Ipsilateral congenital absence of the sternocleidomastoid and trapezius muscles

This is a case report of the ipsilateral absence of the sternocleidomastoid (SCM) and trapezius muscles in two cadavers. The ipsilateral congenital absence of the SCM and trapezius muscles has only been reported once by Benassi and Trabucchi in 1963. Our cases involved a 79‐year‐old woman and a 76‐year‐old woman, both of whom died of cancer. In both instances, the ipsilateral absence of the SCM and trapezius muscles was on the left side and there were no visible signs of trauma or previous surgery. The left‐sided absence of these two muscles was accompanied by the loss of innervation by the spinal accessory nerve and arterial supply; whereas the contralateral SCM and trapezius muscles were normal in their attachments, innervation, and arterial supply. Intracranial examination of one cadaver revealed the bilateral presence of the spinal root of the accessory nerve even though the left SCM and trapezius muscles were absent. Connections of the spinal accessory nerve to the posterior rami of the second through fourth cervical nerves have been previously described. While we were unable to verify this due to previous student dissection, these connections, if present, would complement the motor supply to the upper intrinsic back muscles, thus explaining the left nerve’s existence in the absence of the SCM and trapezius muscles. We were unable to confirm the intracranial presence or absence of the left spinal accessory nerve in the other cadaver due to previous student dissection which may have damaged the structure. In summary, these observations support the congenital absence of the ipsilateral SCM and trapezius muscles, an absence that has been reported only once in the literature.