Caudal Epidural Injection

The sacrum consists of five embryonic fused vertebrae. The coccyx consists of three to five rudimentary vertebral bones and is triangular, with the base attached to the sacrum. The superior articular base attaches to the apex of the V -shaped sacrum at the sacrococcygeal joint, bounded by the sacrococcygeal ligament, which extends dorsally in the midline to cover the sacral hiatus. The sacral hiatus is a natural defect in dorsal aspect of S5 vertebrae, where it meets the S4 vertebra. It is bordered by the sacral cornu laterally and the floor is made of the vertebral body of S5. It contains the coccygeal nerve and the filum terminale. The sacral canal contains the epidural venous plexus down to the level of S4 and some epidural fat. The termination of the thecal sac varies depending on age, and varies between the lower border of the S1 foramen in adults and the S3 foramen in children. This is of significance when judging the optimum placement of the epidural needle tip and avoiding dural puncture.

Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. The corticosteroids regulate a wide range of physiological systems including stress response, immune response, carbohydrate metabolism, protein catabolism, and electrolyte levels. They consist of two types, glucocorticoids and mineralocorticoids. Glucocorticoids such as cortisol control carbohydrate, fat, and protein metabolism, and are anti-inflammatory by preventing phospholipid release and decreasing eosinophil action. Mineralocorticoids, for example, aldosterone, control electrolyte and water levels. It is the glucocorticoids that are used for epidural injections and are routinely described as epidural steroids.

Steroids classically work by the abolition of the rate-limiting step by the enzyme PLA2 to liberate arachidonic acid from cell membranes. Arachidonic acid is then involved in the up-regulation of cyclo-oxygenase (aborted by non-steroidal anti-inflammatory drugs) and lipoxygenase enzyme production. These then increase levels of the hyperalgesic prostaglandins, thromboxanes, and leukotrienes, which are associated with the causation of inflammation and pain. Steroids are also thought to have other actions apart from its effects on the inflammatory cascade. Methylprednisolone has been shown to suppress the transmission in thin unmyelinated C-fibres while not affecting myelinated Aβ fibres. It is thought that this effect is via a direct membrane stabilizing effect as opposed to an indirect action via mediators.

These direct and indirect actions lead to a decrease in intraneural oedema and venous congestion thereby reducing ischaemia and improving pain.

Approaches available to access the lumbar epidural space are the caudal, interlaminar (IL) and transforaminal (TF). Even though the caudal epidural injection technique initially enjoyed significant popularity due to the ease of the technique and low incidence of inadvertent dural puncture, it suffered numerous drawbacks secondary to the perception of the necessity of injecting a substantial volume of fluid to reach the lumbar nerve roots, lack of agreement as to the exact volume of injection, variation in the anatomy, inaccurate placement of needles in substantial proportion of patients (more than 30% even in experienced hands), and inappropriate or uncontrolled spread of anaesthetic solution. However, due to the ease of access and low complication rate, there has been renewed interest in the use of the caudal approach to the epidural space in pain management for administration of local anaesthetics, corticosteroids, adhesiolysis, hypertonic saline neurolysis, and spinal endoscopy.

The advantages of caudal entry into the epidural space include relative ease of entry and minimal risk of inadvertent dural puncture. Other advantages include the fact that caudal epidural in steroid injection is the most studied of all three approaches available to access the lumbar epidural space, namely the interlaminar, caudal, and transforaminal. The caudal space allows access to the epidural space in difficult cases such as post lumbar laminectomy, as well as the ability to introduce a fiberoptic endoscope in to the epidural space through the sacral hiatus.

Other advantages of caudal epidural injections include their reported use in anticoagulated patients or patients who suffer with coagulopathy, with increased safety compared to other interlaminar or transforaminal injections. They also have been shown to be advantageous in post spinal surgery patients; as well as in patients suffering with either chemical or compressive radiculitis at the S1 nerve root. In addition, uncontrolled and anecdotal case reports show that caudal epidural steroid injection may be effective in management of pain related to pelvis, rectum and perineum.

The major disadvantages of caudal entry of the epidural space through the sacral hiatus include the necessity of injection of a substantial volume of fluid to reach higher levels and unrecognized placement of the needle outside the epidural space in a substantial number of cases. Inaccurate placement of the needle has been reported in 9% to 38% of cases when evaluated under fluoroscopic control.

Intravascular uptake was also noted in a significant number of patients. Blood may not always be observed returning from the needle and dependence on negative aspiration is known to be unreliable in the caudal space.

The other disadvantages of caudal epidural steroid injections include higher risk of infection, unreliable spread of local anaesthetic, patchy blocks, and suitability limited only to lower lumbar and sacral pathology.

Infection is a risk in this procedure, as with most interventional treatments of the spine. Full aseptic preparations should be adopted, with special precautions in the immune compromised.

Intrathecal injection can occur despite a reduced risk of the same and may be associated with prolonged and/or high subarachnoid block.

Nerve injury may occur, but is rare.

Bleeding complications
The most important risk factor for bleeding is coagulopathy, either primary or pharmacological. Anticoagulants like warfarin and antiplatelet drugs such as clopidogrel, rivaroxaban and digabatran are contraindications to epidural injections. On the other hand, NSAIDs, including aspirin, do not appear to appreciably increase the risk of epidural bleeding. The discontinuation of medications in patients receiving antithrombotic or thrombolytic therapies before epidural steroid injections is not devoid of risks and the decision must be made after careful consideration of the risks and benefits, in consultation with a specialist. Because of its location at the distal end of the spinal column, its shallow depth (which enables compression) and the fact that it can easily be accessed with a small gauge needle, the caudal approach might be considered when an epidural steroid injection is strongly indicated and the risk of discontinuing warfarin or antiplatelet therapy is high.

Steroid-related side effects
Complications of corticosteroid administration include suppression of pituitary-adrenal axis, hypercorticism, Cushing’s syndrome, osteoporosis, avascular necrosis of bone, steroid myopathy, epidural lipomatosis, weight gain, fluid retention and hyperglycemia.

Anatomical variations of the sacrum and the contents within the sacral canal pose a challenge during caudal epidural steroid injections. Variations in sacral anatomy have been reported to be as high as 10% and resulted in misplaced needles in up to 30% of caudal epidural injections, even when performed by experienced physicians without imaging guidance.

Inadvertent intravascular injection has been reported to range from 2.5% to 9% and negative needle aspiration for blood has been shown to be neither sensitive nor specific. Intravascular injection is also more likely in elderly patients as the epidural venous plexus may continue inferior to the S4 segment in these patients. This provides the rational for the need of performing caudal epidural injections with real-time imaging guidance, in order to maximize the outcome and minimize the complications.