Wasp waist sign: Congenital vertebral fusion

Congenital vertebral body fusion is known as a block vertebra. This usually occurs in the cervical or lumbar region. One of the important factors is distinguishing congenital fusion from acquired fusion. In congenital fusion there is narrowing of the dimensions of the bone at the level of the intervertebral disc – called the ‘waist’ or ‘wasp waist’ sign. In addition, the height of the disc and vertebral body complex is the same as normal for that level, as opposed to acquired fusion in which the height of the disc is lost. A block vertebra is a segmentation anomaly of the vertebrae. There are several classification systems for vertebral segmentation defects. One of the most established is the Klippel-Feil syndrome – which is a rare skeletal anomaly characterised by the fusion of two or more cervical vertebrae. This results in the clinical triad of a short neck, limited range of neck motion and low hairline at back of the head. The fused section of spine is characterised by the ‘wasp waist’ sign. The term wasp waist originates as a fashion term and refers to the waistline created by the wearing of a corset, itself a reference to the narrow waist between the thorax and abdomen of a wasp. Klippel-Feil anomaly was originally described in 1912. Since that time there has been recognition of 4 subtypes with different patterns of cervical fusion and associated fusions at other levels and non-spinal anomalies. There has also been recognition of differing inheritance patterns.