Electromyography, Local Blocks/Injections, Discograms

By admin On Apr 3, 2024

Ovid: Spine

Editors: Bono, Christopher M.; Garfin, Steven R.

Title: Spine, 1st Edition

> Table of Contents > Section I
– Examination and Diagnostics > 3 – Electromyography, Local
Blocks/Injections, Discograms

Electromyography, Local Blocks/Injections, Discograms

Eugene Carragee

Most diagnoses in patients with spinal disorders can be
made on the basis of clinical presentation. Fractures, dislocations,
disc herniations, infections, and malignancies can be strongly
suspected on clinical grounds and most often are confirmed with imaging
studies. In the absence of clear pathologic findings (e.g., in patients
with age-related degenerative changes), ancillary investigations using
special diagnostic tests can be helpful. Some, such as electromyography
(EMG) and nerve conduction studies (NCS), are objective and have
established basic science in support of their use and findings. Others,
such as provocative discography, are controversial and do not have
clear basic science support in confirming a test-specific diagnosis.

NEUROPHYSIOLOGIC STUDIES (ELECTROMYOGRAPHY, NERVE CONDUCTION STUDIES, SOMATOSENSORY EVOKED POTENTIAL)

Neurophysiologic studies sometimes are used in the
evaluation of suspected spinal disorders that manifest neurologic
symptoms. Usually spinal disorders with neurologic symptoms are not a
diagnostic dilemma. The site of pathologic findings on imaging studies
corresponds to the symptoms and signs on neurologic examination. In
these cases, EMG and NCS rarely are indicated for diagnostic purposes.
In patients with less clear relationships between pathoanatomic
findings and clinical symptoms, EMG, NCS, or both may be appropriate.

Common indications for neurophysiologic testing include the following:

To evaluate symptoms and signs of neurologic impairment without clear pathology
To evaluate multiple anatomic
(radiologic) levels of disease in the presence of focal neurologic
signs (use to confirm the level)
To distinguish radicular (nerve root) from peripheral neuropathy
To distinguish spinal cord (myelopathy) from nerve root (radiculopathy) lesions
To evaluate prognosis and estimate acute versus chronic injury

Causes of Nerve Symptoms and Signs

Nerve injury with spinal disorders can occur at multiple
levels and may be associated with or be confused by concomitant
(extraspinal) neurologic dysfunction.

The most common cause of neurologic symptoms in spinal disorders is irritation or compression of spinal nerve root.
Spinal cord injury usually presents with
different signs than root injury (spasticity versus pain/weakness);
however, cord and root injuries can be present at the same time.
Psychological issues are common in
patients with back pain syndromes. It often is difficult to
differentiate true neurologic weakness from lack of volitional effort
to move.

Table 3-1 lists a practical set
of neurologic presentations and common diagnoses. This is not an
all-inclusive list but rather a reflection of the more common causes
and important differential diagnoses in each group.

Nerve Injury and Electromyography and Nerve Conduction Studies

Axonal loss with destruction of the axon and myelin sheath:

Most frequently results in an abnormal EMG examination

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Produces varying range of motor loss (mild to complete)
Commonly results in muscle atrophy
Has a guarded prognosis

TABLE 3-1 PRACTICAL DIFFERENTIAL DIAGNOSIS OF NEUROLOGIC SYMPTOMS IN SPINAL DISORDERS

Weakness and Pain
	Root injury/compression
	Peripheral nerve injury/compression (mechanical injury, diabetic, or toxic neuropathy)
	Intrinsic nerve disease (herpes zoster, nerve tumor)
	Spinal cord injury (unusual)
	Emotional/psychological disturbance (nonneurologic weakness)
	Poor effort (pain or secondary gain issues)
Painless Weakness
	Spinal cord injury or myelopathy
	Brain injury (e.g., motor cortex, internal capsule)
	Intrinsic nerve disease (polio, multiple sclerosis, amyotrophic lateral sclerosis)
	Emotional disturbance (“hysterical paralysis,” fear of injury)
	Root or peripheral nerve injury (unusual)
Predominantly Discoordination, Ataxia
	Brain injury
	Spinal cord injury
	Mild (inapparent) weakness
	Vestibular disease
	Psychological disturbance (flamboyant unsteadiness or collapse while under observation)
Painless Sensory Changes without Weakness
	Peripheral neuropathy
	Psychological disturbance (e.g., “conversion reaction”)
	Root injury (unusual)
	Spinal cord injury or disease (e.g., posterior column disease, vitamin B₁₂ deficiency)
	Brain injury

