Here is something I've found:
Clin Orthop Relat Res. 1992 Feb;(275):258-62.
Delayed recognition of a vascular complication, carotid artery aneurysm, 60 years after operation for muscular torticollis. A case report.
Korovessis P, Michalopoulos B, Vassilakos P.
Orthopaedic Department, General Hospital Agios Andreas, Patras, Greece.
An extremely rare vascular complication, carotid artery aneurysm, developed 60 years after a torticollis operation. The patient's internal jugular vein was completely obstructed proximal to the subclavian vein. There was an associated incomplete obstruction of the ipsilateral common carotid artery on the right aspect of the neck--the site where resection of the whole sternocleidomastoid muscle had been performed when she was one year old. The patient had audible bruits over the right common carotid at the base of her neck but no signs of occlusive cerebrovascular disease. The diagnosis was made mainly with use of technetium radionuclide angiography. To the authors' knowledge, this is the first such report in the English or German literature.
PMID: 1310455 [PubMed - indexed for MEDLINE]
IJV obstructed proximal to the subclavian vein - I haven't got access to the full document, but what sort of obstruction was this? Could it have been obstructed from birth, undiscovered at the time of the treatment? This is where many CCVSI obstructions are found.
An end-arterial branch of the superior thyroid artery supplies the middle part of the sternocleidomastoid muscle. Obliteration of this end artery may be responsible for the development of muscle fibrosis. As an alternative, primary trauma that temporarily and acutely obstructs the veins may lead to intravascular clotting in the obstructed venous tree. In infants, this clotting is evidenced by the development of a sternocleidomastoid mass, which eventually disappears and is replaced by fibrous tissue.
I can't find out which veins they're talking about here. Could an obstructed venous tree, not as a result of trauma but of inherited internal jugular obstructions, lead to the development of a sternocleidomastoid mass? Either way, intravascular clotting doesn't sound good either.