A transmandibular lateral transsphenoidal navigated surgical approach to access a pituitary macroadenoma in a warmblood mare

Abstract

A 16-year-old warmblood mare was referred with a progressive history of behavioral changes and left-sided blindness. Following neuroanatomical localization to the forebrain, magnetic resonance imaging of the head revealed a well-delineated, 4.5 cm in diameter, round pituitary mass causing marked compression of the midbrain and optic chiasm. Euthanasia was recommended but declined by the owners. Veterinary specialists and a human neurosurgeon collaboratively prepared for surgical case management. A novel navigated transmandibular lateral transsphenoidal approach was developed to access the region of the sella turcica and practiced on cadaver specimens. The horse was anesthetized and placed in sternal recumbency with the head above the heart line. Using a cone beam computed tomography (CBCT)-coupled navigation system, a navigated pin traversing the vertical ramus of the mandible and the lateral pterygoid muscle was placed in a direct trajectory to the predetermined osteotomy site of the basisphenoid bone. A safe corridor to the osteotomy site was established using sequential tubular dilators bypassing the guttural pouch, internal and external carotid arteries. Despite the use of microsurgical techniques, visualization of critical structures was limited by the long and narrow working channel. Whilst partial resection of the mass was achieved, iatrogenic trauma to the normal brain parenchyma was identified by intraoperative imaging. With consent of the owner the mare was euthanized under the same general anesthesia. Post-mortem magnetic resonance imaging and gross anatomical examination confirmed partial removal of a pituitary adenoma, but also iatrogenic damage to the surrounding brain parenchyma, including the thalamus.

Keywords: Horse; computer-assisted surgery; neuronavigation; pituitary adenoma; pituitary pars intermedia dysfunction.

Figure 1.

(A) T2w sagittal, (B) T2w transverse and (C) T1w 3D reconstructable magnetization-prepared gradient-echo (MP-RAGE) with contrast medium (gadoteric acid, clariscan ^TM, GE Healthcare AG, opfikon, Switzerland) dorsal magnetic resonance images of the head of a 16-year-old warmblood mare, depicting a large well-delineated pituitary macroadenoma compressing the thalamus, the midbrain, and the optic chiasm.

Figure 2.

Photograph showing the mare positioned in sternal recumbency on the surgery table, with her head placed in an elevated position on a carbon fiber table. The patient tracker required for surgical navigation using an optical tracking system is solidly anchored on the left facial crest using two 3.2 mm Schanz pins.

Figure 3.

Close-up view of a screen shot of the StealthStation S8 depicting the surgical plan on merged cone beam computed tomography and T1w high-resolution 3D reconstructable magnetization-prepared gradient-echo (3D MP-RAGE) transverse plane images. The light blue line represents the core axis of the corridor to be established as transmandibular lateral transsphenoidal surgical approach to the pituitary macroadenoma.

Figure 4.

(A) Intraoperative photograph of the surgical approach performed in the left cheek to gain access to the lateral pterygoid muscle through a circular osteotomy in the vertical ramus of the mandible (rostral is to the left, dorsal is to the top of the image). The buccal branches of the facial nerve are protected by a Penrose drain. (B) Insertion of the sequential muscle dilators (METRx II System, Medtronic) in the lateral pterygoid muscle through the osteotomy to create a working channel to gain access to the basisphenoid bone. Please note the central guidewire, the tip of which is firmly contacting the basisphenoid bone.

Figure 5.

Intraoperative view of the circular osteotomy in the basisphenoid bone, provided by a 30° forward oblique, 4 mm outer diameter, 18 cm standard telescope (Karl Storz Endoscopy, Tuttlingen, Germany) inserted in the metallic muscle dilator. The maxillary branch of the trigeminal nerve (*) is visible in the sulcus nervi maxillaris through the osteotomy.

Figure 6.

Post-mortem T1w high-resolution 3D reconstructable magnetization-prepared gradient-echo (3D MP-RAGE), (A) transverse, (B) sagittal and (C) dorsal plane magnetic resonance images of the head of a 16-year-old warmblood mare, depicting partial removal of the adenoma, but also inadvertent removal of portions of the thalamus.

Figure 7.

Post-mortem transverse slice of the head of the warmblood mare depicting the left-sided transmandibular lateral transsphenoidal surgical approach providing access to the pituitary macroadenoma. Parts of the mass had been removed intraoperatively, as demonstrated by the hemorrhagic area in its left dorsal aspect (right in the image), but iatrogenic damages to the surrounding brain parenchyma are also macroscopically visible. Please note the hemorrhagic tract within the center of the lateral pterygoid muscle, where the sequential muscle dilators were inserted.