VASCULAR VARIATIONS, ANOMALIES, AND CONDITIONS OF THE TEMPORAL BONE
- Vascular variants frequently are not associated with symptoms.
- Computed tomography, including computed tomographic angiography, is the primary tool for evaluating vascular conditions of the temporal bone.
- Acquired vascular abnormalities are more likely to prove dangerous to the patient.
- These conditions are important considerations in the evaluation of a patient presenting with pulse synchronous (pulsatile) tinnitus and seventh and eighth cranial nerve and lower neuropathies.
These vascular conditions run the gamut of minor developmental variations to complex vascular anomalies and malformations. They are generally sporadic and not associated with syndromes. They may cause signs and symptoms but are frequently observed as incidental findings. Dissections and aneurysms of the petrous segment of the carotid artery, if not occurring spontaneously, will usually be acquired as a result of trauma or infection.
The most common vascular variations are those of jugular fossa development seen as incidental findings in many patients imaged with computed tomography (CT) or magnetic resonance (MR) for unrelated issues.
These conditions will often present as a cause of pulse synchronous (pulsatile) tinnitus either with or without a mass visible behind an intact tympanic membrane, which is discussed in more detail in Chapter 109. Vascular conditions may also be associated with a cranial neuropathy producing hemifacial spasm, sensorineural hearing loss, or vestibular symptoms or other signs as well as symptoms such as pain and headache that could herald a venous thrombosis, dissection, or aneurysm.
Some will be encountered incidentally when imaging a patient with aural atresia or other congenital malformation of the temporal bone or on imaging done as part of the evaluation of conductive or sensorineural hearing loss.
APPLIED ANATOMY AND EMBRYOLOGY
Venous anatomy and related bony anatomy:
- Sigmoid sinus, jugular bulb, petrosal sinuses, other major dural venous sinuses, and the vein of Labbé
- Jugular fossa, sigmoid sinus, and bony conduits of the superior and inferior petrosal sinuses (Figs. 104.10 and Fig. 104.60)
Arterial anatomy and related bony anatomy:
- Distal segment of the internal carotid artery (ICA), including its cervical, petrous, and cavernous segments; distal maxillary and middle meningeal artery (Fig. 104.10)
- Carotid foramen and carotid canal; caroticotympanic canal and inferior tympanic canaliculus and foramen spinosum
Other temporal bone anatomy:
- Emphasizing the usual relationships of the carotid and jugular to the cochlea and vestibule, respectively, the facial nerve canal, promontory of the cochlea, and more generally the relationships of these structures within the middle ear cavity (Fig. 104.25)
- An awareness that vascular transdiploic channels are commonly present in the temporal bone, being most common around the mastoid, which may become large developmentally or when needed to handle increased flow volume (Fig. 108.1)
The arterial vasculature of the cranium evolves from remnants of the branchial components of the aortic arch and great vessels. Primitive embryonic vessels without an adult derivative usually regress completely; however, occasionally, anomalous persistence may occur, influencing the evolution of the remainder of the cerebral vasculature.
The hyoid artery or second arterial arch and its principal branch, the stapedial artery, will shape the appearance of the distal external carotid artery circulation and middle meningeal artery (Fig. 104.59A). The stapedial artery supplies the face, orbit (except for the eye), and meninges for some time. Progressive atrophy of the stapedial artery occurs because part of its territory is annexed by the external carotid artery as it forms the middle meningeal and ophthalmic artery. A small caroticotympanic artery is left inferiorly and possibly the superior tympanic artery superiorly. If the stapedial artery fails to regress, it will instead give rise to the middle meningeal artery.
A persistent stapedial artery may occur in association with an aberrant ICA. An absence of the vertical petrosal segment of the ICA leads to enlargement of both the inferior tympanic and caroticotympanic arteries. The flow in the latter is reversed, completing the connection to the horizontal portion of the petrosal ICA (Figs. 108.2–108.4). If a persistent stapedial artery is present, it will leave the carotid from the aberrant segment.1,2
The complexity of cranial venous embryology explains the frequency of isolated venous variations. The primitive sinus transversus, sigmoideus, and internal jugular can be detected at the 35-mm stage, and their development continues into the postnatal period. The morphologic changes of the posterior fossa dural sinuses, emissary veins, and jugular bulb are therefore related to the development of the brain, the shift from fetal circulation to a postnatal type of circulation, and postural hemodynamic changes. The transverse sinuses rapidly enlarge and then decrease in caliber during embryogenesis. These changes may lead to unequal heights, asymmetrical sizes, mild to marked irregularities, septa or fenestrations, or even absence of portions of the transverse sinus. The superior sagittal sinus drains more frequently into the right transverse sinus, probably related to the relatively linear course to the superior vena cava. On the left side, the internal jugular vein takes a bend through the brachiocephalic vein before reaching the superior vena cava.
The sigmoid sinus and primitive internal jugular vein keep a tiny caliber until after birth. The jugular bulb is usually developed by 2 years of age. It appears that formation of the bulb is related to changes in circulation due to the shift from lying positions to erect posture. There may be excessive enlargement and other variations of the jugular bulb as discussed in Chapter 104 (Figs. 104.17 and 104.18). The dural sinuses are frequently larger on the right side.3
Computed Tomography and Magnetic Resonance Imaging
In general, these anomalies are best evaluated with a combination of standard high-resolution temporal bone imaging and CT angiography. Specific protocols by indication appear in Appendixes A and B.
Pros and Cons
Magnetic resonance imaging (MRI) with MR angiography can be used for simple tasks such as confirming an anomalous carotid but is not well suited to definitive evaluation of more complex anomalies. It is equally not well suited to definitive evaluation of the region of the jugular fossa, with this limitation being essentially related to the inability of MRI to provide definitive bone information and because of the variability in the area caused by the common occurrence of bone, air, and fat interfaces. This is further complicated by flow-related artifacts that are often exacerbated by the use of contrast, which is discussed in Chapter 104 (Fig. 104.19). Basic CT angiography improves on this situation quite a bit but is always sufficient.