Sciatica is generally due to a pressing on one of the lumbar or sacral nerve roots.
USMLE® Step 1 style questions USMLE
A 40-year-old man comes to the office because of pain in the right lower back with radiation down the back of the right leg into the foot. He says this started a few weeks ago and has progressively worsened. He denies any previous trauma. His past medical history is significant for hypertension which was diagnosed at his last visit two months ago and is well-controlled on hydrochlorothiazide. Physical examination shows the patient does not have point tenderness in the vertebrae or in the right hip. He has minimally decreased sensation along the back of the right thigh. Muscle strength is 5/5 and symmetric. Which of the following is the most likely cause of his symptoms?
Sciatica is a condition where there’s pain that starts in the lower back and goes down the leg.
Since the pain is associated with injury or compression of the sciatic nerve and follows the path of the sciatic nerve, it makes sense to name it sciatica.
The sciatic nerve is the longest and widest nerve in the body.
It’s formed by the spinal nerves L4, L5, and S1-3, which leave the spinal canal through intervertebral foramen, an opening located between the vertebrae and behind the intervertebral discs.
These nerves travel to the area in front of the sacrum, and join to make the sacral plexus.
All the nerves in the plexus, except S3 are split into two divisions: anterior and posterior.
Anterior divisions of the L4, L5, S1, S2 and the entire S3 nerve create the tibial nerve, while posterior divisions of the L4, L5, S1 and S2 form the common fibular nerve.
These two nerves are bound together by connective tissue and make up the sciatic nerve.
The sciatic nerve then passes beneath the piriformis muscle, through the greater sciatic foramen which is an opening formed by the pelvic bone, sacrospinous, and sacrotuberous ligaments.
It then travels down the back of the thigh to the back of the knee where it splits into the tibial and common fibular nerves.
The sciatic nerve innervates the muscles in the back of the thigh.
Tibial nerve innervates the muscles of the posterior compartment of the leg and intrinsic flexors of the foot.
Common fibular nerve is in charge of the muscles in the anterior and lateral compartments of the leg and intrinsic extensors of the foot.
Now, each spinal nerve is in charge of the sensation of a specific area of the skin, called a dermatome.
Dermatomes of the spinal nerves of the sacral plexus cover almost the entire surface of the thigh, leg and foot.
L4 covers the medial side of the leg, L5 the lateral side.
S1 covers part of the dorsum and the entire sole of the foot, S2 the back of the leg, while S3 covers the back of the thigh.
Skin sensations like touch, temperature, pain, and pressure are carried to the spinal cord and then to the brain where we register the sensations.
Sciatica occurs when there’s irritation to any part of the sciatic nerve or the spinal nerves that form it.
The causes of sciatica can be divided into two groups - spinal, and non-spinal.
Spinal causes are related to the spinal column.
The most common one is intervertebral disc herniation.
The intervertebral discs lie between vertebrae, and act as shock absorbers.
Each disk is made of two parts.
The outer fibrous ring called annulus fibrosus, and the inner gel-like pulp - nucleus pulposus.
Poor posture, traumas, physical activity and strong rotational movement can cause herniations where the disk bulges out in one direction.
If it bulges out towards the center of the spinal cord, it could compress multiple nerve roots on both sides, or laterally, compressing one nerve roots on one side.
In some cases, the nucleus macrophages attack the nucleus pulposus and start secreting inflammatory cytokines - tumor necrosis factor alpha and interleukins 1 and 6.
The inflammation and swelling which result from this reaction compresses the nerves even further.
Another cause of nerve compression is spinal stenosis which is narrowing of the spinal canal or intervertebral foramen.