What is an Epidural?
The term epidural typically refers to the medical procedure of placing anesthetic drugs in the epidural space.
The epidural space is a small area that surrounds the spine and is bordered by two membranes — the fibrous outer ligamentum flavum and the inner dura mater. It’s name is derived from it’s position surrounding the dura — i.e. “epi” or “on” the dura. The dura or dura mater (meaning “tough mother” in Latin), contians the spinal cord and spinal fluid forming the spinal canal.
Human spine showing the epidural space between the outer ligamentum flavim and inner dura mater
This proxity of the epidural space to the spinal cord and its spinal nerve roots is the key to epidural function. Drugs placed in the epidural space are very close to spinal nerves traveling to and from spinal cord and allow access to these nerves and the resulting anesthesia or analgesia.
Two main classes of medicines are typically used in epidurals; local anesthetics and opioids. Local anesthetics are novacaine-like drugs that block nerve transmission and provide “numbness”. Opioids are morphine-like drugs that modify pain signals and our perception of pain. These two types of medicines are often used together, and provide better results compared to using only one drug.
To deliver these drugs to the epidural space, a specially designed needle is carefully passed through the skin, between the bony spine into the epidural space. Drug mixtures can be injected once as a “single shot” or continuously infused through a small catheter, commonly known as an epidural catheter.
An epidural needle and epidural catheter.
Single shot (one time) epidural injection provide only one dose of the drug(s) and therefore last only as long as the specific medicine used. Continuous epidural infusions use a small catheter, as mentioned above, to allow for repeat or continuous infusions of medicines and therefore prolonged epidural therapy. Most epidurals used for labor pain are continuous infusions using an epidural catheter.
Epidural catheters are generally connected to an infusion pump, allowing for controlled, continuous infusions of anesthetic drugs. Commonly, this infusion is a combination of two medicines – a local anesthetic (“numbing” medicine) and a opioid (morphine like medicine). The combination of a local anesthetic and an potent opioid works synergistically to allow the use of much lower concentrations of each medicine than if they were used alone. Benefits of this combination therapy are many and include:
- Fewer and less severe side-effects of the local anesthetic, which can include:
- Weakness (motor nerve block)
- Numbness (sensory nerve block)
- Low blood pressure (autonomic nerve block)
- Local anesthetic toxicity (ringing in ears, metallic taste, lip and oral numbness, light headedness, and very rarely seizures, cardiac disturbances, coma and even death)
- Few and less severe side-effects of the opioid medicine, which can include:
- Nausea/Vomiting
- Itch
- Constipation
- Somnolence (sleepiness)
- Dysphoria (unpleasant feeling or experience)
- Respiratory depression (trouble breathing)
The epidural infusion is typically continued until pain control is no longer needed. The effects, numbness and muscle weakness, typically wear off within 2 hours of discontinuing the epidural medicines.