Introduction
Background
As the nucleus pulposus loses its turgor and the elasticity of the anulus diminishes, the disk bulges outward beyond the vertebral body margins, causing bulging of the disk. Herniation of the nucleus pulposus (HNP) through an anular defect causes focal protrusion of the disk material beyond the margins of the adjacent vertebral endplate, resulting in disk herniation. Trauma is the single most common cause of rupture of the nucleus pulposus through the anulus fibrosus. The result is protrusion or extrusion of the disk material into the vertebral canal. This outcome can be caused by a single event or by repeated trauma. Predisposition to degeneration is also a factor. Alterations in the vertebral endplate cause loss of disk nutrition and disk degeneration.
Other potentiating factors include the following: (1) age, (2) apoptosis, (3) abnormalities in collagen, (4) vascular ingrowth, (4) loads placed on the disk, (5) abnormal proteoglycan, (6) obesity, (7) sedentary lifestyle, and (8) poor physical fitness.1,2,3
Axial CT myelogram of a large, central calcified disk extrusion present at the T5-6 level; it causes severe spinal cord compression.
Right L5 radiculopathy. Sagittal T1- and T2-weighted images show a large, right central disk extrusion at L4-5 that markedly compresses the thecal sac. The extruded disk migrates cranially, compressing the right L5 nerve root.
Pathophysiology
Symptoms of HNP are the result of either HNP through a mechanically weak anulus fibrosus or from tearing of the anulus itself. Subsequent radiculopathy can result from nerve root compression or from an inflammatory process affecting nerve roots (neuritis).1 When the spinal cord is involved at the cervical or thoracic level, myelopathy ensues.1,2,3,4
Herniation is thought to be caused by a defect in the anulus fibrosus, most likely the result of excessive stress applied to the disk. The injury most often occurs on the posterior or posterolateral aspect of the disk. Histologic evaluation reveals that, whatever the cause of the tear, the extruded portion always involves material from the nucleus pulposus. The morphology of the anular fiber bundles plays an important role in the propensity for disk herniation on the posterior aspect of the disk. This directs the herniation toward the exiting and traversing nerve roots.4,5
Frequency
United States
Mild bulging of lumbar and cervical disks are common incidental findings in patients older than 20 years. Such bulging is more common in the lumbar region than in the cervical region. The bulging may or may not be clinically significant. Some patients with bulges are symptomatic, whereas others remain totally asymptomatic.1,5,6
The incidences of cervical radiculopathies by nerve-root level are as follows: C7, 60-70%; C6, 19-25%; C8, 4-10%; and C5, 2%.1
Herniation of the intervertebral disk may occur in any direction. This is a common condition associated with the increasing wear and tear that occurs with aging. Focal trauma exerted on the intervertebral disk is a causal factor.
About 90% of all lumbar disk herniations occur at the L4-5 and L5-S1 levels, about equally.5,6 Herniations at the L3-4 disk are subject to infrequent rupture. Nearly 93% occur inside the spinal canal, 3% are predominantly in the intervertebral foramen, and 4% are extraforaminal or occur far laterally.5 About 20-30% of all HNPs of the cervical region occur at the C5-6 level; 60-75% occur at the C6-7 level.
Symptomatic herniation at the thoracic level is relatively uncommon; thoracic HNP accounts for less than 1% of all diskectomies.2
Mortality/Morbidity
When untreated, partially treated, improperly treated, or even optimally treated, HNPs may result in severe morbidity, characterized by chronic pain syndrome, segmental paresis, atrophy, reflex changes, sympathetic dystrophy with regional dysautonomia, and sphincter dysfunction.
Herniation of a central disk in the cervicothoracic region results in myelopathy with attended paraparesis, spasticity, hyperreflexia, clonus, sensory disturbance in the legs, and sphincter dysfunction. With herniation of a high cervical central disk, respiratory compromise may occur. This, in addition to complications of immobility and associated intercurrent infections, increases the mortality rate.1,2,3
Race
The incidence of HNP is about the same all over the world.
