All About that Base: A Radiologist’s Guide to Skull Base Injury

1. eEdE-141 M Gelbman1, Z Chadnick2, S Lev1 1Nassau University Medical Center, East Meadow , NY 2American University of the Caribbean, Sint Maarten All About that Base: A Radiologist’s Guide to Skull Base Injury

2. Disclosure • The authors have no financial or non-financial disclosures.

3. Goals • Skull base trauma can be quite devastating, often presenting with traumatic brain injury and often with significant injury/morbidity involving extra axial soft tissue structures such as cranial nerves and blood vessels. • We will provide an approach to evaluating skull base CT in the setting of trauma, review of skull base fracture patterns, and provide a review of related injuries and complications involving non-osseous structures.

4. Anatomy • An understanding of general skullbase anatomy and anatomic relationship of nonosseous structures to the bony skullbase will aid in accurate diagnosis of fractures and in predicting secondary injuries and complications.

5. * denotes parts of the central skull base

6. Anterior Fossa • Composed of paired frontal bones and ethmoid bone. Seats the anterior frontal lobes • Olfactory nerves transit the cribriform plates to enter the olfactory bulbs which lie within the olfactory grooves bordered medially by the crista galli and laterally by the lateral lamella. Left Facing Arrow: Crista Galli. Right Facing Arrow Right Lateral Lamella. Vertical Arrow: Left Olfactory Groove. Green Line Marks posterior margin of anterior fossa.

7. Middle Fossa • Formed of the Sphenoid and anterior portions of the paired temporal bones. Seats the temporal lobes and Meckel’s cave. • Many important soft tissue structures including CN 2-6, cavernous sinuses, pituitary gland, and portions of the internal carotids.

8. Posterior Fossa • Formed of the occipital bone and posterior portion of the paired temporal bones. Seats the cerebellar hemispheres and brainstem. • Important soft tissue structures include CN 7-12, petrous portions of the ICA’s, vertebrobasilar arteries, sigmoid transverse, torcula, and occipital sinuses, and the vestibular and auditory apparatuses.

9. Approach • It is imperative to scrutinize thin axial images, sagittal, and coronal reformats in the setting of head trauma. 3D reformats may also be helpful. • Added attention to areas of soft tissue swelling/hematoma, pneumocephalus, sinus or mastoid air cell opacification can increase sensitivity for subtle skullbase fractures. • If a fracture is identified soft tissue structures such as blood vessels which may be secondarily injured should be noted.

10. Anterior Skull Base • Fractures involving the anterior skullbase and frontal bones are of special importance and can frequently be associated with orbital injury, CSF leak, cephalocele, and intracranial infections.

11. Craniofrontal Smash 3-D reconstruction and noncontrast CT image show a severe craniofrontal smash injury with extensive pneumocephalus and cerebral edema.

12. Frontobasilar Fractures • Numerous classification systems of frontobasilar fractures are described in the literature. Of special clinical relevance in this region is whether there is involvement of the anterior and/or posterior walls of the frontal sinuses.

13. Central Skull Base • Transverse/coronal fractures commonly result from high velocity impact to the lateral skull. Can involve the sphenoid sinus. • Oblique fractures through the central skull base are often associated with facial or frontobasilar fractures and more commonly associated with CSF leak.

14. Sagittal reformat demonstrates a coronally oriented linear fracture of the sphenoid body extending through the floor of the sella.

15. Transverse fracture of the Clivus extending to the bilateral carotid canals. Involvement of the carotid canals should prompt the radiologist to recommend CTA to exclude injury to the internal carotid arteries. Transverse/oblique clival fractures: due to high velocity lateral blow to the skull. ICA injury is common. Longitudinal clival fractures: due to axial loading. Vertebrobasilar injury is common.

16. Temporal Bone Fractures • Temporal bone injuries are usually associated with severe head trauma and can be blunt, penetrating, or blast related. • Intracranial pneumocephalus or opacified mastoid air cells can help to localize subtle fractures. • Transverse fractures are traditionally associated with sensorineural hearing loss while longitudinal fractures with conductive. • Delayed complications can include CSF fistulas, infections and tegmen tympani dehiscence with meningoencephaloceles.

17. Longitudinal Longitudinal Fracture of Petrous Portion of Right Temporal Bone Also note the non-displaced linear fracture through the squamous portion of the temporal bone; subcutaenous emphysema and soft tissue swelling should draw attention to this region.

