IS IT TIME TO REVISIT THE "MYTHS" OF NEUROSURGERY?

BOSTON—Some traditional neurosurgical practices have been based on assumptions that have not passed the tests of rigorous evaluation, contended Gregory Dowd, MD, and Deepak Awasthi, MD, from the Department of Neurosurgery, Louisiana State University Medical Center, New Orleans. At the 49th Annual Meeting of the Congress of Neurological Surgeons, they enumerated 10 examples of conventional but "untested" neurosurgical techniques and concepts. "Often based on the assumption of what is best . . . they are handed down from mentor to apprentices in a 'here is what you do' format," said Drs. Dowd and Awasthi. "Although tradition and ritual may put the practitioner at ease, blindly reproducing the errors of a previous generation is not good medicine," they continued. "Only by questioning the scientific foundations of our daily practice can we confidently believe in their merit."

Drs. Dowd and Awasthi conducted a literature search for selected aspects of neurosurgical practice that "although closely held and followed by practitioners, may have unsound foundations." They found "a wide variation of opinion, ranging from subtle differences to frank controversy, concerning neurosurgical practice." They pinpointed 10 practices and premises that have either been disproven or been found to lack a scientific foundation. "These unsound practices may be regarded as neurosurgical myths," they argued.

Head elevation is beneficial in the treatment of elevated intracranial pressure. This belief, which is often put into practice following craniotomy procedures, is based on the notion that head elevation may augment venous drainage from the head with a resultant decrease in intracranial pressure. While this may be true, however, there is also an accompanying greater decrease in mean arterial pressure, Drs. Dowd and Awasthi pointed out.

"Head elevation results in an overall decrease in the cerebral perfusion pressure. For the patient's brain, elevation of the head in bed may mean a decrease in the effective perfusion of the brain. The extreme form of this may result in cerebral ischemia partially resulting from head elevation," they said.

Hyperventilation (induced hypocapnia) is useful to treat elevated intracranial pressure. This notion is based on the fact that carbon dioxide (CO₂) is a potent vasodilator at the cerebral arteriolar bed. Therefore, by reducing the arterial partial pressure of CO₂ (pCO₂), constriction of the vascular bed may both reduce intracranial blood volume and lower intracranial pressure.

"The mechanism of vasoconstriction is based on a differential of the hydrogen ion in the blood to the perivascular cerebrospinal fluid (CSF) spaces," the researchers said. "Over time, equilibration will occur and the vascular effect will diminish. Abrupt normalization of the pCO₂ will cause a reversal of the blood-CSF hydrogen gradient and can lead to rebound vasodilatation in the brain." In damaged tissue, no such control may exist. In the presence of hypovolemia, hyperventilation may result in decreased brain perfusion.

This approach was put to the test in a randomized trial; outcomes were, in fact, worse, in patients with severe brain injury who were treated with hyperventilation. However, hyperventilation may have a role in temporarily reducing intracranial pressure until a more definitive measure can be initiated, the researchers commented.

Glucocorticoids are beneficial in the treatment of brain injury. While steroids have been very beneficial in treating vasogenic forms of edema, they have also been thought to be useful for cytotoxic damage resulting from brain injury. "Data from the use of high-dose glucocorticoid treatment for spinal cord injury suggest an improvement in outcome for this condition. However, similar trials in the brain injury population have failed to show an improved outcome with respect to mortality or functional outcome. In fact, an increase in infectious complications may result from the use of steroids in these patients," the researchers said.

Histamine H₂ blockers (such as cimetidine) should be used to prevent stress gastritis. Patients with severe neurologic injuries and those taking glucocorticoids are at risk of upper gastrointestinal bleeding, which might be fatal or require surgery. While agents that lower gastric acidity are effective in reducing the number and severity of gastric ulcers, neutralization of gastric juices can also cause bacterial overgrowth.

According to Drs. Dowd and Awasthi, "these critically ill patients often have bouts of subclinical gastric reflux and pulmonary aspiration of this fluid. Increased rates of pneumonia have been found in patients receiving H₂ blockers. The resultant increase in pulmonary complications more than offsets the decrease in gastric bleeding." An alternative, suggested the researchers, is sucralfate; it does not change the gastric pH, but reduces gastrointestinal bleeding without increasing the rate of pneumonia.

Shaving the operative site reduces infection. " This seemingly logical concept does not hold true. There is a long-standing bias across cultures that the absence of hair denotes purity. Monks shave their heads. The profession of hair cutting and surgery were closely related in the past as manifest by the barber-surgeon. These early surgeons would shave the site to be operated on," Drs. Dowd and Awasthi commented.

