INVITED REVIEW
Neurophysiologic Intraoperative Monitoring During Surgery for
Tethered Cord Syndrome
Bhojo Khealani* and Aatif M. Husain†‡
Abstract: Tethered cord syndrome (TCS) occurs when the distal spinal cord
is adherent to inelastic tissue. This results in sensorimotor deficits in the
lower extremities, bowel and bladder dysfunction, and musculoskeletal
deformities. Tethered cord syndrome is often found in childhood, but may be
first noticed in adults as well. The symptoms are usually progressive unless
halted by surgical correction of the spinal cord tethering. Surgery for TCS
can be complicated by inadvertent injury to nerves that are either embedded
in the tether or in close proximity to it. In an attempt to reduce this iatrogenic
injury, neurophysiologic intraoperative monitoring is used to identify neural
structures in the surgical field and reduce the risk of injury. Many neuro-
physiologic intraoperative monitoring paradigms have been used in TCS
surgery, including free running and stimulated electromyography of the
muscles of the lower extremities, external anal and external urethral sphinc-
ter electromyography, tibial, clitoral, and dorsal penile somatosensory
evoked potentials, and bulbocavernosus reflex testing. It is widely believed
that neurophysiologic intraoperative monitoring helps reduce morbidity of
TCS surgery, but data supporting this are limited. This article will review the
various neurophysiologic intraoperative monitoring paradigms that can be
used in TCS surgery and discuss the data supporting the use of these
paradigms.
Key Words: Neurophysiologic intraoperative monitoring, Tethered cord
syndrome, Electromyography, Somatosensory evoked potentials.
(J Clin Neurophysiol 2009;26: 76–81)
Tethered cord syndrome (TCS) is constellation of symptomsconsisting of sensory and motor deficits affecting the lower
extremities, bowel and bladder dysfunction, and musculoskeletal
deformities that occurs because of congenital pathology, resulting in
tethering of caudal end of spinal cord to bone or other inelastic
tissue. It is often associated with spinal dysraphism. Tethered cord
syndrome is most commonly diagnosed in children, but presentation
may be delayed until adulthood. Surgery is the mainstay of therapy
to prevent further progression of the neurologic deficits. It involves
careful dissection of the nerve roots to relieve the tethering and
remove the lesion. Because the anatomy is altered, and it is not
always possible to recognize neural tissue visually, the surgery may
result in inadvertent injury of nerve roots leading to iatrogenic
neurologic deficits.
Neurophysiologic intraoperative monitoring (NIOM) is a
commonly used strategy to minimize the chance of iatrogenic injury
to neural structures during surgery for TCS. As with much of NIOM,
there is little data comparing outcomes of TCS surgery with and
without NIOM. There are, however, many case series discussing the
role of NIOM in these surgeries. This article reviews the NIOM
techniques used during surgery for TCS and the data available
supporting the use of these techniques in reducing neurologic
morbidity.
Neurophysiologic Intraoperative Monitoring
Techniques
Tethered cord syndrome affects nerve roots of the cauda
equina, i.e., L2 and beyond. These nerve roots subserve sensorimo-
tor function of lower extremities and pelvis and urinary and anal
sphincters. Thus it is logical to monitor sensory and motor pathways
from and to the legs and sphincter function. In addition to continu-
ous monitoring of these functions, it is also important to identify
neural tissue in filum terminale. Free running and stimulated elec-
tromyography (EMG) of the lower extremity muscles and the
external anal sphincter are the most widely used NIOM modalities
during surgery for TCS. Other techniques such tibial, clitoral, and
dorsal penile somatosensory evoked potentials (SEP), and the bul-
bocavernosus reflex have also been used to monitor sacral sensory
and motor pathways during TCS surgery. These techniques are
discussed in detail later.
Free-running Electromyography
Motor function of lower extremities can be assessed by free-
running EMG. Electromyography is recorded with subdermal needle or
wire electrodes placed in multiple muscles in each lower extremity.
Suggested muscles to monitor are bilateral vastus lateralis, tibialis
anterior, gastrocnemius, semitendinosus, and the gluteus maximus
(Husain et al., 2008). Other muscles can also be used if necessary.
Securing the electrodes in position is important for successful monitor-
ing. The electrodes should be taped properly, and the legs should be
wrapped with bandage to provide additional security.
