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Paediatric Imaging

Sacral agenesis

absence of the sacrum, a part of a broad spectrum of findings included within the term caudal regression syndrome. These range in severity from sirenomyelia to problems such as anorectal malformation. Embryologically the sacrum, conus, filum terminale and sacral nerve roots are formed by a process of canalization and retrogressive differentiation which is usually complete by the 4th week of fetal life. The various manifestations of caudal regression result from disturbance of this normal process during the third week of fetal development. This may reflect a toxic, infective or ischaemic insult and there is a significant association with maternal diabetes.

Sacral agenesis (Fig.1) may be classified radiographically as:

  • type I - partial or complete unilateral sacral agenesis

  • type II - bilateral symmetrical partial sacral defect, normal sacroiliac articulation

  • type III - total sacral absence, ilium articulates with the lowest vertebral body

  • type IV - total sacral absence with fused ilia.

    Symptoms related to sacral agenesis include bowel, bladder or lower extremity motor dysfunction. These symptoms are most marked with types III and IV and such patients are usually non mobile. Type I unilateral partial sacral agenesis results in a scimitar shaped sacrum and may be associated with rectal stenosis and a mass anterior to the sacrum (teratoma or meningocele) in a condition termed Currarino's triad.

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    Fig.1

    Lateral radiograph showing complete absence of the sacrum.
    Sacral agenesis, Fig.1