3D CT Evaluation of Common Crus Aplasia

Abstract

SUMMARY:Aplasia of the common crus is an uncommon congenital anomaly. We present the case of a patient with common crus aplasia and discuss the relevant embryology and the role of 3D CT in evaluation of this rare congenital anomaly.M ost of the patients who present with congenital sensorineural hearing loss (SNHL) do not demonstrate an imaging abnormality. 1 Only 39% of patients with congenital SNHL have a documented imaging abnormality, with large vestibular aqueduct syndrome being the most common of the congenital anomalies to be identified on imaging. 2 Congenital anomalies of the semicircular canals can result in SNHL. Dysplasia of the lateral semicircular canal has been reported to be the second most common cause after large vestibular aqueduct syndrome to be so associated. 3 Common crus aplasia is an uncommon congenital anomaly. 4,5 We report the case of a patient with common crus aplasia and review the underlying pathogenesis. Case ReportA 25-year-old man, a Mexican immigrant, presented to our otolaryngology clinic with difficulty in hearing. The patient's previous medical records could not be traced. On questioning, the patient reported worsening hearing loss since birth. On examination, bilateral SNHL was noted. There were no other features to suggest an association with any syndrome. The patient was referred for a high-resolution CT scan of the temporal bone. Axial and coronal 2D CT scans demonstrated dilated dysplastic lateral semicircular canals with possible dysplastic vestibules bilaterally (Fig 1). Corresponding 3D axial and coronal CT scans confirmed the presence of dilated dysplastic lateral semicircular canals and dysplastic vestibules. However, the 3D CT images raised the possibility of bilateral absence of the common crus, which was not noticed during the initial interpretation on the conventional 2D CT scans (Fig 2). A 3D volume-rendered CT image confirmed bilateral common crus aplasia (Fig 3). DiscussionThe 3 semicircular canals-superior, posterior, and laterallie in orthogonal planes with respect to each other. The posterior semicircular canal and superior semicircular canal share a common limb, the common crus, which is formed by the fusion of the posteromedial end of the superior semicircular canal and the anteromedial end of the posterior semicircular canal. 4 From the point of joining, the common crus runs medially for a length of approximately 4 mm at an oblique angle of approximately 60°to the axial plane until it opens into the superomedial portion of the vestibule.Dysplasia of the semicircular canals is a frequent congenital inner ear malformation, with abnormalities of the lateral semicircular canal being the most commonly identified anomaly noted on imaging. 6 Absence of the common crus or common crus aplasia is uncommon. However, it is one of the known associations of Goldenhar syndrome. 5 Common crus aplasia is usually associated with dysplasia of the lateral semicircular canal. This association can be understood if one is familiar with the embryology of the semicircular canals...