Disc Herniation Imaging - An Overview

Introduction:

Disc herniation is a medical condition in which the nucleus pulposus bulges out of the intervertebral space, compresses the spinal cord or the nerves, and results in pain or dysfunction of the spinal cord (myelopathy). The nucleus pulposus is a rubbery disc made of type ll collagen fibers found in the intervertebral space that acts as a cushion by absorbing the shock. Disc herniation commonly affects the lumbar spine and then the cervical spine. The thoracic spine is less commonly affected by disc herniation. Lumbar disc herniation is common in L4 to L5 or L5 to S1 segments. In addition, C6 to C7 region is frequently involved in cervical disc herniation, and T11 to T12 are commonly affected thoracic spine. The lumbar disc hernia is symptomatic in one to three percent of patients.

What Are the Types of Disc Herniation?

Disc herniation is classified into:

• Protrusion -

  • In protrusion, the base of the disc is broader than the herniation.

  • The outer covering membrane (annular fibers) is intact and found within the intervertebral disc level.

• Extrusion -

  • In extrusion, the base of the disc is narrower than herniation.

  • The herniation may extend below or above the end plates or the nearer vertebrae.

  • In extrusion, the outer covering may completely tear, allowing the nuclear disc passage beyond the annulus.

  • Sometimes the intervertebral disc may be migrated or sequestered. A migrated disc is an extruded disc that moves into an abnormal position. The sequestered disc is the extruded disc that lacks continuity with the parent disc.

Herniations can further be classified as:

• Contained - A herniated disc is found inside the intact annulus fibrosus and posterior longitudinal ligament (ligament attached to the posterior aspect of the vertebral bodies).

• Uncontained -Tear of the outer annulus fibrosus and posterior longitudinal ligament.

What Are the Symptoms of Disc Herniation?

• Sharp, burning, or stinging pain extends from the buttock to the back of the leg.

• Weakness of leg or foot.

• If the herniated disc compresses the adjacent nerves, it causes numbness or a tingling sensation in the areas of the compressed nerve supply.

• Loss of urinary bladder control is common in cauda equina syndrome.

What Are the Risk Factors of Disc Herniation?

Trauma is the most common cause of disc herniation. However, there are certain factors that increase the risk of disc herniation.

• Age.

• Gender.

• Obesity.

• Improper weight lifting.

• Smoking.

• Frequent driving.

• Apoptosis (cell death).

• Vascular ingrowth.

• Abnormalities in collagen.

• Abnormal proteoglycan.

• Increased loads placed over the disk.

What Are the Radiological Findings of Disc Herniation?

• Plain Radiography:

Plain radiographs do not provide accurate information about disc herniation. Plain radiography reveals the only findings of disc herniation, such as loss of vertebral disk height, the gas inside the disc (vacuum phenomenon), and endplate osteophytes. However, they help exclude the differential diagnosis of back pain such as fracture, cancer, or infection.

Myelography shows displacement of thecal sac, elevation or deviation of the root sleeve, and edema in the affected area of disc herniation patients.

• Computed Tomography:

CT (computed tomography) scan is superior in the diagnosis of disc herniation than myelography alone. When wear and tear occur on the vertebral bodies, the vertebral disc degenerates, and intervertebral spaces become narrowed. This leads to an abnormal relationship with the nearer vertebral bodies resulting in hypertrophy or sclerosis of the uncinate process. When the neural foramen becomes narrowed, it results in radiculopathy (a condition in which nerve roots are compressed). CT shows the hypertrophied uncinate process and osteophytes in cervical degenerative disc disease.

A computed tomography scan usually demonstrates a thoracic disk as it contains calcium. In a CT scan, the clearly defined mass surrounded by epidural fat at the side of the dural sac is indicative of herniated disk.

• Magnetic Resonance Imaging (MRI):

Herniation nucleus pulposus appears hypointense, with uneven protrusion of the disk materials outside their confines in MRI. On sagittal T2-weighted images, high signal intensity is seen in the posterior annulus. T1 MRI images are obtained using short TR (repetition times) and TE (time to echo) times. T2 images are obtained through longer TE and TR times. At the earlier stage, disc bulging is seen as a loss of disc concavity. Moderate bulging appears as non-focal protrusion of the disc contents outside the vertebral borders.

Radial tear of the annulus fibrosus (fibrous membrane covering nuclear pulposus) is shown as increased signal intensity on T1 and T2 weighted images. Radial tear of annulus fibrosus is accompanied by the bulging annulus, loss of disc height, destruction of endplates, and herniation of nucleus pulposus. After contrast administration, the extruded fragments may appear as ring-like enhanced structures.

What Are the Differential Diagnosis for Disc Herniation?

• Neurinoma - A tumor of the peripheral nervous system, also called a schwannoma.

• Metastasis - The spread of cancer from its primary site to other parts of the body.

• Discal Cyst - A fluid-filled cystic lesion found in the spinal cord that communicates with the adjacent vertebral disc.

• Mechanical Back Pain - It occurs due to any stress, disc, or nerve injury.

• Epidural Abscess - The infection of the space between the bones of the spinal cord and their lining membrane.

• Epidural Hematoma - The collection of blood in the space between the bones of the spinal cord and their lining membrane.

• Diabetic Amyotrophy - A complication of diabetes that causes pain and muscle loss in the thighs, legs, hips, and buttocks.

• Osteophytes - A smooth bony growth on the spine or around the joints.

• Cauda Equina Syndrome - The compression of the collection of nerve roots (cauda equina) results in loss of movement and sensation.

• Synovial Cyst - A fluid-filled cyst found in the joints.

Conclusion:

Disc herniation often does not cause pain, and sometimes if the pain occurs, it may gradually decrease within a few weeks. However, imaging is required in disc hernia if the symptoms of the hernia exist for more than six weeks. Disc hernia is more common among the age group of 30 to 50 years old. MRI is the standard imaging technique required for disc herniation. The diagnostic value of a CT scan combined with myelography is superior to CT alone.