INTRODUCTION
CM of the ear are usually the result of defects in the embryogenesis of the auditory system that compromises normal fetal development. The overall incidence of ear malformations is 1:3800 newborns [1], however, this figure varies greatly depending on the compartment affected and its etiology. CM may have a genetic or non-genetic background and they could appear in a syndromic context (Treacher-Collins, Pierre Robin, Goldenhar, or Kippel-Feil syndromes as the most common)[2].
Ear malformations may be silent but in most cases, they present as conductive hearing loss and/or sensorineural hearing loss early in childhood. Radiologists should be familiar with the anatomy and wide spectrum of malformations of the auditory system and work together with neurotologists and pediatricians to assure a global approach. The early diagnosis of these anomalies is mandatory to provide early treatment and to enhance a child’s language and speech development.
EMBRYOLOGY
The temporal bone develops from two separate primary structures, the pars branquialis, which arises from the first and second branchial arches and the first branchial groove, and the pars otica, which develops from the auditory vesicle and the adjacent mesenchyme [3]. The external and middle ear anomalies usually appear combined as they both develop from the pars branquialis. The inner ear develops independently from the pars otica.
ANATOMY
The external ear can be structurally split into two sections:
- Auricle: an expanded and irregular cartilage that projects laterally from the head.
- External auditory canal (EAC): an S-shaped tubular structure that connects the auricle with the tympanic membrane. It is typically 2.5 cm in length and 6-10 mm in width. The lateral one-third is a fibrocartilaginous tube and the medial two-thirds are surrounded by bone.
The external ear is easily accessible to direct examination, usually by pediatricians or otolaryngologists. The clinical history and otoscopy are in most cases sufficient to diagnose and treat many of the diseases of the external ear. However, in some particular scenarios, especially when middle ear anomalies are suspected or when there is any kind of hearing impairment, imaging can be crucial for reaching an accurate diagnosis.
The middle ear extends from the tympanic membrane to the lateral wall of the inner ear. It consists of the tympanic cavity, a pneumatized box-like structure that houses the ossicular chain, and the antrum. It is subdivided into three spaces: epitympanum, mesotympanum, and hypotympanum. This cavity is bounded superiorly by the tegmen tympani and the jugular wall inferiorly. The medial wall contains the oval and round windows and the prominence of the tympanic segment of the facial nerve.
The bones of the middle ear are the auditory ossicles: the malleus, incus, and stapes.
- Malleus: also known as the hammer, is the largest and the most lateral of the ear bones. It attaches to the tympanic membrane through the handle.
- Incus: the middle one. It connects the malleus and the stapes.
- Stapes: is the smallest and the most medial one.
The inner ear is the most medial compartment of the ear and is located within the petrous portion of the temporal bone. It consists of a membranous labyrinth surrounded by a bony labyrinth (the cochlea, vestibule, and semicircular canals).
The cochlea is a spiral-shaped structure with 2.5 turns (the basal, middle, and apical turns), and they get progressively smaller from basal to the apical one. These turns spiral around a central bony axis called the modiolus. In the inside of the cochlea, there is an osseous spiral lamina that divides the spiral into the scala vestibuli and the scala tympani, connected by the helicotrema at the apex. The vestibule is an ovoid structure located superior and posterior to the cochlea and unites with the semicircular canals (superior, posterior and lateral).