Spinal anesthesia is a technique that numbs the lower half of your body by injecting a small amount of anesthetic into the fluid-filled space surrounding your spinal cord. It’s one of the most common alternatives to general anesthesia for surgeries on the legs, hips, abdomen, and pelvic area, and it allows you to stay awake while feeling nothing below the injection site.
How Spinal Anesthesia Works
Your spinal cord is surrounded by a protective membrane and bathed in cerebrospinal fluid, the clear liquid that cushions both the brain and spinal cord. This fluid sits in a space called the subarachnoid (or intrathecal) space. During spinal anesthesia, a thin needle is guided between the bones of your lower back and into that space. The anesthesiologist confirms correct placement when cerebrospinal fluid flows freely from the needle, then injects a small dose of numbing medication directly into the fluid.
The medication spreads through the cerebrospinal fluid and blocks nerve signals traveling from the lower body to the brain. This shuts down pain sensation, touch, and muscle movement in a predictable zone from roughly the waist or chest down, depending on the surgery. Only a tiny volume of medication is needed, often less than 2 milliliters, because it’s delivered right to the nerve roots rather than absorbed through tissue.
When It’s Used
Spinal anesthesia is a standard choice for procedures on the lower body. That includes cesarean sections, hip and knee replacements, hernia repairs, bladder and prostate surgeries, and operations on the legs and feet. It’s often preferred over general anesthesia for these surgeries because it avoids the risks of putting someone fully to sleep, such as airway complications, and it typically causes less nausea afterward.
What the Procedure Feels Like
You’ll either sit on the edge of the bed with your back rounded or lie on your side in a curled position. Both postures open up the spaces between the bones of your spine, making needle placement easier. In the sitting position, you may be asked to let one leg hang off the side of the bed and curl your back forward like a “C.” If you’re lying down, a pillow is placed between your knees and along your back for support.
Before the spinal needle goes in, you’ll get a small numbing injection in the skin of your lower back, similar to what a dentist gives before dental work. When the spinal needle is placed, you may feel pressure or a brief sensation of pushing, but it shouldn’t be painful. The injection itself takes only seconds.
Within minutes, your legs will start to feel warm, tingly, and heavy. The tingling gives way to full numbness that gradually moves upward to the level your surgeon needs. Most people describe the transition as their legs feeling very heavy and then simply “disappearing” from their awareness. You’ll remain fully conscious throughout the surgery, though mild sedation is often given to help you relax. A drape blocks your view of the surgical site.
How Quickly It Works and How Long It Lasts
Spinal anesthesia takes effect faster than almost any other regional technique. Numbness typically begins within one to five minutes of injection, with full surgical-level block established shortly after. The duration depends on which anesthetic is used. Shorter-acting agents wear off in roughly two and a half to three hours, while longer-acting ones can last three to four hours or more. Your anesthesiologist selects the drug based on how long the surgery is expected to take.
How Sensation Returns Afterward
As the block wears off, feeling and movement return gradually from top to bottom. You’ll first notice sensation coming back in your upper thighs and hips, then progressing down toward your feet. Motor function, your ability to move and lift your legs, returns on a similar timeline. In studies of knee replacement patients, full sensation and movement returned within roughly two and a half to three and a half hours, depending on the medication used. Shorter-acting agents brought full recovery about 45 to 50 minutes faster than longer-acting ones.
The bladder muscle responsible for urination is one of the last to recover. This means you may not be able to urinate normally for a few hours after surgery, and a catheter is sometimes used in the meantime. You won’t be allowed to stand or walk until you can move your legs well and your blood pressure is stable, which the nursing team will test before letting you up.
Common Side Effects
The most frequent side effect is a drop in blood pressure. This happens because the nerves that control blood vessel tone in the lower body are temporarily blocked, causing blood to pool in the legs. Studies report that anywhere from 25% to 75% of patients experience some degree of low blood pressure after a spinal, with most large studies putting the figure between 33% and 58%. The anesthesia team monitors blood pressure continuously and treats drops quickly with fluids or medications given through your IV, so most episodes are brief and well-controlled.
A slower heart rate can also occur for similar reasons, since some of the nerves affected help regulate heart rhythm. Nausea, shivering, and itching (if certain pain-relief additives are used) are other common but temporary effects.
Post-Dural Puncture Headache
Because the needle punctures the membrane surrounding the spinal fluid, a small leak of that fluid can occasionally cause a distinctive headache afterward. This “spinal headache” is typically worse when sitting or standing upright and improves when lying flat. The risk depends on needle design and size. Modern pencil-point needles, which spread the membrane fibers apart rather than cutting them, have reduced the incidence significantly. In the highest-risk group (pregnant women), spinal headache occurs in about 0.8% to 5% of cases. For other patients, rates tend to be at the lower end of that range. Most spinal headaches resolve on their own within a few days, though a simple procedure to seal the leak can be done if the headache persists.
How Spinal Differs From Epidural
The two techniques are easy to confuse because both involve a needle in the lower back, but they work differently in important ways.
- Where the medication goes: A spinal injection delivers medication directly into the cerebrospinal fluid. An epidural places medication into the epidural space, which sits just outside the membrane that contains that fluid. The epidural space is a few millimeters closer to the skin.
- Dose and speed: Because spinal medication is injected right into the fluid bathing the nerves, it requires a much smaller volume (often under 2 mL versus 10 mL or more for an epidural) and works within minutes. Epidurals take longer to reach full effect, usually 15 to 20 minutes.
- Duration and flexibility: A spinal is typically a single injection that lasts a set period. An epidural involves threading a thin catheter into the space so medication can be topped up continuously or repeatedly. This makes epidurals better suited for labor pain, where relief may be needed for many hours, and for post-surgical pain control that extends well beyond the operating room.
In some cases, the two are combined. A “combined spinal-epidural” uses the spinal injection for rapid surgical numbness and leaves an epidural catheter in place for extended pain relief afterward.
Who Should Not Have Spinal Anesthesia
Spinal anesthesia is not an option for everyone. It’s generally avoided in patients with severe bleeding disorders or those on blood-thinning medications that can’t be safely paused, because a needle near the spinal cord carries a risk of bleeding in that space. Active infection at the injection site, significant spinal deformities that make needle placement unsafe, and certain heart valve conditions that can’t tolerate a sudden blood pressure drop are other reasons it may be ruled out. Your anesthesiologist reviews your medical history and medications beforehand to determine whether a spinal is appropriate for you.