Paravertebral epidural injection in the L3-L4 interspace.

In the illustration, a 'single shot' epidural injection is made in the Lumbar 3-4 interspace using a paravertebral approach.

The bony and ligamentous structures of the region are shown in the lateral view image. An epidural (Tuohy ) needle is inserted in the sagittal plane, with its bevel pointing cephalad.

For the paramedian approach, local anaesthetic is infiltrated 15mm lateral to the cephalad end (upper border) of the vertebral spinous process below the selected interspace. The Tuohy needle is inserted at 90⁰ to all skin planes. The paravertebral muscles are penetrated until the bone of the vertebral lamina is encountered. The depth of needle insertion is noted. The needle is partially withdrawn and redirected cephalad and towards the midline until the previous depth is attained. Once the ligament is entered, the syringe is attached and the needle is slowly advanced while searching for loss-of-resistance.

The required dose of local anaesthetic solution is now injected, and can be seen spreading through the epidural space - which envelops the spinal cord. If a local anaesthetic is injected, the effect of the drug is 'concentrated' at the nerve roots as they emerge from the spinal cord. This in contrast to the effect of opioids, where the effect is exerted within the spinal cord. The epidural space can also be approached via the caudal route.