Adjunct Diagnostic Studies – I Examination and Diagnostics

Eugene Carragee

Heidi Prather

T. Joseph Malbrough

Most diagnoses in patients with spinal disorders can be made on the basis of clinical presentation. Fractures, dislocations, disk 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 or structural findings that mirror the clinical presentation precisely, ancillary investigations using special diagnostic tests can be helpful. Electromyography (EMG) and nerve conduction studies (NCS) are objective tests and have established basic science in support of their use and findings. Others, such as provocative discography, remain somewhat 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. This test is not required in cases where the clinical presentation, including the physical examination, matches the structural disorder. In patients with less clear relationships between pathoanatomic findings and clinical symptoms, EMG, NCS, or both may be appropriate. Further, in patients with suspected or undiagnosed comorbidities (e.g., polyneuropathy) quantifying nerve function can be helpful in prognosticating recovery following surgery and to help set patient expectations regarding treatment outcomes. Unusual disorders including amyotrophic lateral sclerosis and some myopathies can be confusing in presentation in patients with coexisting spine pain. EMG and NCS studies will identify these disorders and can better direct necessary care.

Common indications for neurophysiologic testing include the following:

To evaluate signs and symptoms of neurologic impairment without clear pathology
To evaluate multiple anatomic (radiologic) levels of disease in the presence of focal neurologic signs (used to confirm the level). That is to confirm which, if any, MRI findings are actually causing physiologic changes in the nerve root.
To distinguish radicular (nerve root) symptoms from peripheral polyneuropathy or entrapment neuropathy
To distinguish spinal cord (myelopathy) from nerve root (radiculopathy) lesions
To distinguish a plexopathy from a radiculopathy
To identify physiological radiculitis not associated with an identifiable structural change
To evaluate prognosis and estimate acute versus chronic injury
To assess for a neuromuscular junction disorder, neuropathy, or myopathy in patients with weakness discordant with their imaging findings

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 a spinal nerve root.
Spinal cord injury usually presents with signs of central nervous system disorder (spasticity vs. pain/weakness); however, cord and root injuries can be present at the same time.
Psychological issues are common in patients with back pain syndromes. At times, it is difficult to differentiate true neurologic weakness from lack of volitional effort to move.

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
Myopathies
Motor neuron disease
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 (cerebellar, basal ganglia)
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

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 (axonotmesis):

Most frequently results in an abnormal EMG examination
Produces varying range of motor loss (mild to complete)
Commonly results in muscle atrophy
Has a guarded prognosis

Demyelination without axonal loss (neuropraxia):

Results in only mild or no muscle weakness
Uncommonly results in atrophy
Has a good prognosis with treatment (e.g., surgical decompression)
Usually results in a normal EMG examination and the compound motor action potential on NCS will be normal distal to lesion after 7 to 10 days

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. However, the biggest factor in obtaining accurate results is the skill of the performing physician.

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 and demyelinating disorders.
When positive, EMG can indicate the root or roots involved but not the precise site of pathology, particularly in the lumbar spine due to the cauda equina.
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 of the identified level.
EMG is diagnostic of a radiculopathy if abnormalities are present in two or more muscles innervated by the same nerve root and different peripheral nerves.
Some findings have lag time after injury.
- □ ∼1 week for most proximal muscles
- □ ∼6 weeks for distal extremity muscles
- □ Thus, there is potential for false-negative results in early examination
- □ Spontaneous activity refers to abnormal activity at rest, including fibrillations, positive sharp waves, and fasciculations. These are indications of acute denervation of the tested muscle.
Acute injury—fibrillation/fasciculation potentials, positive sharp waves.
Chronic injury—giant unit potentials with a long duration secondary to regeneration via axonal sprouting; >30% of motor units are polyphasic.

TABLE 3.2 COMPARISON OF BASIC FEATURES IN COMMON ELECTROPHYSIOLOGIC DIAGNOSTIC TESTS

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

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.