Sex
The male-to-female ratio is approximately 1:1.
Age
HNP may be observed with MRI in 10% of asymptomatic individuals younger than 40 years and in 5% of those older than 40 years.7,8 Degenerative disk disease (DDD) is most common after age 40 years.
Anatomy
Disk and ligaments
Each disk is composed of 2 parts: the nucleus pulposus (the inner, central, soft part of the intervertebral disk) and the anulus fibrosus (the thick outer part). A longitudinal ligament attaches to the vertebral bodies and to the intervertebral disks anteriorly and posteriorly; the cartilaginous endplate of each disk is attached adjacent to the bony endplate of the vertebral body. The anterior longitudinal ligament (ALL) provides a tension band to resist forces applied in extension; it is a stronger ligament than the posterior longitudinal ligament (PLL). The PLL is not as strong as the ALL; it provides a tension band to resist flexion forces. The PLL strongly attaches to the anulus fibrosus. The incidence of tears of the PLL is high among cases of free-fragment disk herniation.4,5,6
The nucleus pulposus consists of proteoglycan and, specifically, hyaluronic long chains, which have highly hydrophilic branching side chains. They have a strong avidity for water molecules and therefore hydrate the nucleus or the center of the disk.
The anulus fibrosus forms a ring around the nucleus pulposus. It also attaches to the vertebral bodies above and below. As the nucleus pulposus loses its turgor and elasticity, the disk bulges outward beyond the margins of the body of the vertebra.
The anular bands are subdivided into inner fibers and outer Sharpey fibers. The inner fibers are connected to the cartilaginous endplate, whereas the outer fibers are attached to the vertebral body.
A meningeal branch of the spinal nerve, better known as the recurrent sinuvertebral nerve, innervates the area around the disk space. This nerve exits from the dorsal root ganglion and enters the foramen, where it then divides into a major ascending branch and a lesser descending branch.
The disk receives most of its nutrition by diffusion. It has a low metabolic rate. The disk itself is not supplied by lymphatics or blood vessels.
Nomenclature and classification of lumbar disk pathology
The North American Spine Society (NASS), the American Society of Neuroradiology (ASNR), and the American Society of Spine Radiology (ASSR) are joint task forces overseeing the standardization of the terms used to define conditions of the lumbar disk. This article reflects the work of those task forces. Although the principles and most of the definitions used in this article could easily be applied to the cervical and dorsal spine, the focus is on the lumbar spine.6
Normal disk
The term normal applies to young disks that are morphologically normal, without consideration of the clinical context. This class does not include disks with degenerative, developmental, or adaptive changes that may be considered clinically normal in some contexts (eg, disks that have undergone typical changes associated with aging, scoliosis, or spondylolisthesis would not be considered normal). However, the bilocular appearance of the adult nucleus resulting from the development of a central horizontal band of fibrous tissue is considered a sign of normal maturation.
Degenerative and/or traumatic pathology
Degenerative and/or traumatic changes in the disk represent broad category that includes the subcategories of anular tear, herniation, and degeneration. To characterize disks as degenerative and/or traumatic does not imply that trauma is necessarily a factor or that degenerative changes are necessarily pathologic, as distinct from changes associated with the normal aging process.
The term anular tear or, more properly, anular fissure refers to separations between anular fibers, the avulsion of fibers from their vertebral body insertions, or breaks through fibers that extend radially, transversely, or concentrically so as to involve 1 or more layers of the anular lamellae. The terms tear and fissure reflect the spectrum of such lesions and do not imply that the lesion is the result of trauma.
Degeneration may include any or all of the following: real or apparent desiccation; fibrosis; narrowing of the disk space; diffuse bulging of the anulus beyond the disk space; extensive fissuring (eg, numerous anular tears) and mucinous degeneration of the anulus; defects and sclerosis of the endplates; and the occurrence of osteophytes at the vertebral apophyses. A disk with 1 or more of these degenerative changes can be further classified as spondylosis deformans, which possibly represents changes in the disk associated with the normal aging process, or intervertebral osteochondrosis, which is possibly the consequence of a more clearly pathologic process.
