Ankle Clonus and Wakeup Tests During Posterior Spinal Fusion: Correlation With Bispectral Index

Abstract not available. Introduction provided instead.

Spinal cord injury and its resultant neurologic deficits are recognized complications of posterior spinal fusion (PSF) for correction of scoliosis. Spinal cord injuries are identified with various methods, including intraoperative monitoring using somatosensory evoked potentials (SSEPs) and/or motor evoked potentials (MEPs), intraoperative wakeup test, and demonstration of ankle clonus.^1-4

The wakeup test, originally reported (in the 1970s) to be a means of monitoring spinal cord integrity, involves gradually decreasing anesthesia depth until the patient is able to follow commands and voluntarily move the lower extremities^5,6; then anesthesia is returned to its previous level, and the surgery is completed. Although this test and potential intraoperative awareness are discussed with the patient before surgery, recall may occur during the test. In addition, as anesthesia lightens, hemodynamic changes may cause bleeding, and excessive patient movement may cause bodily harm or dislodge intravascular catheters or even the endotracheal tube. Given these issues, alternative means of monitoring spinal cord integrity are desirable.

The ankle clonus test was the first test to be used to assess spinal cord integrity during surgery. Ankle clonus is a neurologic sign that is usually considered pathologic but can normally appear during emergence from general anesthesia.^7,8 During the normal awake state, descending inhibitory fibers prevent clonus in response to an ankle stretch. As the patient emerges from general anesthesia, lower motor neuron function returns before descending inhibitory cortical fibers regain their normal function, thereby disinhibiting the lower motor neurons and resulting in the ability to elicit clonus. If the spinal cord has been damaged, flaccid paralysis will be present, thereby preventing spinal reflexes, including ankle clonus.³ The ankle clonus reflex is elicited by rapid dorsiflexion of the foot followed by continued application of pressure to hold the foot in slight dorsiflexion. Rhythmic contractions of the gastrocnemius muscle result in repetitive plantar flexion of the foot.

Compared with the wakeup test, the ankle clonus test can usually be elicited before the patient regains consciousness, at a deeper level of anesthesia.⁹ In this article, we describe the cases of 3 adolescents whose SSEPs or MEPs changed during intraoperative monitoring for PSF. In these cases, the Bispectral Index (BIS) monitor (Aspect Medical Systems, Inc., Newton, Mass) was used to judge the depth of anesthesia and to provide numeric data regarding the anesthesia level at which ankle clonus can be elicited, versus the point at which the patient is able to voluntarily move the lower extremities.