EMG BLIND SPOTS: MONONEUROPATHIES

Anthony Chiodo, MD, MBA

University of Michigan Health System

AAPMR Meeting, San Diego

November, 2014

Mononeuropathy

• Clinical syndrome of sensory and/or motor abnormalities in the distribution of a peripheral nerve

• Does not distinguish neuropraxia, axonotmesis, neurotmesis

Differential Diagnosis of Mononeuropathies

• Radiculopathy• Plexopathy• Myelopathy• Central Etiology• Myofascial pain• Just because patient has a certain constellation of

symptoms does not mean that they don’t have a mimicking diagnosis instead

Gold Standard

• Definitive determiner• Ultimate • Ideal• Reference measurement procedure• Measure of comparison for all other tests of the same

problem or disorder

What is the Gold Standard?

• We see patients with classic symptoms with normal studies• We see patients with different symptoms who have

abnormal studies• We see patients with symptoms of a different problem who

have “superimposed” MN• We see people who we screen with no symptoms with

abnormal nerve conduction studies• How useful is clinical presentation as a gold standard?• How useful is physical examination as a gold standard?

Blind Spot #1 in Mononeuropathies

• Patients with symptoms that have normal studies• Does the patient have a mimic?• Lengthens the diagnostic evaluation• How far to go in searching for an elusive diagnosis

• If not, treat what you think• How far do you treat?• Risk/benefit analysis may be hard to calculate with subjective

data only: Who’s the driver???

• Patients without symptoms that have abnormal studies• Can follow over time

Trouble with NCS

• Needle examination not commonly helpful• Sensitivity depends on the cut off used• Greater the sensitivity, the lower the specificity• IN OTHER WORDS, GREEN LIGHT FOR SURGICAL

TREATMENT• Greater the specificity, the lower the sensitivity• So, just because the nerve conduction studies are normal,

does that rule out nerve abnormality as a source of the patient’s complaint?

• In general, does not make a good gold standard

Sensitivity and Specificity

• Sensitivity: TRUE POSITIVE RATE• % Identified with the condition• True positive/(True positive + False negative)• Probability of Positive Test if you do have the condition• Specificity: TRUE NEGATIVE RATE• % identified without the condition• True negative/(True negative + False positive)• Probability of Negative Test if you don’t have the

condition

Case 1

• 54 presents with one year history of progressive numbness and tingling in the left 1st-3rd digits

• Symptoms worse first thing in the morning and with fine motor activities

• Notes no weakness• Physical examination: 2+ reflexes, strength 5/5, intact pin

sensation, positive Tinel’s, negative Phalen’s

Normal NCS, SymptomsNormal needle exam, responds to use of carpal tunnel splint

Amp: RT

Amp: LT

Latency: RT

Latency: LT

CV: RT

CT: LT

Median Sensory 2 24 3.8

Ulnar sensory 5 19 3.4

Median palm 30 2.3

Ulnar palm 25 2.2

Median motor 9.5 3.7 51

Ulnar motor 9.7 2.4 59

Highly Specific

• Just because it is highly specific does not mean that all patient’s with abnormal nerve conduction studies have clinical findings consistent with mononeuropathy

• 41 year old presents two weeks ago with new onset right sided neck pain and RUE numbness after fall

• MRI shows right C5-6 disc herniation• Physical examination: 2+ reflexes, 5/5 strength, non-

localizing sensory loss to light touch and pin

Abnormal NCS, No SymptomsNeedle examination is normal

Amp: RT

Amp: LT

Latency: RT

Latency: LT

CV: RT

CV: LT

Radial Sensory Forearm 32 2.3

Median Sensory 2 12 17 3.9 3.5

Ulnar Sensory 5 16 2.7

Median Sensory Palm 23 2.4

Ulnar Sensory Palm 19 1.9

Median Motor Wrist 7.5 6.4 3.9 4.3 48

Ulnar Motor Wrist 8.0 2.8 51

Screening to Predict CTSWerner, M+N, 2001.