Demyelination without axonal loss:

Results in only mild or no muscle weakness
Uncommonly results in atrophy
Has a good prognosis with treatment (e.g., surgical decompression is better)
Usually results in a normal EMG examination

Weakness from deconditioning, poor cooperation, and psychological factors do not influence nerve injury patterns on EMG or NCS.

Severe paralysis attributable to root injury on physical examination cannot be normal on EMG.
EMG and NCS data, when carefully acquired and interpreted, are objective and independent of patient effort.

Common Electrodiagnostic Procedures

EMG, NCS, H-reflex, and somatosensory evoked potential (SSEP) are compared in Table 3-2.

Electromyography

EMG is the most useful test for radiculopathies.
When positive, EMG can indicate injury distribution and level.
Level of injury is estimated by usual innervation pattern of extremity muscles.
Overlap and anatomic variation of innervation are common.
Best estimation is to within one or two segments.
Some findings have lag time after injury.
- 1 week for most proximal muscles
- 6 weeks for distal extremity muscles
- Potential for false-negative results in early examination
- Spontaneous activity
  refers to abnormal activity at rest, including fibrillations and
  fasciculations. These are indications of denervation of the tested
  muscle.
Acute injury—fibrillation/fasciculation potentials
Chronic injury—giant unit potentials

Contraction activity refers
to the shape, amplitude, and duration of nerve firing and the number of
phases of electrical potential compared with a normal muscle unit.

Giant amplitude potentials indicate renervation.
Some estimation of severity can be made (not very accurate).

H-Reflex

H-reflex is a monosynaptic spinal reflex specific for S1.
H-reflex is abnormal immediately after injury.
It is abnormal with S1 radiculopathy and corresponds to ankle reflex on physical exam.
It also is abnormal in advanced age,
tibial or sciatic nerve injuries (e.g., trauma, hip arthroplasty), and
peripheral neuropathies (e.g., diabetic).
H-reflex cannot differentiate acute from chronic injury.

Nerve Conduction Studies

NCS measure amplitude and speed of an applied signal traveling along a peripheral nerve.
Velocity can be normal in radiculopathy proximal to the dorsal root ganglion.
Amplitude should be normal, unless massive or multilevel axonal loss has occurred.
When abnormal, it is usually due to peripheral neuropathy.

Somatosensory Evoked Potential

SSEP measures conduction between a large peripheral nerve and the cerebral cortex or the spinal cord.
SSEP has poor detection of radiculopathy lesions.
It is sensitive to certain spinal cord
pathway lesions, including traumatic spinal cord injury or tumor,
compressive myelopathy, and multiple sclerosis.
Deficits in motor pathways may be missed.

DIAGNOSTIC ANESTHETIC INJECTIONS

Diagnostic anesthetic injections are diagnostic procedures that rely on the premise that if pain is relieved by an anesthetizing

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injection, that structure is a likely cause of the patient’s pain. This
method is used most commonly in the evaluation of nonspecific back
pain. Local injections to the zygapophyseal (facet) joint, sacroiliac
(SI) joint, spondylolytic defects, or selective nerve root blocks at
the neural foramen are the most commonly used. Criteria for a positive
result are arbitrary but can be estimated as the degree of subjective
pain relief (e.g., 50%, 75%, 100%). In most cases, the predictive value
of positive or negative blocks is not known because of the lack of an
objective gold standard measure to confirm the result.

