The Side effects
In addition to blocking the nerves which carry pain, local anaesthetic drugs in the epidural space will block other types of nerves as well, in a dose-dependent manner. Depending on the drug and dose used, the effects may last only a few minutes or up to several hours. This results in three main effects:
Loss of other modalities of sensation (including touch, and proprioception)
Loss of muscle power
Loss of function of the sympathetic nervous system, which controls blood pressure
Pain nerves are most sensitive to the effects of the epidural. This means that a good epidural can provide analgesia without affecting muscle power or other types of sensation. The larger the dose used, the more likely it is that the side-effects will be problematic.
For example, a labouring woman may have an epidural running during labour which is providing good analgesia without impairing her ability to move around in bed. She requires a Caesarean section, and is given a large dose of epidural bupivacaine. After a few minutes, she can no longer move her legs, or feel her abdomen. Her blood pressure is noted to be lower and she is given an intravenous infusion of ephedrine or phenylephrine to compensate. During the operation, she feels no pain.
Very large doses of epidural anaesthetic can cause paralysis of the intercostal muscles and diaphragm (which are responsible for breathing), and complete loss of sympathetic function, even to the heart itself, causing a profound drop in heart rate and blood pressure. This requires emergency treatment, and usually general anaesthesia. This happens because the block height is too high ("high block") and the epidural is blocking the heart's own sympathetic nerves, as well as the phrenic nerves, which supply the diaphragm.
It is considered safe practice for all patients with epidurals to be confined to bed to prevent the risk of falls.
The loss of the sensation of needing to urinate may require the placement of a urinary catheter for the duration of the epidural.
Opioid drugs in the epidural space are very safe (as well as effective). However, very large doses may cause troublesome itch, and rarely, delayed respiratory depression.
Back pain is occasionally reported after epidural insertion, and the epidural may be blamed. However, there is no good evidence linking epidural insertion to back pain. In women who have recently given birth, the incidence of back pain in those who had epidurals is not different from those who did not.
Occasionally, spurious studies appear which suggest that epidural analgesia during childbirth is responsible for behavioural problems in the baby, such as delay in establishment of breast feeding. The evidence for such conclusions is extremely poor.