18. Transverse 3D rendering in a different patient demonstrates a nondisplaced transverse fracture of the temporal bone.

19. Posterior Skull Base • Fractures of the posterior skull base may typically result from a lateral blow or a blow to the occiput.

20. OcciptalCondyleHypoglossal CanalAxial MIP image demonstrates a comminuted non-displaced left sided occipital bone fracture with involvement of the left occipital condyle and os of the left hypoglossal canal and extension to the foramen magnum. This patient was assaulted with a baseball bat.

21. Occiptal Condyle Coronal reformat in a different patient with an isolated left occipital condyle fracture. Occipital condyle fractures are often subclassified: Type I: Compression from axial loading. Type II: Posterior skull base fracture with extension into the condyle. Type III: Avulsion fractor involving the insertion of the alar ligament.

22. Non-Contrast CT Limitations • It is imperative to recognize the limitations of non contrast CT in detecting non osseous sequela of skull base trauma. • Thorough understanding of the anatomic relationships of various soft tissue structures to the skull base will help to predict such injuries and indicate when follow-up advanced imaging, such as CTA/MRA, MRI, or scintigraphy, is advised.

23. Categories of Complications • Vascular • CN injury • Dural injury • Infection • Intraorbital injury • Auditory/Vestibular injury

24. Vascular Complications • Arterial Transection • Dissection/Occlusion • Pseudoaneurysm • AVF • C-C fistula • Sinus thrombosis

25. Dissection Axial T2 weighted images in a patient s/p MVC demonstrate replacement of L. vertebral artery flow void with high T2 signal (arrow). There is bright signal in the median and left paramedian medulla compatible with secondary ischemic changes.

26. Dissection • Dissection: Expanding hematoma within the wall of the vertebral artery, carotid artery, or within intracranial branches. • Can cause ischemic or embolic strokes. • Imaging: Crescentic T1 hyperintensity within expected region of vessel lumen. Angiography: Tapered narrowing or occlusion. “String sign.” May see intimal flap. Conventional Angiography is gold standard.

27. Occlusion Digital subtraction angiogram shows extracranial vertebral artery occlusion in a different patient with occipital/cervical dislocation.

28. Bilateral Carotid Occlusion Axial NECT in 10 y/o M demonstrates diffuse cerebral edema and the cerebellar reversal sign. Cerebral angiogram showed delayed, slow and persistent filling of the bilateral internal carotid arteries, with no contrast seen intracranially. There is prominent filling of the external carotid arteries with a nasopharyngeal blush (corresponding to the “hot nose” sign seen on brain death scintigraphy studies).

29. Sinus thrombosis • Depressed calvarial or skull-base fractures can directly injure underlying venous sinuses, this can lead to sinus thrombosis and secondary infarcts. Propagation of the clot into a cortical vein can cause secondary intraprenchymal hemorrhage.

30. Axial NECT images demonstrate hyperdensity within the straight sinus, left transverse sinus, and torcula. A secondary intraparenchymal hematoma is seen in the left parietal region.

31. Dural AVM Axial MRA source images demonstrate vascular lesion in the region of the right sigmoid sinus. Left vertebral artery DSA injection demonstrates an AVM nidus fed by branches of the AICA Lateral ICA injection demonstrates filling of AVM from the meningohypophyseal trunk with deep venous drainage.

32. AVF/DAVM • In the setting of post traumatic sinus thrombosis, neovascularization or enlargement of underlying physiologic AV shunts can occur leading to an acquired fistulous malformation (Dural AV fistula/Dural AVM). • Presence of reflux into cortical veins increases the risk of hemorrhage and the benefit of treatment.

33. Enlargement of the left superior ophthalmic vein on CECT (left) suggests the possibility of a CC fistula in this 42 year old trauma patient. DSA (right image) with filling of the left cavernous sinus and superior opthalmic vein during contralateral CCA injection confirms the presence of an indirect CC fistula.

34. Carotid-Cavernous fistula • Both direct and indirect CC fistula can result from skullbase trauma; CC fistula should be considered in the setting of sphenoid bone fractures. • Direct CC fistula: Laceration of the cavernous segment of the ICA causes a direct high-flow fistula with the cavernous sinus. • Indirect CC fistula: Dural AV fistula between the cavernous sinus and branches of the ipsilateral and/or contralateral ICA and/or ECA. • Secondary signs on cross sectional imaging: dilated superior opthalmic vein, dilated petrosal sinuses, proptosis, enlarged extraoccular muscles. • Gold standard imaging modality is catheter angiography.