TABLE 3-2 COMPARISON OF BASIC FEATURES IN COMMON ELECTROPHYSIOLOGIC DIAGNOSTIC TEST

		EMG	NCS	H-Reflex	SSEP
Root level specific		Moderate (1-2)	No	S1 only	No
Immediately positive after nerve injury		No	Yes	Yes	Yes
	with spinal cord injury	No	No	No	Yes
	with moderate radiculopathy	Yes	No	Yes	No
	with massive or multiple root injury	Yes	Yes/no	Yes	Yes/no
	in peripheral neuropathy	No	Yes	Yes	Yes/no
Different in acute versus chronic injury		Yes	No	No	No
EMG, electromyography; NCS, nerve conduction study; SSEP, somatosensory evoked potential.

There are theoretical problems with anesthetic injections as a diagnostic technique, as follows:

Injections proximal to a lesion may block
afferent pathways. Pain from peroneal nerve entrapment may be relieved
with a selective nerve root block of L5.
Injections distal to a lesion (e.g.,
sciatic nerve anesthetic in root compression) also can give pain
relief, albeit through an unclear mechanism. More worrisome is the
overlap of segmental innervation of the spine from a single dorsal root
ganglion. A single sensory afferent neuron may have branches that
converge from two or more spinal segments (discs, bone, facets).
Anesthetic injections to a painless site may modulate the input from a
distant painful site, giving a misleading result.
Placebo effects unrelated to presence or absence of a lesion may be 30% with spinal injections.

Facet Joint Injections

Estimates of facet joint pain incidence in chronic low
back pain (LBP) range from 15% to 40% based on diagnostic anesthetic
blockage. These studies have no gold standard to confirm the diagnosis,
however. Facet stimulation in normal subjects can induce pain in
certain referred patterns with modest reproducibility, but this typical
pattern has not been confirmed in symptomatic subjects. Experimental
facet joint pain (distention with injection) can be blocked with
simultaneous local anesthetic blockade of the medial branches of the primary dorsal rami above and below the stimulated facet.

There is a poor correlation between clinical symptoms,
signs, or radiographic evidence of facet arthrosis and response to
anesthetic facet blockade. The failure to identify any reliable
clinical syndrome or radiologic finding with facet block results may
indicate the following:

The painful lesion being locally
anesthetized simply is not detectable by imaging studies and has a
variable clinical presentation.
The test does not identify a true
clinical entity of any sort, and the response is related to secondary
pain pathways or perception.

Differential Anesthetic Blocks

Some authors suggest the use of sham injections or
injections with short-acting and long-acting anesthetics as a means of
increasing the specificity of diagnostic injection. Using this method,
one group found ablation procedures relatively effective. These authors
also reported, however, no difference in duration of pain relief when
short-acting versus long-acting anesthetics were used in this trial
(both about 4 to 5 hours). These findings reinforce the argument that
diagnostic blocks are poorly understood pharmacologically and
neurophysiologically.

Sacroiliac Joint Injections

Diagnosis of SI joint problems usually is clear in cases
of trauma (SI joint dislocation, Malgaigne’s fracture), infection, or
inflammatory disease; diagnostic injections usually are not needed or
performed. With less clear presentation, SI joint injections are
prompted by suggestive signs on physical exam. A positive response to
provocative maneuvers (Faber and Galen tests) does not correlate,
however, with a positive response to an SI joint anesthetic injection.
In one small series, 20% to 30% of selected chronic LBP patients
responded to a single or differential (short-acting versus long-acting
agents) anesthetic block. The predictive value of SI joint blockade is
unknown because of a lack of an objective gold standard test (i.e.,
pain resolution is subjective).

In summary:

Patients who respond to SI joint
injections do not always have a characteristic history or response to
provocative maneuvers on physical exam.
It is not clear whether patients who respond to SI injection with some pain relief represent only subjects with

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“true” SI joint pathology, or whether they are a diverse group with various local, regional, or complex pain sites.
It has not been shown that a specific
pathology found on imaging studies is seen only in subjects with
response to SI joint anesthetic injections.

