The purpose of this study was to report the sonographic findings of neonatal coccygeal abscess, previously not described.

Eighteen neonates (5-18 days old) presented with swelling in the coccygeal area and by either open drainage (n=13) or follow-up after antibiotic therapy (n=5), this was diagnosed as coccygeal abscess. We retrospectively reviewed the size, shape, location, echo pattern and marginal characteristics of the abscesses, as seen on sonography, as well as their intradural content and relationship with the spine. Additional MR images (n=5) were separately reviewed.

Mean longest diameter of the abscesses was 1.5cm (range, 0.8-2.3); they were oval or round and located in the subcutaneous fat layer. Echogenicity compared with surrounding fat varied : in nine patients it was isoechoic, and in nine, hypoechoic. Internal echogenicity was homogenous in 14 patients and heterogeneous in four, and in seven cases, the margin of the abscess was well demarcated. Intradural structure and bony spines were normal, and the possibility of spinal dysraphism, could thus be excluded. All cases except one were correctly diagnosed by sonography and clinical findings å on sonography, the echogenicity of one lesion was exactly the same as that of lipoma, and it was thus misdiagnosed. In cases where sonography revealed an isoechoic mass, the use of MR excluded the possibility of lipoma. Three of five cases showed marginal or diffuse enhancement on contrast enhanced MR images.

Coccygeal absesses were confined to the subcutaneous fat layer and were either iso- or hypoechoic compared with surrounding fat. In neonates, abscess formation in the coccygeal area is possible, and coccygeal abscess should therefore be included in the differentiation of coccygeal masses.