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

Atrophy of the suboccipital muscles has been observed in patients who experience headaches following whiplash-type injuries and tension-type headaches⁽³⁾. Needle electromyography (EMG) is a diagnostic technique that can be used to determine the underlying cause of muscle atrophy. The EMG procedure involves inserting a needle electrode into each suboccipital muscle to record and analyze electrical signals, allowing clinicians to differentiate between atrophy from disuse versus atrophy from denervation – an important factor in determining course of treatment. Each muscle is identified using anatomical landmarks prior to needle insertion⁽³⁾. The atlantomastoid is thought to contribute to head extension if present bilaterally and lateral flexion and/or rotation of the head if present unilaterally^(1,4); thus, the variant muscle shares common functions with the typical suboccipital muscles. The authors suggest that the atlantomastoid, when present and identified by ultrasound, should be considered for EMG analysis.

Manual therapists may use an invasive technique known as dry needling; a method of pain relief involving the insertion of solid filament needles into specific myofascial trigger points⁽⁵⁾. Dry needling of the suboccipital muscles is used to treat cervicogenic headache and often results in positive outcomes including increased range of motion and significant reductions in local and widespread pain⁽⁵⁾. If variant muscles like the atlantomastoid have the potential to play an etiological role in chronic pain, detection using POCUS prior to intervention could result in a new target in pain management. Furthermore, treatment for chronic tension-type headache involves a variety of modalities primarily focused on the suboccipital region⁽⁶⁾. One of the most common treatment methods is suboccipital release, a non-invasive osteopathic manipulation technique that targets the suboccipital musculature⁽⁷⁾. It is recommended that patients undergo a complete musculoskeletal exam of the neck prior to a suboccipital release, as concern for neurovascular compromise is a contraindication for the procedure⁽⁸⁾. Since variations in suboccipital musculature are closely associated with compression of neurovascular structures such as the vertebral artery and the greater and third occipital nerves⁽²⁾, POCUS evaluation during the initial exam may be beneficial in determining patient suitability prior to cervical manipulation. If present, the atlantomastoid lies in close proximity to certain neurovascular structures of the posterior cervical region. The occipital artery courses between the lateral border of the obliquus capitis superior and the medial border of the atlanto-mastoid prior to ascending obliquely towards the external occipital protuberance⁽²⁾. The greater, lesser, and third occipital nerves travel medially relative to the atlantomastoid to innervate the posterior scalp. At the current time, the relationships between these nerves and the atlantomastoid have not been discussed in the literature, however the authors postulate that the lesser occipital nerve in particular could be in close proximity to the atlantomastoid. Additionally, the third segment of the vertebral artery lies in close relationship to three bony landmarks: the transverse process of C1, the posterior tubercle of C1, and the tip of the mastoid process. As the atlantomastoid originates from the transverse process of C1 and inserts on the mastoid process, it would lie in close proximity to the third segment of the vertebral artery

Clinicians performing interventional procedures in the suboccipital region, like ultrasound-guided greater occipital nerve (GON) blocks and botulinum toxin injection into the suboccipital muscles to treat pain related to compressed nerves, muscle spasm, occipital neuralgia, and chronic tension-type headache, could benefit from knowledge of the atlantomastoid muscle because of its relationships to key anatomical landmarks and neurovascular structures in the region⁽⁴⁾. The procedure for ultrasound-guided GON blocks typically involves the use of the obliquus capitis inferior as the primary anatomical landmark to identify the GON prior to needle injection⁽⁹⁾. However, muscular variations in the suboccipital region have been shown to cause alterations in the pathway of the GON⁽¹⁾, further illustrating the value of POCUS identification of muscular variants that may influence the treatment approach if discovered prior to performing the procedure. Ultrasound-guided injection of a local anesthetic-steroid combination into the suboccipital muscles has been shown to be an effective management strategy for post-dural puncture headache⁽¹⁰⁾, again indicating the potential for the atlantomastoid to be considered a target for treatment, when present.

This cadaveric POCUS simulation indicates that the atlantomas-toid muscle variant, when present, can be identified during scanning by using the obliquus capitis inferior, the transverse process of C1, and the mastoid process as landmarks. This letter aims to help foster interest in the atlantomastoid muscle and encourage POCUS practitioners to consider the muscle’s relevance in the diagnosis and treatment of chronic pain conditions.