35. Cranial Nerve Injuries • Cranial nerves may be compromised directly or indirectly following trauma. • Injuries to the base of the skull can damage nerves as they emerge from the brain or brainstem. • Mechanisms of injury include crushing, penetrating, stretching and traction forces.

36. Orbital apex fractures can involve the optic canal and/or superior orbital fissure (SOF) with associated injuries to the optic nerve or SOF contents (including Cranial Nerves 3,4,V1,6) This trauma patient with a left orbital apex fracture developed sudden loss of vision. Traumatic optic neuropathy can result from fracture fragments or hematoma compressing CN2. Transection of CN2 can be readily identified on CT images by discontinuity of the nerve.

37. CN I injury • Shearing injuries and fractures at the cribiform plate can lacerate the olfactory nerves. • Inferior frontal hematomas can compress CN I. This patient with multiple frontal contusions and a fracture of the right cribriform plate went on to develop anosmia.

38. Complications Related to Violation of Dura and Other Barriers • CSF leak • Meningitis • Abscess • Cephalocele • Cholesteatoma

39. CSF Leak Sagittal and coronal reformats of CT cisternogram performed in a trauma patient with CSF rhinorrhea. A bony defect at the roof of the left sphenoid sinus is seen with transit of intrathecal contrast from the basal cisterns into the left sphenoid sinus.

40. CSF Leak • Consider in the setting of anterior or central skull base fracture. • May present with CSF rhinorrhea or otorrhea. Can be confirmed with fluid analysis for β2-transferrin. • If CSF leak is suspected in setting of multiple fractures, or a discrete fracture is not identified, localization can be performed with CT or radionuclide (Tc-99 DTPA or In-111 DTPA) cisternography. • Treatment: Often resolve within 1-2 weeks with conservative management although surgical intervention is required if persistent.

41. Meningitis B C A Axial contrast enhanced CT (A, B) and axial contrast enhanced T1W MR (C) demonstrate leptomeningeal pattern of enhancement with diffuse enhancement within the sulci.

42. Meningitis • Direct spread of infection via dural/bony defects. • Gold standard for diagnosis is LP with CSF analysis. • Imaging: May be normal. May show leptomeningeal enhancement pattern on CECT and T1+Gd. Imaging is most useful in demonstrating complications such as abscesses. • Treatment: generally antibiotics.

43. Epidural Abscess with Mastoiditis Axial T1 post contrast MR demonstrates a rim enhancing collection with convex borders resulting in mass effect of the temporal lobe. Fluid is noted in the right mastoid cells on axial T2 MR image.

44. Abscess Direct spread of infection via bony and dural defects can lead to: • Epidural Abscess • Subdural Empyema • Intraparenchymal abscess. • Ventriculitis. Imaging: Subdural/Epidural Empyema: Peripherally enhancing fluid collection with diffusion restriction. Treatment: surgical drainage

45. Cephalocele Coronal CT reformat demonstrates a left orbital roof “blow-in” fracture with suggestion of herniation of left frontal lobe tissue into the orbit. Sagittal T1 MRI image confirms the presence of an orbital encephalocele.

46. Cephalocele • Refers to herniation of intracranial contents via a bony defect. Two types: • Meningocele: Herniation of meninges and CSF. • Encephalocele: Herniation of brain tissue.

47. Cholesteatoma Axial Noncontrast CT through the temporal bone shows scutum erosion and destruction of the long process of the incus.

48. Cholesteatoma • Can cause cranial nerve dysfunction, headache, mixed conductive hearing loss, otorrhea. • Keratin filled and encapsulated by stratified squamous epithelium. • Acquired cholesteatomas- arise from retraction pockets in the pars flaccida or in the pars tensa of the eardrum, grow into the middle ear.

49. Injury to the Auditory and Visual Apparatus • Skull base fractures can be associated with a myriad of injuries to the intraorbital structures and to the conductive and sensorineural auditory system.

50. Traumatic ossicular dislocation An air-filled space is seen interposed between the left incus and malleus in this patient with a transverse temporal bone fracture.