A commonly used time window for free-running EMG is 100
milliseconds/division (1 second/full screen). This allows better rec-
ognition of spontaneous discharges. High-frequency filters and low-
frequency filters are set at 10 Hz and 2 to 5 kHz (Minahan and
Mandir, 2008). Suitable sensitivity settings are 50 !V to 2 mV; this
can be adjusted depending on amplitude of the recorded EMG
activity (Husain et al., 2008).
Spontaneous EMG activity appears as a result of neural
irritation or injury. This may occur from mechanical trauma or a
change in temperature or osmolality. The spontaneous activity may
occur as brief bursts of motor units or neurotonic discharges. A
neurotonic discharge is a high frequency discharge of motor units,
which may be sustained or unsustained (Fig. 1). Brief bursts of
motor units and unsustained neurotonic discharges suggest minor
From the *Department of Medicine (Neurology), The Aga Khan University,
Karachi, Pakistan; †Department of Medicine (Neurology), Duke University
Medical Center; and ‡Neurodiagnostic Center, Veterans Affairs Medical
Center, Durham, North Carolina, U.S.A.
Presented, in part, at the annual meeting of the American Clinical Neurophysiol-
ogy Society, Savannah, GA, February, 2008.
Address correspondence and reprint requests to Aatif M. Husain, M.D., Box 3678,
202 Bell Building, Duke University Medical Center, Durham, NC 27710,
U.S.A.; e-mail: aatif.husain@duke.edu.
Copyright © 2009 by the American Clinical Neurophysiology Society
ISSN: 0736-0258/09/2602-0076
Journal of Clinical Neurophysiology • Volume 26, Number 2, April 200976

trauma, whereas sustained neurotonic discharges suggest significant
and possibly irreversible injury (Minahan and Mandir, 2008).
Stimulated Electromyography
Compound muscle action potentials (CMAP) can be obtained
by stimulating neural tissue (nerve roots and spinal cord) and
recording from muscles. Stimulated EMG is recorded from the same
needle or wire electrodes used for recording free-running EMG. An
appropriate time window for recording CMAP after stimulation of
the nerve roots or spinal cord is 10 milliseconds/division (100
milliseconds/full screen) (Husain and Ashton, 2008). Filter and
sensitivity settings are the same as for free-running EMG.
Both monopolar and bipolar probes can be used for stimula-
tion. Bipolar stimulators are preferred because monopolar stimula-
tion leads to greater current spread and possible activation of nearby
neural structures. Triggered stimulation is used, which allows easier
recognition of the CMAP as it appears at the same location on the
screen (Fig. 2). The stimulus duration is 0.1 to 1 milliseconds.
The stimulation intensity used to elicit a CMAP depends on
the neural structure being stimulated. Normal nerve roots often
require less than 1 V for activation, whereas stimulation of spinal
cord usually requires less than 10 V (Kothbauer et al., 1994).
Pathology can result in higher stimulation thresholds. The filum
terminale (which usually does not have neural elements) cannot be
stimulated even with high current intensities. Some authors have
advocated using current intensities as high as 100 V when identify-
ing filum terminale; if a CMAP is not elicited at this intensity, they
recommend sectioning the filum terminale (Quinones-Hinojosa et al,
2004; von Koch et al., 2002).
Though identification of neural tissue is the main objective of
stimulated EMG during TCS surgery, identifying the CMAP acti-
vation threshold after stimulation of the spinal cord before and after
release of the tether may provide prognostic information. A higher
stimulation threshold after untethering compared with before is
associated with postoperative worsening neurologic function (Hu-
sain et al., 2006).
FIGURE 1. Neurotonic discharge from the right hamstring muscle, indicative of nerve injury. Sweep duration 1 second full
screen. LQ, left quadriceps femoris; LAT, left anterior tibialis; LMG, left medial gastrocnemius; LH, left hamstring; RQ, right
quadriceps femoris; RAT, right anterior tibialis; RMG, right medial gastrocnemius; RH, right hamstring.
FIGURE 2. Triggered CMAP from the left medial gastrocnemius muscle from stimulation of the left L5 nerve root. Sweep duration
100 milliseconds full screen. LQ, left quadriceps femoris; LAT, left anterior tibialis; LMG, left medial gastrocnemius; LH, left ham-
string; RQ, right quadriceps femoris; RAT, right anterior tibialis; RMG, right medial gastrocnemius; RH, right hamstring.
Journal of Clinical Neurophysiology • Volume 26, Number 2, April 2009 Tethered Cord Syndrome Monitoring
Copyright © 2009 by the American Clinical Neurophysiology Society 77