What's the Difference Between Epidural and Spinal Anesthesia?
Basic Types of Anesthesia
General anesthesia, which means you are unconscious for your surgery
Regional anesthesia, which numbs a large region of your body by placing the numbing medicine at a large nerve or group of nerves. Epidurals and spinals numb the lower half of the body. Nerve blocks numb an area supplied by the numbed nerve. For example, a femoral nerve block, placed at the top of the thigh, numbs the (front and sides of the) leg down to the knee.
Local anesthesia is placed directly under the skin at the spot to be numbed. For example, to remove a mole, the numbing medicine is injected around the mole.
Neuraxial Anesthesia - Spinals and Epidurals
Spinal and epidural anesthesia fall into the category of neuraxial anesthesia. Neuraxial refers to the spinal column and related structures.
Broadly, epidurals and spinals may be categorized as regional anesthetics, as well, since they do numb a region of the body by targeting nerves that supply that area.
By the mid-1880s, cocaine was known to be a useful local anesthetic. After some experimentation, cocaine was injected into the spinal fluid for anesthesia in 1885. In 1899, spinal anesthesia was used in the United States for the first time. Early success was intemittent, probably in part because the medicine sometimes didn't make it to the spinal space. An epidural, with less numbing, was achieved instead.
Over the next decade, advances were made in the understanding of spinal anatomy. The importance of dural puncture was understood and spinals became more popular and successful.
So what was and is the difference between spinal and epidural anesthesia? And, how do we use this information to provide anesthesia by both routes for different situations?
The answer lies in the anatomy of the spinal column and the relationship of the epidural and subdural spaces.
Anatomy of the Spinal Column
When you feel the bones in the back, you are feeling structures called the spinous processes. These protrude out from the body of the vertebrae. The photo shows a side view of the vertebral bodies. Because this is a side view, the spinous processes that you can feel as you run your finger down your back are shown on the right.
Layers of the Spinal Column
In performing spinal and epidural anesthesia, an understanding and ability to visualize the spinal anatomy is essential. As we pass the needle from the outside to the desired layer, we do much of it by "feeling" these layers with the needle. With experience, we know how skin feels as opposed to ligaments, for example. Here are the layers we pass.
Skin covers the spinous processes and there is a variable amount of fat and subcutaneous (under the skin) tissue underneath. Beneath the subcutaneous tissue is a ligament that runs up and down from the top to bottom of the spinal column, the supraspinous ligament. Next, the interspinous ligament binds the spinal bones together while allowing flexibility.
The ligamentum flavum is the last tough layer encountered before the epidural space. This ligament is very tough and we can feel this ligament "grab" he needle, in many cases.
The Loss of Resistance Technique of Epidural Anesthesia
When performing epidural anesthesia, we use what is called "loss of resistance" technique in most cases.
- As described above, the ligamentum flavum is tough and resistant to needle passage.
- When we reach that layer, we put a syringe with (usually) saline on the end of the needle.
- We then apply pressure to the hub of the syringe.
- When the needle is in the ligament, the hub resists or bounces back against our applied pressure.
- As the needle is very slowly advanced in millimeter-scale measures, we continue to apply pressure.
- As the needle exits the ligamentum flavum and enters the epidural space, there is a noticeable loss of resistance on the hub of the needle and the saline is easily injected.
This feel of loss of resistance is how we know we are in the epidural space. We also confirm that we aren't past the epidural space into the spinal (subdural) space by checking to make sure CSF doesn't flow back out of our needle.
The Epidural Space
The epidural space between the ligamentum flavum and the dura (see below) is only a few millimeters thick. This space contains
- Epidural veins
- Connective tissue
- Nerves exiting the spinal cord
It does not contain any spinal fluid.
Again, this space is only a few millimeters, so if it seems like the anesthesiologist is taking his or her time finding it, they probably are. It is a delicate procedure to place an epidural and find that tiny space in there!
The next layer in, is the dura covering the spinal fluid around the spinal cord. We want to enter that space for a spinal, but not during an epidural. Piercing the dura and entering the spinal space when desiring epidural anesthesia causes unwanted complications and side effects.
When the epidural space is found, a catheter (thin tube) can be threaded through the epidural needle to the epidural space. The needle is then removed and the catheter can be used to deliver more medication to the epidural space if needed. This is the technique most often used during labor and delivery.
Model of Spinal Cord Meninges
The Coverings of the Spinal Cord
Just in front of the epidural space (when approached from the back for these techniques) is the dura.
The spinal cord is said to have 3 layers of coverings or membranes. The pia mater is a thin layer directly over the cord. The arachnoid membrane is a web-like (thus, the name) covering outside of that. The outermost layer is the dura.
The dura mater is a thick, tough covering that surrounds the spinal cord. Inside the dura is also the CSF--the cerebrospinal fluid (spinal fluid for short). The CSF covers and protects the brain and spinal cord. It is also found circulating in the brain's "hollow" spaces, called the ventricles.
As indicated, when spinal anesthesia is desired, the dura is intentionally punctured and medication is injected directly into the cerebrospinal fluid.
Because spinals don't require finding the loss of resistance at the epidural space, they are actually easier to perform in most cases. The dura is punctured, which can often be felt as a distinct "pop," and CSF flows back in the spinal needle. This is confirmation that the correct space has been entered and the medication is placed.
Spinals are usually done as "one-shot" techniques. Catheters aren't generally used with spinal anesthetics.
The dose of medications usually is about 10 times lower for spinal anesthetics since it is placed directly into the fluid around the spinal cord and nerves.
People often worry that medicine is being put "into the spinal cord." The spinal cord runs down the back and ends as a distinct cord at a level above where the spinal anesthesia is placed. At the part of the low back where spinals are done, the cord has ended and threads of nerves extend down into that level. The nerves may be irritated occasionally by the needle or the medicine, but the spinal cord will not be pierced since there is no cord there.
CSE - Combined Spinal Epidural
There is also a technique called a CSE or combined spinal-epidural anesthesia.
This anesthetic is just as the name implies—medication that will act immediately is placed into the spinal fluid. Then a catheter is put into the epidural space so that medication can be run continuously or added as needed.
For this technique, the loss of resistance method to find the epidural space is used. When the epidural needle (usually called Tuohy needle) is in place, a smaller, skinnier spinal needle is placed through the epidural needle. The spinal needle goes in and pierces the dura. When CSF is found, medicine is placed through the spinal needle into the fluid. The spinal needle is removed and a catheter can be threaded through the epidural needle that is still in place. After the catheter is placed, the epidural needle is also removed.