• 77 workers with positive NCS but asymptomatic• Auto parts manufacturer, spark plug manufacturer, paper container

manufacturer, insurance company• Antidromic median and sensory responses to fingers 2 and 5 at 14 cm• Followed up to 70 months• Previous follow up to 17 months showed no difference between groups• 70% follow up rate• 23% with clinical symptoms of CTS compared to 6% of normal

screened (p = 0.01)• Not related to a change in nerve conduction studies!!!• Age, BMI and repetitive work were risk factors

How many studies do you do?

• Increase sensitivity?• Decrease specificity?• Increase sampling error?

How Technique Impacts Your Blind Spot (#2)

• The greater the error, the less findings are similar to standards

• AVOIDING ERRORS MAKES THE BLIND SPOT SMALLER

• Common causes of error in NCS• Temperature• Measurement, especially inching• Stimulus intensity

Will Imaging Save Us?

• In 2014, advanced CT, MRI and ultrasound are all very sensitive tests: Lumbar DDD, rotator cuff syndrome

• However, none have proven very specific• Lots of clinically normal patients with very abnormal

imaging studies. • So, if the image is abnormal, is it really correlative to the

patient’s pain complaint or is it just coincidental?

Interaction of Ultrasound ImagingBeekman, M+N, 2011.

• Seven of 14 studies in a critical review• Ulnar studies at the elbow: uses EMG/NCS diagnosis as

gold standard. Patients not studied if had symptoms and negative EMG/NCS

• Clinical criteria: Weakness of FDP/FCU OR hand intrinsic weakness with sensory changes in the fingers and hand, including DUC

Patients Controls Sensitivity Specificity

Maximal diameter in 2 locations > 2.4

84 45 81 91

CSA 2 locations > 8.8

33 14 46 NR

CSA 3 locations > 8.3

26 30 (B) 100 93

Diameter Ratio

27 20 NR NR

CSA 3 locations > 10

38 36 88 88

Diameter 3 locations

36 21 (B) 83 81

CSA two location and echotexture

38 23 (B) 54 96

Parameters for Positive Test

• Ulnar nerve thickening at the elbow: cross-sectional area or transverse diameter• 8.3 to 11 mm2 cut offs• Influence by controls: self, others, both arms in controls

• Maximal location• Predetermined locations (2-4)• Swelling ratio• Comparison to cubital tunnel CSA

Other nuances

• Echotexture interpretation• Inner fascicular structure

Causes

• Subluxation• Seen in healthy controls and no systematic comparison

• Snapping of the medial head of the triceps• Accessory muscles• See in 11% of cadavers, no systematic comparison

• Ganglia• Osteophytes• Tumor

CTS: NCS vs. ImagingDeniz, NS, 2012.

• 69 women with symptoms:• Motor weakness or• Positive Flick sign, median hypoesthesia, positive Tinel’s,

Phalen’s and reverse Phalen’s

• Negative work up for peripheral nerve disease• EMG/NCS: AANEM guidelines• Sensory studies to digits 1,2,3• Motor studies

• Ultrasound (54), CT (39) and MRI (50)• Both hands tested

Sensitivity Specificity

EMG 90.9 81.2

Ultrasound 83.7 78.6

CT 67.6 86.7

MRI 65 80

Guideline: Ultrasound in CTSCartwright, Muscle + Nerve, 2012

• 4 class I articles• Three had clinical findings and abnormal NCS• One had clinical findings and positive response to

conservative treatment• Three used opposite side as control if asymptomatic with

normal NCS, one used other patients

Class I Study Results

CTS Controls Sensitivity Specificity Area

Improved 40 40 100 93 9

#1 64 33 83 73 10

#2 78 23 82 87 10

#3 132 32 97 98 8.5

Anomalous Innervation: Blind Spot #3

• Martin-Gruber Anastomosis• Accessory Fibular (peroneal) Nerve• The All Ulnar Hand

Martin-Gruber

• Median to ulnar crossover of ulnar innervated muscles of the hand

• Can explain decreased motor evoked amplitude of the ulnar motor response stimulated at the elbow (false conduction block)

• Can explain increased motor evoked amplitude of the median motor response stimulated at the elbow

Martin-Gruber: Muscles Affected

• Innervate FDIH 21/22• Innervate Hypothenar 9/22• Innervate Thenar 3/22

Accessory Fibular Nerve

• Can explain increased motor evoked amplitude of the fibular motor response stimulated at the knee

QUESTIONS?Thank you!