cadaveric brains were dissected to explore the associated brainstem anatomy. Three-dimensional modeling and tractography with 7T magnetic resonance imaging allowed visualization of key fiber tracts. The longest edge of the ST was the medial edge (mean [SD], 12.9 [2.9] mm), while the longest edge of the VT was the right lateral edge (16.2 [3.8] mm). The mean surface area of the ST was 36.1 (14.4) mm² measured under full retraction to expose the lateral recess of the fourth ventricle. After full retraction, the area of the VT increased from 38.4 (30.0) mm² to 129.4 (50.9) mm² (P = 0.01). The ST allows access to the cerebellomedullary cistern, distal p3 and proximal p4 segments of the posterior inferior cerebellar artery, and ipsilateral foramen of Luschka. The VT allows access to the caudal loops and distal p3 segments of the posterior inferior cerebellar arteries and the upper rhomboid fossa. The ST and VT allow reliable access to the fourth ventricle, dorsal pons, medulla oblongata, and gracile/cuneate regions. The ST can be used during the suboccipital telovelar approach for inferior peduncular and cuneate lesions, and the VT allows access to gracile, trigonal zone, and rhomboid lesions through either the suboccipital transvermian or transventricular approach.