The atlantomastoid: a muscle variant relevant to point-of-care ultrasound?

The atlantomastoid is a variant muscle of the suboccipital region, originating from the transverse process of the atlas (C1) and inserting on the posterior aspect of the mastoid process of the temporal bone (Fig. 1 A). The atlantomastoid is supplied by the occipital artery and innervated by the suboccipital nerve(1); the former is a branch of the external carotid artery and the latter is formed by the dorsal ramus of C1. A recent study determined the muscle’s prevalence to be 15%(2). Because of the atlantomastoid's proximity to the four typical suboccipital muscles and certain neurovascular structures of the posterior cervical region, detection of the atlantomastoid muscle using ultrasound may be beneficial for clinicians performing interventional procedures in the suboccipital region, and for manual therapists that manipulate the cervical spine. Summarized in this letter is a point-of-care ultrasound (POCUS) simulation that was conducted on a cadaveric specimen, whereby the mastoid process, the transverse process of C1, and the obliquus capitis inferior served as anatomical landmarks to aid in the imaging of the muscle. This letter aims to bring attention to the atlantomastoid variant and highlight its potential clinical importance for POCUS practitioner