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An excruciating, burning, stabbing pain in the area of the lower back and pelvis, radiating down to the thigh and eventually ending at the foot with or without any signs of a wound, infection, trauma, etc., occurs in a common clinical condition known as Sciatica, which is a disease of the sciatic nerve at its origin in the sacral plexus or along its course. This can make a person completely immobile and bedridden, which is a miserable condition. Hence, the sacral plexus is an important neural network that plays an unparalleled role in our movement and locomotion. In the following words, we will try to understand about sacral plexus, its formation, course, area of supply/innervation, its branches, and ultimately diseases related to it and their management.
SacralPlexus (Plexus Sacralis)
First of all, we have to know what a plexus is in neuroanatomy. Plexus is an interconnected network formed by spinal nerves that supply the limbs. We know that the vertebral column has been divided into 5 parts on the basis of its location. In the neck or cervical region, there is the cervical part which is composed of cervical vertebras, similarly in the trunk/back region, it’s a thoracic part; in the abdomen, it’s the lumber part; in the upper pelvis, it’s sacral part, and in the lower pelvis it’s a coccygeal part. Spinal nerves, when emerging from the spinal cord, their motor and sensory routes join each other, and as a mixed nerve, they come out of the vertebral canal through intervertebral foramina. Then they divide into anterior and posterior divisions, also known as ventral and dorsal rami, respectively. Anterior divisions of spinal nerves that supply the limbs form interconnected networks (Note that dorsal rami do not take in plexus formation). In the sacral region, this network is known as the “sacral plexus,” which supplies the lower limb through its multiple branches. Similarly, there are other plexuses, i.e., “brachial plexus,” which supplies the upper limb, “coccygeal plexus,” and “cervical plexus”.
Origin of the Sacral Plexus
The sacral plexus originates from the ventral rami of L4 (only a part of it), L5, S1, S2, and S3 spinal nerves. These are also called roots of the sacral plexus. Roots L4 and L5 join to form a big nerve known as the lumbosacral trunk, which then joins sacral roots. A small part of S4 also contributes to the formation of the sacral plexus. These ventral rami further divide into anterior and posterior divisions. These divisions ultimately give rise to branches. Nerve fibers from different roots get distributed in branches such that one branch may contain fibers from several roots.
Location, Course, and Relations
The lumbosacral trunk descends down over the ala of the sacrum and joins S1. Before joining, it is separated from S1 by the superior gluteal artery and vein. S1 and S3 are separated from each other by the inferior gluteal vessels. The plexus is separated from the posterior pelvis wall through the piriformis muscle and pelvic fascia. On the ventral aspect, there are internal iliac vessels and ureters along with the sigmoid colon on the left side. These relations are important because these nerves can get injured during any surgical procedure involving the ureters or the colon.
Area of Supply and Functions
The sacral plexus provides both sensory and motor innervation to part of the pelvis, the posterior aspect of the thigh and hamstring muscles (muscles of the back of the thigh), most of the lower leg, and the foot. This includes cutaneous as well as motor muscular innervation along with autonomic innervation to vessels and joints.
Motor functions include an extension of the thigh at the hip joint, lateral rotation and abduction of the thigh, flexion at the knee joint, medial rotation of the leg in a semi-flexed position, lateral rotation of the leg in a semi-flexed position, dorsiflexion of the foot, inversion and eversion of the foot, maintenance of the arches of the foot, plantar flexion of the foot, and movements at the metatarsophalangeal joints and interphalangeal joints.
Sensory Functions include the transmission of sensory input (i.e., touch, pressure, pain, warmth, cold, etc.) via different receptors from the areas of the cutaneous innervation. It also transmits proprioceptive information via proprioceptive fibers innervating the joints, which provides information about the position, extent of flexion/extension or medial/lateral rotation, the extent of the stretch at the joints, etc. Autonomic supply to the vessels helps in maintaining the vessel’s caliber to maintain blood pressure.
Branches or nerves arising from the sacral plexus include:
The sciatic nerve is the thickest nerve in the body. Its root value is L4, L5, S1, S2, S3. Basically, it has two parts: The tibial part, which arises from anterior divisions of ventral rami of L4, L5, S1, S2, and S3, and the Common peroneal part, Which arises from dorsal divisions of ventral rami of L4, L5, S1, S2. Soon after its origin, it passes from the greater sciatic foramen and enters into the gluteal region. It gives no branches in this region, runs downward with a slight medial convexity, and enters into the back of the thigh. Here it gives branches to the hip joint (articular branches) and muscular branches to the Hamstring muscles, i.e., semimembranosus, semitendinosus, biceps femoris, and ischial head of the adductor Magnus’s muscle. Adductor Magnus has two heads of origin which differ in their innervation. The Adductor part is supplied by the obturator nerve.