Herniation
Herniation is defined as a localized displacement of disk material beyond the limits of the intervertebral disk space. The disk material may be nucleus, cartilage, fragmented apophyseal bone, anular tissue, or any combination thereof. The endplates of the vertebral body define the disk space cranially and caudally; the outer edges of the vertebral ring apophyses, exclusive of osteophytic formations, define it peripherally.
Localized displacement in the axial (horizontal) plane is classified as focal, in which less than 25% of the disk circumference is involved, or as broad-based, in which 25-50% of the disk circumference is involved. When 50-100% of the circumferential disk tissue extends beyond the edges of the ring apophyses, the disk is classified as bulging; such a condition is not considered a form of herniation. Likewise, diffuse adaptive alterations of disk contour secondary to adjacent deformity, as occurs in cases of severe scoliosis or spondylolisthesis, may not be herniation.
Herniated disks may take the form of protrusions or extrusions, depending on the shape of the displaced material. In cases involving protrusion, the greatest distance in any plane from the edges of the disk material beyond the disk space is less than the distance between the edges of the base in the same plane.
Because the PLL often constrains posteriorly displaced disk material, imaging may portray disk displacement as a protrusion on axial sections and as an extrusion on sagittal sections; in such cases, the displacement should be considered an extrusion.
Herniated disks that occur in the craniocaudal (vertical) direction through a break in the vertebral body endplate are referred to as intravertebral herniations.
Protrusions may be focal or broad based. The distinction is arbitrarily set at 25% of the circumference of the disk. Protrusions with a base less than 25% (90°) of the circumference of the disk are focal. If disk material is herniated, so that the protrusion encompasses 25-50% of the circumference of the disk, it is considered a broad-based protrusion.
Extrusion
A disk is classified as an extrusion if (1) any distance between the edges of the disk material beyond the disk space is greater than the distance between the edges of the base measured in the same plane, or (2) there is a lack of continuity between the disk material beyond the disk space and the material in the disk space.
Extruded disk material that has no continuity with the disk of origin may be further characterized as sequestrated. A sequestrated disk is a subtype of extruded disk. By definition, a sequestrated disk can never be classified as a protruded disk. Disk material that is displaced away from the site of extrusion, regardless of the presence or absence of continuity, may be called migrated, a term that is useful for interpreting images because it is often impossible to show continuity on imaging.
Containment and continuity
Herniated disk material can be either contained — a term that refers to the integrity of the outer anulus covering the disk herniation — or uncontained. In cases of contained herniation, fluid that is injected into the disk does not leak into the vertebral canal.
Displaced disk fragments are sometimes characterized as free. A fragment should be considered free, or sequestrated, only if there is no continuity of disk material between the fragment and the disk of origin.
The terms migrated disk and migrated fragment refer to displacement of disk material away from the opening in the anulus through which the material extrudes. Some migrated fragments are sequestrated, but the term migrated refers only to the position and not to continuity. Regarding the PLL, displaced disk material may be described as subligamentous, extraligamentous, transligamentous, perforated, subcapsular, or submembranous.
Canal compromise of less than one third of the canal at that section is classified as mild. Compromise of one to two thirds is classified as moderate, and compromise of over two thirds is classified as severe. The same grading system can be applied to foraminal involvement.
Composition of the displaced material may be characterized by terms such as nuclear, cartilaginous, bony, calcified, ossified, collagenous, scarred, desiccated, gaseous, or liquified.
From central to right lateral in the axial (horizontal) plane, the location may be defined as central, right central, right subarticular, right foraminal, or right extraforaminal.
