The spinal cord runs down the vertebral column within a membranous tube called the dural sac. This sac also contains fluid that bathes and nourishes the spinal cord. The epidural space is the space between the dural sac and the bones of the vertebral column. Nerves from the lower limbs enter the vertebral column and pierce the dural sac to reach the spinal cord. For various reasons these nerves can become irritated and cause pain in the lower limbs.
The term “epidural steroid injection” refers to the injection of a corticosteroid into the epidural space as a means of treating the pain caused by irritation of these nerves.
The chief effect of an epidural steroid injection is to reduce inflammation and thereby pain. Most patients will receive good relief for several weeks to months, with a small proportion obtaining longer lasting benefit. Some patients experience little or no pain relief, and a few patients experience side effects or an increase in pain as detailed below.
With any operation or injection procedure there are potential risks. In the case of an epidural injection of steroid this risk is extremely small.
There are potential risks with any invasive procedure. In the case of an epidural injection headaches are relatively common. Severe headaches are however, uncommon but may occur if the dural sac is punctured at the time of the injection. This is said to occur in approximately 1 in every 200 patients. In most cases the headaches subside in a few hours, but sometimes persist for a few days, very rarely for weeks.
Occasionally patients develop a hot flush or a rash if they are allergic to one of the additives in the steroid preparation. This invariably subsides within a matter of hours or days.
If the anaesthetic injected with the steroid spreads too far up the spinal canal it could cause a temporary paralysis of the muscles which help breathing. This complication is referred to as an ascending block and is very rare. If it does occur the doctor performing the injection may need to assist you with your breathing for an hour or so until the anaesthetic wears off. There have been extremely rare cases of permanent paralysis recorded after spinal injections. This is thought not to be a realistic risk, but has been reported in the literature.
With any injection through the skin, it is possible for bacteria to gain entry into the body and cause an infection. The risk of this is extremely small, less than 1 in 1,000.
Any injection into the dural sac may cause the inflammatory condition (arachnoiditis) mentioned above. This is a very rare complication and in most cases does not cause any symptoms.
With any injection into the spinal canal it is theoretically possible that the needle could damage a nerve, but the risk of this occurring is extremely small. The needle is inserted very slowly and if the lining of the nerve was touched it would cause pain in the leg. The doctor would then change the position to avoid any risk of damage to the nerve itself.
In some patients their blood pressure falls at the time the epidural injection is carried out, and it is for this reason that the anaesthetist takes the precaution of placing a needle in one of your arm veins. This is so that your blood pressure can be controlled by the administration of intravenous fluids or medication if necessary.
Epidural steroid injections can be administered by an anaesthetist in hospital or targeted under x ray guidance and administered by a radiologist or by your spinal surgeon under radiological guidance in the operating theatre.