Going downwards, it divides into its two terminal branches: 1) tibial nerve, which supplies muscles of the back of the leg (e.g., gastrocnemius, soleus, etc.), knee joint, skin via the sural nerve, and also some vessels. 2) Common peroneal nerve, which supplies the skin of the calf, knee joint, and biceps femoris muscle. It divides into two branches deep and superficial peroneal nerves which supply the skin of the leg and foot, and muscles of the front, medial and lateral sides of the leg (e.g., Tibialis anterior, Extensor of the foot, etc.)
Superior Gluteal Nerve
The superior gluteal nerve is actually a branch of the lumbosacral plexus with the root value of L4, L5, and S1. It has roots derived from both the lumbar and sacral plexuses. It enters the gluteal region through the greater sciatic foramen and supplies three muscles; the gluteus Medius, the gluteus minimus, and the tensor fascia latae.
Inferior Gluteal Nerve
It is a branch of the sacral plexus that enters the gluteal region via the greater sciatic foramen and supplies exclusively the Gluteus maximus muscle. Its root value is L5, S1, and S2.
Posterior Cutaneous Nerve of the Thigh
Its root value is S1, S2, and S3. Just like the sciatic nerve, it leaves the pelvic area through the greater sciatic foramen and enters into the gluteal region. It supplies the posterior two-thirds of the skin of the scrotum (in males), labium majus (in females), and posteroinferior quadrant of the gluteal region.
Nerves to Quadratus Femoris, Obturator Internus
These are the small branches of the sacral plexus that leave the pelvis through the greater sciatic foramen and enter into the gluteal region. The nerve to quadratus femoris (root value L4, L5, S1) supplies the quadratus femoris, gamellus inferior muscle, and hip joint. Neve to Obturator muscles (root value L5, S1, S2) reenter the pelvic cavity through the lesser sciatic foramen and supply the obturator internus and gamellus superior.
Its root value is S2, S3, and S4. It leaves the pelvic cavity for a while through the greater sciatic foramen and enters into the perineum. It does not supply any structure in the gluteal region. The perineum supplies the rectum via the inferior rectal nerve, scrotum, labium, and muscles (e.g., bulbospongiosus, etc.). It also supplies the skin of the penis in males via the dorsal nerve of penis and clitoris via the dorsal nerve of the clitoris.
Perforating Cutaneous Nerve
The root value for this nerve is S2, S3. The route of this nerve is different from other branches of the sacral plexus as it does not course through the greater sciatic foramen; rather, it leaves the pelvic cavity by piercing the sacrotuberous ligament. It supplies cutaneous innervation to the posteroinferior quadrant of the gluteal region.
Diseases of the Sacral Plexus, its Branches, and Their Management
There are many diseases that affect the nervous system. Some of them are systemic, which affect the nervous system as a whole, and some are localized to particular nerves. We will limit our discussion to the sacral plexus and its branches.
It is a general term referring simply to an injury to the nerves of the sacral plexus. At times, both the lumbar and sacral plexus are involved because of their close proximity, in that case, it is called lumbosacral plexopathy (LSP). There are many causes of sacral plexopathy ranging from direct injury to a nerve, accidental trauma resulting in a crush injury, birth trauma, stretch injury, tumors pressing the nerves, hepatic or vertebral dislocation, damage to the nerves evoked by radiation therapy, drugs, toxins to infections.
Patients with sacral plexopathy may present with pain in the lower back, buttock region radiating downwards, numbness in the lower limb, weakness, and sensory changes. Presentation depends upon the roots involved, extent, and severity of the injury.
Treatment includes pain suppression using pharmacological agents like Gaba analogs etc. Steroids may also be used. If the underlying cause is an infection, it is treated using antibacterial or antiviral agents. Damages caused by chronic diseases like diabetes Mellitus and chronic inflammation can be treated by treating the parent disease. If there is a tumor, bone spur, or dislocated bone pressing the nerve, it can be surgically removed/corrected to restore nerve function.
As previously described, Sciatica is a condition in which there is a shooting, stabbing, burning pain originating in the lower back and gluteal region and radiating downward in the back of the thigh, lateral side of the leg, and dorsal surface of the foot, i.e., along the course of cutaneous distribution of the sciatic nerve. The severity of the pain depends on the extent of the injury. Pain may be mild or very sharp, like an electric current. This pain is often so severe that a person becomes immobilized and bedridden. Pain worsens with movement, coughing, sneezing, sitting for a long time, and exercise. There is numbness and often inflammation of the affected leg.