This practice later became a tradition at some institutions. The patient's head was shaved the night before surgery with a straight razor. Even the expertise with which the shaving was performed was taken into account, and residents who caused patients to bleed were thought to possess inadequate surgical skills.

In their literature search, Drs. Dowd and Awasthi discovered a correlation between risk of infection and the length of the interval between shaving and surgery. In addition, some reports suggested that refraining from shaving yields low infection rates comparable to the best reported series. "This evidence should make the surgeon question the benefit of even this seemingly worthwhile technique," they noted.

Following anterior cervical discectomy, a fusion procedure is required. This "myth," Drs. Dowd and Awasthi explained, stems from the early use of fusion to treat cervical discogenic disease. Surgeons later continued to use a fusion procedure to avoid postoperative kyphotic deformity at the surgical level. When anterior cervical discectomy was performed alone, the results were comparable to discectomy with fusion for cervical discogenic disease, and three trials comparing the two procedures reported similar outcomes.

"Although there is a higher rate of interbody fusion in the group of patients treated with a fusion procedure for cervical discogenic disease, the patient satisfaction among both groups is the same. Thus, it appears that fusion is not mandatory for a favorable result following anterior cervical discectomy," the researchers concluded.

Congenital disorders such as neural tube defects and hydrocephalus are not preventable. Neural tube defects are known to be associated with environmental factors such as malnourishment and low socioeconomic status. Most recently, because folate deficiency has been pinpointed as a cause, neural tube defects are now considered very preventable.

With respect to hydrocephalus, treatment options have improved over the past century, but an understanding of CSF circulation has been slower to evolve. Cerebral imaging has demonstrated that infants with hydrocephalus often have intraventricular hemorrhage that may be related to precipitous delivery or bleeding from the germinal matrix. Advances in obstetrics and neonatology have lowered the incidence of hemorrhage and hydrocephalus in relatively healthy infants. However, the persistence of hydrocephalus in very premature and very sick infants attests that this problem is still unresolved.

Primary brain tumors do not usually metastasize. This "myth" is supported by studies of leptomeningeal tissue showing that "this structure acts as a barrier to invasion and therefore to distant spread," Drs. Dowd and Awasthi explained. "However," they continued, "with longer survival of patients harboring primary brain tumors and new techniques used in their treatment, evidence of distant disease has become more common. Spread throughout the subarachnoid space has been well documented for ependymoma and primitive neuroectodermal lesions, but recently meningiomas and gliomas have been found to metastasize by the same route. Growth at distant sites may also result from medical treatment."

Knowledge of neuroanatomy will permit the surgeon to avoid iatrogenic neurologic deterioration. Because it was thought that the nervous system was "hard wired," scientists believed that lesions could be localized reliably on the basis of clinical findings. Target regions were believed to be surrounded by clinically silent areas in which the surgeon could operate with relative safety.

However, the development of sophisticated imaging has shown that in many cases the abnormality is distant from the region predicted on clinical grounds. The work of Penfield and Ojemann, for example, showed a wide distribution of important functions, even extending into areas that had been thought to be "safe."

"This important field of functional mapping will provide the surgeon with specific information for the individual patients. Thus, the surgeon cannot confidently localize eloquent cortex on the basis of surface landmarks alone," the researchers said.

This is true for the peripheral nervous system as well, exemplified by the variability of motor function arising from specific spinal nerves. In one reported study, sectioning of spinal nerves during removal of a paraspinal tumor resulted in deficits in only 10% to 33% of cases. "This is presumably due to the chronic compensation by neighboring nerves that occurs during the slow growth of these tumors," the researchers said.

The brain is the surgical domain of the neurosurgeon. This belief is slowly changing with the development of technology such as computed tomography and magnetic resonance imaging that are implemented by clinicians other than neurosurgeons.

"This means that the contents of the skull are no longer a 'black box' that only the neurosurgeon can see into and that the workup of a patient is largely done by other specialties. Thus, treatment of a given pathologic entity is open to whichever surgeon is willing to attempt the procedure," Drs. Dowd and Awasthi commented. In addition, the development of superior surgical techniques performed by related surgical specialists has earned them preeminence in certain areas, they contended. This is a trend, they stressed, that is likely to continue and underscores the need for neurosurgeons to remain involved with emerging technology.

—Lynda Charters
Contributing Writer

Suggested Reading
Dowd GC, Wirth FP. Anterior cervical discectomy: Is fusion necessary? J Neurosurg. 1999;90(1 Suppl):8-12.