Pivotal role of obliquus capitis inferior in torticaput revealed by single-photon emission computed tomography

Torticaput is the most common primary form of cervical dystonia (CD). Obliquus capitis inferior (OCI) plays a major role in ipsilateral rotation of the head. The present study aimed to use single-photon emission computed tomography (SPECT/CT) to determine the involvement of OCI in torticaput and in torticaput associated with no–no tremor. We retrospectively analyzed the SPECT/CT images of 60 patients with torticaput as the main abnormal posture and ranked the affected muscles. The affected muscles in patients with no–no tremor were also ranked. The correlation between the radioactivity of OCI and the thickness of OCI measured by ultrasonography was analyzed. The agreement between SPECT/CT and electromyography in detecting OCI was also analyzed. After sternocleidomastoid muscle (81.7%), OCI was the second most affected muscle (70.0%) in torticaput, followed by splenius capitis (63.3%). In 23 patients with no–no tremor, OCI (78.3%) and sternocleidomastoid muscle (78.3%) were the most frequently affected muscles, followed by splenius capitis (69.6%). Furthermore, bilateral muscle involvement was commonly seen in patients with no–no tremor, especially for OCI (12/23) and sternocleidomastoid muscle (11/23). A positive correlation was found between the radioactivity and thickness of OCI ( r = 0.330, P < 0.001). The total agreement rate between SPECT/CT and electromyography in the diagnosis of OCI excitement was 94.0%, with kappa value = 0.866 ( P < 0.001). OCI plays a critical role in torticaput and no–no tremor. SPECT/CT could be a practical tool to help clinicians detect abnormally excited OCI.