The most common cause of sciatica is disc prolapse, in which material (nucleus pulposus) from the intervertebral disc radiates out and compresses one or more roots forming a sciatic nerve. Other causes may include a vertebral dislocation, bone spur on the spine, direct injury to the nerve, spinal stenosis (narrowing of the spine where sciatic nerve exits the spinal canal), or a tumor that produces pressure. Neuritis, inflammation of neurons, triggered in response to infections (i.e., bacterial, viral) or autoimmune conditions, can cause sciatica.
Treatment of sciatica includes both pharmacological and surgical. If the cause is disc prolapse, bone spur or tumor, etc., surgery may result in complete resolution of pain. In case of neuritis and other injuries, medicines like non-steroidal anti-inflammatory drugs NSAIDs (e.g., aspirin, ibuprofen, or naproxen, etc.), tricyclic anti-depressants, steroids, and muscle relaxants can be used. After pain suppression, rehabilitation and restoration of the functions of the damaged nerve can be effectively done by physiotherapy.
Foot drop is another disorder that results from injury to the spinal nerve roots supplying the dorsiflexion of the foot, sciatic nerve, common peroneal nerve, or the deep peroneal nerve, leads to paralysis of the muscles that cause dorsiflexion of the foot. As a result, the unopposed effect of planter flexors and the effect of gravity lead to plantar flexion of the foot, which is known as foot drop. This foot drop results in disturbed gait, difficulty in walking, and weakness. While walking, the patient cannot clear the ground on the affected side, which results in a waddling gait or may lead to accidental falls, which have their own complications.
Treatment follows the underlying cause. Sometimes the injury is so severe that the nerve is cut down; this cannot be reversed with either surgery or medicine. Other causes may be treatable with surgery or medicines. Physical therapy is a very good option in the rehabilitation of the patient of any disease involving nerve damage.
Gluteus Medius and minimus maintain the normal gait of a human being. In a normal person during walking, when one limb is off the ground, the two glutei of the opposite side stabilize and raise the pelvis on the side which is off the ground. If these are paralyzed due to any injury to the nerve innervating them (i.e., inferior gluteal nerve branch of sacral plexus), this stabilizing action is lost. During walking, the pelvis on the unsupported side is not raised, and it drops; this is known as positive Trendelenburg’s sign which results in a lurching gait. If the glutei of both sides are involved, this results in a waddling gait.
Sciatic Nerve Block
Nerve block means anesthetizing a nerve. A sciatic nerve block is a common procedure in routine clinical practice. This block is performed for surgeries at or below the knee joint level. This results in local anesthesia and analgesia of the leg. In extreme sciatic pains, a sciatic nerve can be performed to relieve that pain, but this is not a treatment of choice for this condition.
The sacral plexus is an interconnected network of ventral primary rami of spinal nerves L4, L5, S1, S2, S3, and a small part of S4. It provides cutaneous innervation to the part of the skin of the gluteal region, scrotal area, and skin of the dorsum of the penis in males and the labial area along with the skin of the clitoris in females, skin of the back of the thigh, skin of the leg and dorsum of the foot.
Cutaneous functions include the transmission of sensory information, i.e., touch, pressure, pain, temperature, and proprioception.
Motor supply includes innervation to all three Glutei muscles, hamstring muscles, and muscles of the leg and foot. Motor functions include the extension at the hip joint, lateral rotation and abduction of the thigh, flexion at the knee joint, Dorsi/Plantar flexion at the ankle joint, movements at the joints of the foot, i.e., metatarsophalangeal and interphalangeal joints, and maintenance of the arches of the foot.
Important branches are the sciatic nerve, superior and inferior gluteal nerves, lateral and perforating cutaneous nerves of the thigh, nerves to the quadratus femoris and obturator internus, and pudendal nerve.
Disease-related to the sacral plexus are sacral plexopathy, sciatica, foot drop, and gait disorders. Various etiologies for these diseases include direct injury to the nerves, neuritis, radiation therapy, pressure due to tumors, spinal stenosis, bone spur, disc prolapse, vertebral or hip dislocation, and infections.
Patients suffering from the above-mentioned diseases, except for gait disorders, commonly present with symptoms of mild to excruciating pain, numbness, and weakness of the affected limb. Treatment includes both pharmacological using NSAIDs, steroids, nerve blocks, tricyclic antidepressants and antibiotics (in case of infections), etc., and surgical. Along with these curative measures, rehabilitation is done using physiotherapy.