Clinical Practice Guideline
Developed for the
Aerospace Medical Association
by their constituent organization
Overview: The intervertebral disc is comprised of three parts. The nucleus pulposis (NP) is a mucoprotein gel located in the posterocentral portion of the disc. Surrounding the NP is the annulus fibrosis (AF), concentric laminated bands of fibrous tissue. And finally, cartilaginous end-plates separate the disc from the vertebral body. HNP occurs when the NP herniates through the AF, usually in a posterior or posterolateral fashion. This can only occur when the AF is disrupted either by violent trauma or recurrent microtrauma. A history of trauma has been elicited in over 50% of those with lumbar HNP.
Approximately 80% of HNPs occur in the lumbar region. The age-incidence curve for lumbar HNP peaks in the second through fourth decades of life and the incidence is higher in males than females. Other pathologic lesions (i.e. spondylosis and spinal stenosis) have been noted in 50% of the cases with lumbar HNP. Approximately 20% of HNPs occur in the cervical region and 20-33% of these have concurrent lumbar disc involvement. The age-incidence curve for cervical HNP peaks in the fifth through sixth decades of life and the incidence is higher in males than females. Less than 1% of the HNPs occur in the thoracic region. The age-incidence curve for thoracic HNP peaks in the fifth through sixth decades of life and the incidence is equal for both sexes.
Symptoms from HNP vary from asymptomatic to paraplegia. Herniated disc matter may compress nerve roots, vascular structures, and even the spinal cord (above L-2). Symptoms such as pain, numbness, paresthesias, weakness and muscle wasting are usually the presenting findings. However, progression of disease may lead to quadriparesis, paraplegia, and emergent conditions such as cauda equina syndrome and conus syndrome.
Aeromedical Concerns: Inability to perform flying duties may be a result of baseline symptoms (pain, weakness). Sudden incapacitation and permanent disability are significant concerns, particularly in a high-G environment or during ejection. And finally, the forces applied to the intervertebral disc under high-G stress may lead to accelerated progression of disease. Following surgical treatment of HNP, concerns are raised regarding vertebral joint stability and subsequent catastrophic failure of the vertebral column, depending on the type and nature of the procedure. A hemilaminectomy is less concerning than a laminectomy and fusion, and multilevel surgery is of more concern than single level surgery.
Recurrences and frequent removal from aviation duties may have adverse effects on aviation missions, particularly in the military. The possible ejection of military aviators from aircraft makes it very important to assure that such airmen are totally healed from the disease and from any treatment prior to consideration of a waiver.
Medical Work-up: Diagnosis of HNP is based on characteristic symptomatology, confirmed by radiologic studies. The predominant studies used to document HNP are myelography, CT and MR imaging. HNP is confirmed by the demonstration of herniation of disc material (size and direction) and distortion/displacement of the thecal sac, epidural fat, and/or nerve root. However, repeated studies have failed to delineate a direct relationship between the degree of herniation and the amount of symptoms. In fact, 35% of individual over the age of 40 years have been noted to have various anomalies of intervertebral discs while remaining asymptomatic.
Standard treatment for HNP consists of conservative therapy, surgery, and chemonucleolysis. Conservative therapy (i.e., bedrest, pain medication, and physical therapy) has resulted in symptomatic improvement in 30-95% of HNPs. Several studies have documented regression and even complete resolution in up to 50% of HNPs within 6 months. Surgical resection of HNP is effective in 90% of carefully selected HNPs but up to 30% of those who undergo surgery have unsatisfactory results (extradural fibrosis/scarring, recurrent HNP, and spinal stenosis secondary to bone overgrowth). Surgery should be limited to: 1) severe intractable pain not responding to conservative therapy, 2) significant neurologic deficit (most notably weakness) that is at risk for becoming permanent, and 3) frequently recurring episodes of radiculopathy that causes significant functional impairment. In addition, operative complications such as nerve root or cord injury and postoperative joint instability may occur. Chymopapain injected into the HNP results in enzymatic hydrolysis of the chondromucoprotein portion of the HNP. Although this procedure has been in use for twenty years, long-term effects from chemonucleolysis are unknown; this treatment modality has fallen into disfavor in the past few years. Several studies have documented a 20-30% failure rate with this procedure and complication rates similar to surgery.
Aeromedical Disposition (military): Waiver can be considered following the successful treatment of HNP. As noted above, the treatment may be conservative and/or surgical. Initial evaluation should include history and physical examination to ensure that the examinee is asymptomatic and able to perform duties without limitations. If history or physical elicits findings, further studies (such as nerve conduction study and electromyelogram) may be needed for further evaluation. Also, a consultation from an orthopedic surgeon should clear the examinee for flying duties and delineate prognosis. Finally, imaging studies (CT or MRI) should document structural resolution of the HNP. The type of aircraft flown is very important in an aeromedical disposition for HNP patients.
Aeromedical Disposition (civilian): In the civilian environment aeromedical certification is more likely than for the military. For consideration, an individual must provide a complete orthopedic or neurosurgical evaluation. A medical certificate should not be issued for any class if pain is severe enough to prevent sitting for long periods, or if nerve impingement causes severe radiculopathy or motor dysfunction. Obviously, the use of narcotic analgesics or muscle relaxants is contraindicated in the aerospace environment. A medical certificate may be issued if symptoms have resolved with treatment (conservative or surgical), and appropriate written medical documentation has been provided. Select cases may be forwarded for to a FAA-selected consultant for review and recommendations.
Waiver Experience (military): A military waiver file lists 343 members with the diagnosis of HNP. Of those evaluated, over 95% received waivers for flying duties, two thirds of whom were corrected with surgery or chemonucleolysis. Occasionally, categorical waivers are issued in order to preclude flying in high-G performance or ejection seat aircraft. Waivers are considered usually 3 to 6 months after successful treatment. (A thorough history and physical exam should be performed to assure no persistent neurologic or range of motion deficits). Recurrent symptoms after treatment would preclude waiver consideration.
Waiver Experience (civilian): As of January 2001 there were 3,329 First-class, 2,627 Second-class, and 5,888 Third-class airmen with issued medical certificates in the FAA for all variety of HNP, laminectomy, and cervical disk surgery. It should be noted that 5-20% of individuals undergoing surgery will have significant recurrent pain, and these aviators will most likely pose risks in the aviation environment. Mild HNP will most likely not create aviation problems after conservative therapy has concluded.
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October 9, 2001