Presentation
Radicular pain is characterized as a unilateral, lancinating, dermatomal pain. Such pain is often exacerbated by coughing, sneezing, or Valsalva maneuvers. It is also exacerbated by maneuvers that stretch the affected nerve root.
Passive straight-leg raising leads to stretching of the sacral and lower lumbar roots. Passive flexion of the neck stretches the cervical roots. Spinal movements that narrow the intervertebral foramina (extension and lateral flexion of the neck) can also aggravate root pain. This is common with extradural lesions but is rare with intramedullary lesions.
Clinical signs may include dermatomal hypesthesia or hyperesthesia, segmental paresis, amyotrophy, reflex changes, and fasciculations. In the case of herniation of a lumbar disk, the patient may experience sphincter disturbances in addition to a painful back and radicular pains. Bilateral symptoms and signs suggest that the disk material has protruded centrally; sphincter dysfunction is more likely to be the result of a central herniation than a lateral herniation.
The location of the ruptured disk determines where a person has symptoms. Most ruptured disks are in the lower back and cause low back pain. If the ruptured disk presses on a nerve, pain may be felt in the buttocks, legs, and feet. This pain, which usually affects only 1 leg, is known as sciatica.
L5 root involvement causes weakness in foot and toe dorsiflexion, whereas an S1 lesion leads to depressed or absent ankle jerk and weakness of plantar flexion. In either case, restriction in movement of the spine, local tenderness, paraspinal spasm, and/or a positive Lasègue sign (which is observed on a straight leg-raising test) may be evident. Involvement of the L4 root results in a depressed or absent knee jerk; such involvement may also cause weakness and atrophy of the quadriceps and, occasionally, the adductors.
In the event of cervical HNP, patients may present with neck and radicular arm pain that is exacerbated by neck and head movement. The disk causes referred pain between the scapulae in the upper middle part of the back. Headache originating at the base of the neck is common. Pressure on a nerve may cause numbness and burning or weakness in the arm and hand. Pressure on the spinal cord in the neck causes symptoms of weakness in the legs, electric shocks down the spine, and numbness or poor coordination; such findings are of concern because they indicate that spinal cord injury is developing.
Lateral herniation of the disk can result in segmental motor, sensory, and/or reflex changes, usually in the involved root level on the affected side. The C6 and C7 roots are more commonly involved. With a more centrally directed herniation, central cord syndrome may develop, resulting in myelopathy, characterized by spastic paraparesis and a sensory disturbance in the legs; myelopathy is sometimes accompanied by impaired sphincter function.
Vertebral pain is characterized by aching pain that is localized to the point of spine involvement. Such pain is a result of the compressive process; it is often accompanied by joint tenderness. Vertebral pain is common with vertebral extradural lesions such as HNP, neoplastic lesions, and inflammatory lesions. Vertebral pain is infrequent or rare with intramedullary or intradural extramedullary lesions.
Central (funicular) pain is common with intramedullary lesions; it is a deep, ill-defined, and painful dysesthesia that is usually distant from the affected spinal cord level. The exact mechanism of such pain remains uncertain, though it is probably related to dysfunction of the spinothalamic tract, relative to the posterior column. Central pain is often of little value in localizing the lesion.
Complications of cervical and lumbar HNP may include the following:
- Intractable neck and back pain
- Intractable radicular pains down the arm or leg
- Muscle contraction headache
- Regional hyperesthesia, hypesthesia, dysesthesia, motor weakness, sensory loss, reflex changes, and/or sympathetic dystrophy
- Sphincter dysfunction
- Myelopathy, paraparesis, and quadriparesis
- Complications of immobility
- Respiratory compromise, even death
Preferred Examination
Preferred examinations for the evaluation of disk herniation include the following: MRI of the spine (lumbosacral, thoracic, or cervical images), bone scanning, and CT scanning of the spine (lumbosacral, thoracic, or cervical images).7,9,10,11,12,13
Additional tests, when indicated, include assessments of any of the following, alone or in combination: erythrocyte sedimentation rate (ESR), serum glucose level, prostate-specific antigen (PSA) concentration, alkaline phosphatase value, serum protein immunoelectrophoresis result, and urine findings for Bence Jones protein.
