VCP 411: Medicine Section

Approach to the atlanto-occipital site in sheep

APPROACH TO THE ATLANTO-ACCIPITAL SITE IN SHEEP

•  A 2.5 cm² area of skin over the midline occipital region of the skull is clipped and surgically prepared.
• Local anaesthetic is injected subcutaneously at the centre of the triangle which joins the caudal aspect of the occipital protruberance and the wings of the atlas (CI).
• Size of needle required
  • Lambs < 30 kg 1 inch 20 gauge
  • Lambs and ewes > 30 kg 1 ½ inch 19 gauge
  • Rams 2 inch 19 gauge
• The selection of the site described may result in the point of the needle striking the caudal aspect of the occipital bone at a depth of about 1 cm. if this occurs, the needle should be almost withdrawn and the hub directed more rostrally with the point of the needle redirected more caudally.
• Slight resentment is occasionally encountered when the needle penetrates the meninges. Within two to five seconds CSF appears at the needle hub and 1 to 2 ml can be slowly withdrawn by gently syringe aspiration and transferred into an EDTA Vacutainer.
• It is important to break the seal of the syringe before fixing it to the needle hub and to hold the needle hub firmly anchored against the skin in order to avoid any jerking movements when withdrawing the syringe plunger during sampling. Any movement may dislodge the needle point from the subarachnoid space.