Anatomical variation of the insertion of scalenus anterior in adult human subjects: implications for clinical practice.

The frequency of an abnormally anterior insertion of scalenus anterior to the scalene tubercle on the superior aspect of the 1st rib was examined as a possible contributory factor to venous compression syndromes and to difficulties in cannulation of the subclavian vein. The insertion of scalenus anterior was exposed bilaterally in 21 adult cadavers and its position determined. The position of the scalene tubercle was also determined in a sample of 50 dried 1st ribs. The results were normalised for height. Three of the cadavers had position values that were above or at the upper point of the 95% confidence limits for the sample, indicating an abnormally anterior insertion. Two dried 1st ribs had values just below the lower 95% confidence limits, implying an abnormally posterior position. In the 3 cadavers with an abnormally anterior position, the subclavian artery and vein followed an abnormally anterior and inferior course with respect to the clavicle, and in 1 the subclavian vein was stenosed bilaterally at the costoclavicular angle. An abnormally anterior insertion of scalenus anterior may be a cause of venous compression syndromes in the root of the neck and give rise to difficulties when undertaking cannulation of the subclavian vein.