Temporal Bone



Temporal Bone





TEMPORAL BONE FRACTURES


KEY FACTS

Longitudinal type



  • Most common (70% to 80%) type of temporal bone fractures; results from blows to the temporo-parietal region.


  • Results in conductive hearing loss secondary to ossicular chain dislocations (most common: incudostapedial [distance between these ossicles is >2 mm] and malleoincudal dislocations).


  • Tympanic membrane is often perforated, increased incidence of postfracture cholesteatoma.


  • Facial palsy (10% to 20%) is delayed (due to swelling of descending facial nerve and entrapment within bone canal) and generally resolves spontaneously.


  • Air in the temporomandibular joint is an indirect sign of temporal bone fracture.


  • Air in the labyrinth is indicative of underlying fracture.

Transverse type



  • Second most common (10% to 20%) type of temporal bone fracture (some authors believe that mixed or complex fractures are more common than either).


  • Results from frontal or occipital blows.


  • Produces sensorineural hearing loss and vertigo due to involvement of the otic capsule and/or transaction/concussion of 8th cranial nerve.


  • Facial palsy is more common than with transverse fractures and usually is permanent and due to nerve transection.






FIGURE 30-1. Axial computed tomography (CT) shows longitudinal fracture (arrow) and separation of malleus and incus.






FIGURE 30-2. Axial CT, in a different patient, shows air (arrow) in vestibule due to a fracture.







FIGURE 30-3. Axial CT, in a different patient, shows missing incus due to complete dislocation. Only the head of the malleus is seen.






FIGURE 30-4. Axial CT, in a different patient, shows mild malleoincudal separation (arrow) and air cell opacification.






FIGURE 30-5. Axial CT, in a different patient, shows transverse fracture (arrow) extending to basal turn of cochlea.



SUGGESTED READING

Swartz JD. Temporal bone trauma. Semin Ultrasound CT MR 2001;22:219-228.



VESTIBULAR SCHWANNOMA


KEY FACTS



  • Most common posterior fossa tumor in adults and second most common intracranial extra-axial tumor after meningioma in adults.


  • 75% to 80% of masses in the cerebellopontine angle cistern are vestibular schwannomas.


  • They arise from Scarpa ganglion (glial-Schwann cell junction) in the superior division of the vestibular nerve.


  • They are more common in females aged 40 to 60 years.


  • Bilateral eighth nerve schwannomas = NF-2.


  • Most common symptoms: sensorineural hearing loss, tinnitus, headache, and disequilibrium.


  • Facial nerve palsy is uncommon as this nerve is fairly resistant to pressure.


  • 5% to 22% of vestibular schwannomas are atypical by imaging and have associated arachnoid cysts or central necrosis or are partially or completely cystic.


  • Main differential diagnosis: meningioma, facial nerve schwannoma, metastasis, viral infection (Ramsay Hunt syndrome), lipoma (low signal on fat suppression magnetic resonance (MR) images)






FIGURE 30-6. Axial postcontrast T1 shows typical “ice cream cone” shape of large and mostly homogeneous enhancing left vestibular schwannomas.






FIGURE 30-7. Axial postcontrast T1, in a different patient, shows a mostly cystic right vestibular schwannoma.






FIGURE 30-8. Axial T2, in a different patient, shows a large and bright tumor.






FIGURE 30-9. Axial constructive interference in steady state (CISS) image, in a different patient, shows the left-sided schwannoma to be dark; note the decreased signal from fluid in inner ear structures probably due to increased proteins.







FIGURE 30-10. Axial CISS image, in a different patient, shows a small left tumor in fundus of internal auditory canal and probably extending to the basal turn of the cochlea at insertion of modiolus.






FIGURE 30-11. Axial postcontrast T1, in a different patient, shows an enhancing left intracana-licular schwannoma.






FIGURE 30-12. Axial postcontrast T1 image, in a different patient, shows tiny tumor (arrow) in fundus of the right internal auditory canal.






FIGURE 30-13. Axial postcontrast T1 image in a different patient with a purely intravestibular schwannoma (arrow).



SUGGESTED READING

Davidson HC. Imaging evaluation of sensorineural hearing loss. Semin Ultrasound CT MR. 2001;22:229-249.



FACIAL NERVE SCHWANNOMA


KEY FACTS



  • Account for 5% of facial nerve palsies, particularly unilateral (the remainder are of viral or posttraumatic etiology).


  • Onset of facial nerve palsy is slow and progressive.


  • Tend to arise in the geniculate ganglion but may involve any of its segments.


  • Identification of extension along the labyrinthine segment of the facial nerve is important in making the correct presurgical diagnosis as it is only schwannomas that result in significant thick-ening and enhancement in this region.


  • Main differential diagnosis: facial nerve hemangioma, perineural tumor spread, vestibular schwannoma, viral neuritis (Bell palsy and Ramsay Hunt syndrome), and meningioma.






FIGURE 30-14. Coronal postcontrast T1 shows enhancing schwannoma involving the descending and intraparotid (star) portions of cranial nerve VII.






FIGURE 30-15. Axial postcontrast T1, in the same patient, shows the tumor in the mastoid portion (arrow) of the facial nerve.







FIGURE 30-16. Axial CISS image, in a different patient, shows dark signal in a facial nerve tumor involving the ICA (arrow) and the region of geniculate ganglion (arrowhead).






FIGURE 30-17. Corresponding postcontrast T1 shows enhancement throughout the tumor.






FIGURE 30-18. Axial postcontrast T1, in a different patient, shows schwannoma in the internal auditory canal (arrow) and geniculate ganglion (arrowhead).






FIGURE 30-19. Coronal postcontrast T1, in the same patient as 30-18, shows tumor in geniculate ganglion (arrowhead).



SUGGESTED READING

Phillips CD, Bubash LA. The facial nerve: anatomy and common pathology. Semin Ultrasound CT MR. 2002;23: 202-217.



ENLARGED ENDOLYMPHATIC SAC (LARGE VESTIBULAR AQUEDUCT) SYNDROME


KEY FACTS



  • 1% of patients with congenital sensorineural hearing loss have abnormalities detected by imaging studies; the most commonly recognized one is probably a large vestibular aqueduct.


  • Normal vestibular aqueduct extends from the vestibule to the posterior aspect of the petrous bone and contains the endolymphatic duct whose function is equilibration of endolymphatic fluid pressure.


  • CT: dilated vestibular aqueduct (vestibular aqueduct should be no wider than a semicircular canal or more than 1.5 mm at its midpoint).


  • A large vestibular aqueduct may be associated with cochlear anomalies (from Mondini to absence of the modiolus).


  • Enlargement of the endolymphatic sac may occur in the presence of normal-sized aqueduct, and it may present as a mass in the cerebellopontine angle region and is better seen on magnetic resonance imaging (MRI).


  • Fluid levels and occasional contrast enhancement may be seen within the large endolymphatic sac.






FIGURE 30-20. Axial CT shows large vestibular aqueduct (arrow), large vestibule (star), and dysplastic cochlea with absent modiolus and abnormal middle and apical turns.






FIGURE 30-21. Axial CT, in a different patient, shows moderately dilated vestibular aqueduct (arrow).






FIGURE 30-22. Axial CT, in a different patient, shows minimally dilated vestibular aqueduct (arrow).






FIGURE 30-23. Axial CISS image, in a different patient, shows bilaterally mildly enlarged endolymphatic sacs (arrows) and dysplastic cochleas.

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Sep 8, 2016 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Temporal Bone

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