PROVOCATIVE DISCOGRAPHY

Discography first was developed as a method to identify
herniated discs in the lumbar spine. It coincidentally was noted that
sometimes during injection of the contrast material into the disc the
patient’s usual sciatic symptoms were reproduced. With continued
experience, it also was reported that familiar back pain could be
reproduced during disc injection. The finding that disc injection in
some patients seemed to reproduce their usual and typical back pain has led to use of discography as an evaluative maneuver in determining the cause of chronic LBP.

Technique

Discography technique has evolved over time. Compared
with original methods, injections use less irritating contrast dye, are
directed by image guidance, and employ limited pressure. Discography is
performed by injection of a water-soluble, radiopaque dye under
fluoroscopic guidance into several intervertebral discs.

The central portion of the disc is
penetrated percutaneously by a long fine-gauge needle through an
introducer (large-gauge needle) in a “two-needle” technique. This can
be done from a posterolateral approach in most cases. At L4-5 and
L5-S1, the needle usually is bent into a gentle curve before
introduction.
Discography should be performed with
minimal sedation and local anesthetic at the skin puncture site. The
patient needs to be comfortable but alert and able to describe the pain
response to injections.
The dye is injected slowly into the
nucleus of several lumbar discs with the patient blinded to the timing
and site of injection.
The distribution and extravasation (if
present) of the dye in the disc is noted, as is the patient’s clinical
response to injection.
Two key features of clinical response are noted:
- Pain intensity:
  The patient is asked whether each injection is painful and is asked to
  rate the pain against a standardized scale (e.g., 0 to 5, 0 to 10,
  “none” to “unbearable”).
- Concordancy:
  The discomfort provoked by the injection, if any, is rated as similar
  (concordant) or dissimilar to the usual LBP. A patient with similar or
  exact pain reproduction usually is considered to have a concordant pain
  response.

Criteria for a Positive Discogram

Basic criteria: A positive test includes “significant pain” with injection and a reproduction of usual pain.
Walsh criteria:
Based on a study in 1990 by Walsh et al, a positive response is defined
as 3/5 or 6/10 and patient describing the pain as bad or worse. These
authors also required that an observable pain reaction or pain behavior
be present.
Other criteria: The following are used to limit the risk of false-positive injections:
- Control disc injection:
  Many discographers require at least one disc injection to be negative
  before an adjacent disc can be considered positive. It is not clear in
  the literature what a “negative” disc means (e.g., painless with
  injection, not “significantly painful,” or not concordantly painful).
  This confusion has led most practitioners to include the disc above and
  below the suspected painful disc as a control levels.
- Exact pain reproduction:
  Some authors recommend that only “exact” pain reproduction be
  considered a positive response. Clinical and experimental studies have
  shown that patients with regional pain, not coming from the
  intervertebral disc, may confuse disc and regional pain and report both
  as exactly concordant with disc injections.
- Annular disruption:
  Some authors require the dye to reach the outer anulus. Morphologically
  normal discs or discs without fissuring to the outer anulus cannot be
  positive by this criteria.
- Pressure-controlled injection:
  Some authors propose that disc injections be limited to less than 80 to
  100 psi and that positive responses to low-pressure injections (<15
  to 20 psi) be differentiated from positive responses that occur only at
  higher pressures. This criterion was based on an observation of
  differential outcomes in a small cohort of low-pressure positive discs,
  which since have been termed chemically sensitive (rather than mechanically sensitive).
  Experimental studies in subjects without chronic LBP show less frequent
  positive injections at these low pressures, but still a false-positive
  injection rate at low pressures ranging from 10% to 30%.

Specificity of Discography

The unresolved question remains: Does a positive
injection of a disc, by whatever criteria, definitively identify the
disc as a cause of chronic LBP? Because provocative discography relies
entirely on the subjective reporting of pain perception, the pain
intensity and concordancy are subject to the neurophysiologic
modulation of pain pathways, which may amplify or downregulate
nociceptive signals from the stimulated disc.

