The sacral hiatus is the caudal opening of the sacral canal, formed by the failure of fusion of the S4 and S5 laminae. It is covered by the sacrococcygeal membrane and flanked by the sacral cornua (the rudimentary articular processes of S5). The sacral hiatus is the entry point for caudal epidural block, through which local anaesthetic is injected into the sacral epidural space to anesthetise the lower sacral and lumbar nerve roots.
| Articulations |
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|---|---|
| Landmarks |
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| Ossification | Results from failure of the laminae of S5 (and sometimes S4) to fuse in the midline during sacral ossification. |
The sacral hiatus is the target for caudal epidural block, used for paediatric postoperative analgesia, treatment of caudal radiculopathy, and sacral nerve root blocks. The cornua are palpated as the bony landmarks flanking the hiatus in the midline just above the coccyx. In approximately 6% of individuals the hiatus is completely absent due to complete sacral laminar fusion. Fluoroscopic or ultrasound guidance confirms needle placement within the sacral canal. The sacral hiatus is also relevant in caudal anaesthesia for haemorrhoidectomy and perineal surgery.
With the person lying face down or on their side, find the tip of the coccyx in the gluteal cleft and move about 5 cm upward. Feel for two small bony bumps side by side (the sacral cornua); the soft dip between them is the sacral hiatus, the entry point for a caudal epidural. The two PSIS and the hiatus roughly form an equilateral triangle.
Anatomical variation with complete fusion of S4-S5 laminae (absent hiatus) or a narrow hiatus produces failed caudal epidural block when needle access to the sacral canal is obstructed; ultrasound identification of the sacral hiatus before needle insertion identifies this variation and guides decision to proceed with alternative lumbar epidural approach.
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