Myelography with CT scanning is usually indicated in the following conditions: when surgery is contemplated in cases of spinal stenosis; with lateral recess stenosis; with multiple abnormal disks; with spondylolisthesis; with possible neoplasm; and after severe trauma.9
Limitations of Techniques
Regarding limitations of MRI, some individuals with implanted devices (eg, pacemakers) or with metal in their body may not be able to undergo MRI because pacemaker dysfunction and/or electrode heating are possible adverse effects of MRI. Although changing the gradient fields can induce currents in leads, the radiofrequency (RF) pulses are typically the cause of inappropriate pacing.10,14 The physician ordering MRI and the MRI staff must determine whether it is safe for the patient to be examined with MRI.
Patients undergoing MRI lay on a table that slides into the MRI unit. MRI of the head or neck area requires the patient's head and upper body to enter a narrow canal. Individuals with claustrophobia may require light sedation to improve their ability to tolerate the closeness of the machine. Although open MRI is an option for patients with claustrophobia, open MRI units are usually equipped with weak magnets (strength measured in Tesla units), which may prevent depiction of fine details. To obtain the best images, the patient must not move during the examination. Some patients and small children undergoing MRI may require sedation to ensure that they remain still.
MRI is generally avoided during the first 12 weeks of pregnancy, though the hazards of MRI use during this period have not been proven.
Although conventional radiographs probably show cortical bone better than MRIs do, the bone marrow is best imaged with MRI, and MRI is superior for demonstrating most bone abnormalities. CT is preferred for unstable patients with severe bleeding. MRI may not always be useful for distinguishing between tumor tissue and edema fluid, and it does not depict calcium well when it is present in a tumor.
Severe obesity limits MRI of the spine.
Postoperative scarring and/or edema can limit results of the study.
Differential Diagnoses
Other Problems to Be Considered
Infectious etiologies
Diskitis
Epidural, subdural, or intradural abscess
Mechanical etiologies
Cervical stenosis
Cervical zygapophyseal (facet) arthropathy
Metabolic etiologies
Osteomalacia
Rheumatologic etiologies
Ankylosing spondylitis
Enteropathic arthritis
Diffuse idiopathic skeletal hyperostosis (DISH)
Polymyalgia rheumatica (PMR)
Reiter syndrome
Traumatic brachial plexopathy
Normal variants and other
Anular tear
Brachial plexitis or plexopathy
Cervical myofascial pain
Cervical spondylosis
Cervical sprain and strain
Complex regional pain syndromes
Conjoint nerve root
Dilated nerve root sleeve
Ependymoma
Facet fracture
Fibromyalgia
Lumbar plexitis or plexopathy
Lumbar spondylosis
Lumbar sprain/strain
Meningioma
Muscle injury
Nerve root avulsion
Neurofibroma
Osteoarthritis
Osteoporosis, primary
Osteoporosis, secondary
Paget disease
Parathyroid disease
Perineural cysts
Psoriatic arthritis
Radiation-induced brachial plexopathy
Rheumatoid arthritis
Rotator cuff disease
Schwannoma
Thoracic outlet syndrome
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References
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Further Reading
Keywords
herniated disk, herniated disk disease, herniated disc, herniated disc disease, disk herniation, disc herniation, disk disease, disc disease, protrusion, extrusion, sequestration, prolapse, slipped disk, spinal stenosis, ruptured disk, ruptured disk, herniated nucleus pulposus, herniation of the nucleus pulposus, HNP, degenerative disk disease, DDD, sciatica, Dallas classification of diskography, diskography, anulus fibrosus, anulus fibrosis, annulus fibrosus, annulus fibrosis, nucleus pulposus, treating disk herniation, treatment of disk herniation, herniated disk treatment, back pain treatment, treating back pain