The concept of modulation of pain perception should be
considered. Pain may begin at a local structure (e.g., disc, muscle,
ligament), but the transmission of the nociceptive signal is modulated
along the neural axis (i.e., local nerves, autonomic nerves, dorsal
root ganglion, dorsal horn and ascending tracts, thalamic and
associated processing areas of the cortical and higher/complex pain
processing regions) (Figs. 3-1, 3-2, and 3-3).

Discography in Asymptomatic Subjects

Walsh et al studied 10 healthy young men with little
disc degeneration and no known psychological or chronic pain
predisposing to pain amplification. One of 10 subjects had pain at 6/10
level (10% significant pain) with disc injection (Fig. 3-4).
Carragee et al studied 10 subjects asymptomatic for LBP with known
degenerative disc disease and no chronic pain processes: 10% had
significant pain with injection. In another study, Carragee et al
studied 16 subjects asymptomatic for LBP but with distant chronic pain
syndromes and increased psychological distress. Of subjects, 56% (9/16)
had significant pain with injection.

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Figure 3-1
Schematic of pain signal pathways for low back pain (LBP) with
modulation from signal amplification factor and downregulators. DRG,
dorsal root ganglion.

Figure 3-2
Scenario of Pain Amplification Pathway: Low back pain (LBP) from
minimal degenerative changes in a patient with multiple chronic pain
syndromes, chronic narcotic habitation, depression, and workers’
compensation claim (social disincentive). DRG, dorsal root ganglion.

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Figure 3-3
Scenario of down regulation pathway. Low back pain (LBP) from heavy
physical training in well-motivated and conditioned soldiers. DRG,
dorsal root ganglion.

Further studies of disc injections in subjects with no
or minimal discogenic pain have shown the following risk factors for
amplified pain response to disc injections:

Figure 3-4
Percent of asymptomatic subjects with pain (≥6/10) on disc injection in
different subgroups. Note increasing risk factors for false-positive
injections. Asx, asymptomatic; DDD, degenerative disc disease; HIZ,
high intensity zone.

Increased psychological distress
Nonlumbar chronic pain syndrome (concurrent)
Disputed compensation claims
Previous discectomy at the injected disc
History of persistent clinically benign backache

All of these factors can increase the risk of
false-positive injections if found in subjects with chronic LBP. These
subjects may be at higher risk for a positive disc injection at levels
that are not the primary cause of their chronic LBP.

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Discography as Predictor of Clinical Outcomes

Calhoun et al, in an uncontrolled comparison of patients
undergoing spinal fusion-based discography and imaging versus imaging
studies alone, reported a moderate increase in success in the
discography group. More recently, using a historical control design,
adding discography to the preoperative evaluation was not found to
improve outcomes.

Summary

Provocative discography is an invasive method used to
pressurize a suspected disc and elicit a pain response. The specificity
of discography in determining the cause of LBP is unknown.
False-positive rates seem to vary with psychological distress, chronic
pain states, compensation issues, and certain morphologic changes
(annular disruption). Studies are conflicting whether the use of this
technique can improve clinical outcomes of various interventions in
patients with chonic LBP (Table 3-3).

TABLE 3-3 PRACTICAL GUIDE TO DISCOGRAPHY USE

Best case utility
	Negative discogram to determine end of fusion in deformity or other clear pathology
	Positive, single-level disc in subject without risk factors for false-positive injection (e.g., normal psychological profile, no chronic pain behavior or history, no compensation issues)
Unclear utility
	Positive two-level discs, but no risk factors
	Postoperative discs, but otherwise no risk factors
	Intermediate (at risk) psychological profile, single level
Poor utility
	Spine with multilevel pathology
	Abnormal or very chronic pain behavior
	Abnormal psychometric findings
	Disputed compensation cases