ECR 2021 / C-10686
Landmarks and pitfalls on temporal bone anatomy: an easy approach.
Head and neck, CT-High Resolution, MR, Education, Education and training
C. A. F. Coelho Neto, M. Sarpi, L. Duarte, M. R. T. Garcia, S. A. Souza; São Paulo/BR
Findings and procedure details
- ANATOMIC LANDMARKS, PITFALLS AND VARIATIONS ON TEMPORAL BONE:
The Tympanicum Foramen: also called foramen of Hushcke, it represents an ossification defect in the anteroinferior aspect of the external auditory canal. It usually closes before five years old, so be careful not to call it persistent before this age. It is important to recognize this variation mainly because it can mimic a fracture on trauma evaluation.
The Tympanic membrane: a paper thin membrane located at the fundus of the external auditory canal, separating the external ear from the middle ear. When normal, is barely seen on CT.
- The Ossicular Chain: consists of three tiny bones located on the tympanic cavity (malleus, incus and stapes) responsible for amplificating the vibrations of the tympanic membrane and transmitting them to the cochlea, where it will be interpreted as sound.
- The Ossicular Suspensory Ligaments: there are four suspensory ossicular ligaments: superior malleal, lateral malleal, anterior malleal and posterior incudal. They are hardly seen on normal CT exames and may be thickened on tympanoesclerosis secondary to chronic inflammatory process.
- The Famous Ice Cream Cone: the mastoid axial plane, including the malleus head and incus body, depicts the incudomalleolar joint and looks like an ice cream cone. It is import to know how the normal joint looks like to avoid misinterpreation on subtle luxations on trauma evaluation.
- The Cochleariform Process: a thin osseous proeminence which separates the eustachian tube from the canal for the tensor tympanic muscle. It may present a bulging contour and be thickened (diameter superior than 2,3 mm) at the sclerotic phase of otospongiosis. These findings are useful to avoid misinterpretation when hypoattenuating foci are absent in patients with hearing loss with clinical suspicion of otospongiosis.
- The Cochlear Cleft: a curvilinear radiolucent area of incomplete endochondral ossification in the otic capsule adjacent to the cochlea. It is commonly found in children but it may also be seen in adults. It must be recognized as it can mimic otosclerosis on a temporal bone CT.
Persistent Stapedian Artery: a rare congenital vascular anatomy that may be a incidental finding or related to pulsatile tinnitus. The imaging clues, especially on CT, are a soft tissue proeminence along the cochlear promontorium and an absent foramen spinosum.
- Cochlear-facial Dehiscence: a recently described entity that may be a incidental finding or be associated with third window symptoms. It carachterized by lack of bone separating the labyrinthine segment of the facial nerve and the cochlea.
- High Hiding Jugular Bulb and Dehiscences: a high riding jugular bulb is characterized when its roof extends superiorly than unusal. The criterias for a high bulb vary on literature. On clinical practice, due to easy and quick assessment, we use the basal turn of the cochlea as a reference in our institution. A dehiscence indicates the absence of osseous tissue between the bulb and the adjacent structures, such as the vestibular aqueduct, the internal auditory canal, the tympanic cavity and the labyrinth. The findings may be asymptomatic or be associated with symptoms like pulsatile tinnitus.
- The Labyrinth: structure responsable for hearing and balance, consisting of the osseous labyrinth, well seen on CT, which houses the membranous labyrinth, filled with high T2WI fluid on MRI. Includes the cochlea, vestibule and semircular canals.
- Vestibular Aqueduct: osseus structure of the innear ear that contains the endolymphatic sac. On CT we can delimitate the aqueduct while the MRI depicts the endolymphatic duct and sac. The enlarged vestibular aqueduct syndrome is the most common inner ear malformation related to congenital sensorial hearing loss.
- Anatomic Variants Related to Tinnitus: some anatomic constitucional variatons may be seen in patients with tinnitus and may be related to symptoms in absence of other defined causes. They include pneumatized petrous apex, high riding jugular bulb, pneumatized squamous temporal bone and pneumatization around the carotid canals.
- Sinus Tympani and Facial Nerve Recess: important landmarks on the tympanic cavity which my be blind spots for surgeons on tympanic cavity lesions, such as cholesteatomas.
- The Prussak Space: important landmark on the tympanic cavity. The pars flaccida cholesteatoma originates in this space, which is limited superiorly by the lateral malleal ligament, inferiorly by the short process of the malleus, medially by the malleus neck and laterally by the tympanic membrane pars flaccida. Cholesteatomas on this site may enlarge the Prussak space, dislocate the ossicular chain medially and erode the scutum.
- Internal Auditory Canal Diverticulum: focal lucency along the anteroinferior margin of the internal auditory canal. It may be related to cavitary otosclerosis or represent an anatomic variant (without hearing loss).
- Fissure or fracture? Many linear structures on the temporal bone are linear and may mimic fracture lines, such as the occipitomastoid suture, petro-occipital fissure, tympanosquamous fissure and the persistence of the tympanomastoid fissure. It is important to always compare with the contralateral side on trauma evaluation to avoid misinterpretation.
- The Stylohyoid Complex: craniocervical structure with 4 main components, including the styloid process, the stylohyoid ligament, the lesser cornua of the hyoid and superior portion of the hyoid body.
The calcification of the complex my be asymptomatic or cause the Eagle Syndrome, which includes symptoms related to cranial nerve or arterial impingement, with dysphagia as the most commmon manifestation.
The reference value of normality for the length of the complex is 3,0 cm, and Eagle syndrome is more prevalent when it measures 4,0 cm or more. But on daily pratice, we measure the calcified complex in every symptomatic patient.