Kawalilak LT, Tucker RL, Greene SA.
Introduction/Purpose: Epidural anesthesia was first administered to veterinary patients as early as 1885 and continues to be widely used in practice today. The volumes used in epidurals were established as early as 1968 and vary according to the extent of local anesthesia required: 0.14mLlkg for the tail and perineal area and 0.2mLlkg for areas up to the thoracolumbar junction. However, these volumes are based on clinician judgment and clinical responses, such as loss of weight support, flexor reflex, and response to nociceptive stimuli , rather than actual measurement of the volume of the epidural space. Studies into the migration of known epidural volumes have mainly been limited to the use of methylene blue dye in live animals that are then euthanized and necropsied. This study aims to investigate if computed tomography (CT) is an accurate modality for visualizing solutions in the epidural space. It also aims to confirm the long held belief that a 0.2mLlkg volume injected into the lumbosacral epidural space reaches the thoracolumbar junction in the majority (>80%) of dogs.
Methods: Ten clinically normal, adult, non-pregnant, mixed-breed dogs euthanized for reasons other than the current study were obtained within five minutes of euthanasia. A lumbosacral epidural was performed using a 7.5cm, 22-gauge spinal needle and an injection of iodinated contrast (iohexol, volume of 0.2mLlkg body weight) was carried out over a period of thirty seconds. Injection into the epidural space was confirmed by a loss of resistance test. A CT scan of the thoracic to the sacral vertebrae was performed using 3mm transverse, contiguous slices. The cranial extent of the contrast solution column was measured in number of vertebral bodies on multiple image reconstructs by one observer (LTK) and confirmed by a board-certified radiologist (RLT).
Results: The 0.2mLlkg dose of radio-opaque contrast travelled to the thoracolumbar junction (seven or more vertebral bodies) in eight of the ten dogs studied. In two dogs, the contrast only travelled to the junction of the first and second lumbar vertebrae , or roughly six vertebral bodies. The contrast column was well-visualized in all planes. The contrast column travelled primarily in the dorsal epidural space in all cadavers , though contrast was also visualized in the lateral and ventral epidural spaces.
Discussion/Conclusion: This study confirms that CT in combination with iodinated contrast provides an excellent modality for observing the spread of a solution injected into the lumbosacral epidural space. This study also confirms the long standing belief that a 0.2mLlkg volume injected into the epidural space reaches the thoracolumbar junction in the majority of dogs and has the potential to provide both anesthesia and analgesia to areas caudal to this landmark.