Case reveal


  • A soft tissue attenuating, moderately contrast-enhancing mass is identified within the left trigeminal ganglion (yellow arrows), which extends through the foramen lacerum, the orbital fissure and alar canal. This mass is expansile, most severely at the level of the orbital fissure and alar canal (green arrows), causing severe, ill-defined lysis of the surrounding sphenoid, palatine and lacrimal bones as well as a moderate amount of amorphous periosteal proliferation extending onto the sphenoid and palatine bones. Additionally, the mass is medially deviating the left lateral wall of the sphenopalatine sinus. The mass is well-defined at the level of the trigeminal ganglion and becomes ill-defined more distally at the level of the trigeminal nerve branches. 
Figure 1
Figure 2
  • Within the left conchofrontal sinus, a moderate amount of fluid to soft tissue attenuating material with a gas interface (meniscal sign). This fluid increases in severity over the duration of the study, from mild on pre-C images to moderate post-C.
Figure 3
  • Bulging the mucosa of the dorsal aspect of the left dorsal conchal sinus, there is a small soft tissue attenuating, non contrast-enhancing, broad-based mass.
Figure 4
  • In the rostral aspect of the left rostral maxillary sinus, there is a small, rounded region of fluid soft tissue attenuation.
Figure 5
  • Multiple well-defined, round regions of geographic lysis surrounded by sclerosis are present within the subchondral bone of the zygomatic process of the temporal bone, involving both temporomandibular joints. Bilaterally centered within each temporomandibular joint space, there are thin, curvilinear regions of mineral attenuation.
Figure 6


  1. Left trigeminal nerve sheath tumor such as neurofibrosarcoma, with associated osteolysis. An infectious process, such as granulomatous neuritis, cannot be ruled out (Figure 1 and 2).
  2. Left frontal sinus mass with left rhinitis. Consider cyst, mucosal hyperplasia, granuloma or hematoma. The progressive nature of the sinus fluid may represent hemorrhage or edema (Figure 3 and 4).
  3. Left rostral maxillary sinus cyst (Figure 5).
  4. Bilateral temporomandibular joint osteoarthrosis with intra-articular mineralization, likely representing meniscal mineralization (Figure 6).

Case outcome

The patient was euthanized. Necropsy declined. The patient also had a history of SCC removed 1.5 years ago of the left lower eyelid.

A little bit more…

  • Trigeminal nerve sheath tumors in horses are rare with only few reports available.
  • They can affect all animal species but are more commonly seen in dogs and cattle.
  • Neurofibrosarcoma and schwannoma are the two most commonly reported tumor types.
  • Trigeminal nerve sheath tumors are described to be severe locally aggressive in dogs.
  • While bilateral lesions may occur, unilateral disease is most common.
  • Beltran et al. described unilateral neuritis of the trigeminal nerve with similar imaging characteristics as neoplasia and should be a considered differential diagnosis.
  • Interestingly, a case report by Wilson et al. describes a peripheral nerve sheet tumor diagnosed as an invasive squamous cell carcinoma. Our case had a history of SCC removed off the left lower eyelid 1.5 years before presenting to our hospital. The presented imaging features are similar to our case suggesting that a similar disease process may be considered.
The right trigeminal nerve ganglion is moderately to severely enlarged when compared to the contralateral side (Wilson et al.)
The right trigeminal nerve ganglion is moderately to severely enlarged with subsequent expansion and thinning of the bone of the alar canal (Wilson et al.)
  • MRI is the preferred modality to assess the brain and associate neuromuscular bundles due to its higher resolution of the soft tissue structures; however, CT also provides sufficient detail to guide a diagnosis.
  • The clinical sign of trigeminal tumors in dogs are well described and include masticatory muscle atrophy, reduced facial and corneal sensation with absent palpebral reflex.
  • In comparison to dogs, the mandibular branch of the trigeminal nerve exits more caudally in horses at the foramen lacerum.
  • While there are no reported therapy considerations in horses, radiation and surgical section has beed described in dogs with variable efficacy.


  • Beltran, Elsa, et al. “Imaging diagnosis—unilateral trigeminal neuritis mimicking peripheral nerve sheath tumor in a horse.” Veterinary Radiology & Ultrasound 57.1 (2016): E1-E4.
  • Stokes, Rebecca, et al. “Surgical and oncologic outcomes in dogs with malignant peripheral nerve sheath tumours arising from the brachial or lumbosacral plexus.” Veterinary and Comparative Oncology 21.4 (2023): 739-747.
  • Wilson, Amie, et al. “Invasive squamous cell carcinoma causes trigeminal nerve dysfunction in a horse.” Equine Veterinary Education 35.6 (2023): e451-e456.
  • Bagley, R.S., Wheeler, S.J., Klopp, L., Sorjonen, D.C., Thomas, W.B., Wilkens, B.E. et al. (1998) Clinical features of trigeminal nerve-sheath tumor in 10 dogs. Journal of the American Animal Hospital Association,  34,  19–25.
  • Gonçalves, R., Malalana, F., McConnell, J.F. & Maddox, T. (2015) Anatomical study of cranial nerve emergence and skull foramina in the horse using magnetic resonance imaging and computed tomography. Veterinary Radiology & Ultrasound,  56,  391–397.