NCS IN PRACTICE-1

This 34 years old lady, a known case of bilateral cervical rib, who had undergone right sided cervical rib resection in year 2003, came to us for nerve conduction studies.
According to history provided, she was doing well after the operation. But for last 2 years, she started feeling pain in left hand, and for last 2-3 months, she was also complaining of pain in her right hand.

Following are her findings on NCS testing (shown in figure):

Following is her brief report:
This 34 years old FEMALE, case of bilateral cervical rib, right side operated in 2003, was evaluated for pain in both hands with NCS, and the following observations were made:
MNCS:
Bilateral median latencies increased. Right median amplitude decreased. Latencies, amplitudes and conduction velocities were normal in bilateral ulnar nerves.
SNCS:
Bilateral median latencies increased, and conduction velocities decreased. A relative decrease in right median SNAP amplitude noted. Latencies, amplitudes and conduction velocities were normal in bilateral ulnar nerves.

IMPRESSION:
Abnormal study, showing bilateral moderately severe carpal tunnel syndrome.

Clearly, our study revealed the cause of her pain as CTS, and not TOS.

INTRODUCTION TO POLYSOMNOGRAPHY

INTRODUCTION TO POLYSOMNOGRAPHY
Polysomnography or sleep study is done to assess a patient with suspected sleep related disorders. Common components assessed by sleep study are:
1. EEG –to assess various stages of sleep and their association with disorders (e.g. REM sleep behaviour disorder or RBD is likely to be seen in REM sleep).
2. Respiratory functions – including effort, air flow, pulse oximetry (e.g. in obstructive sleep apnea, we expect cessation of airflow through the nose or mouth with persistence of effort)
3. Limb movements –by placing electrodes over tibialis anterior muscles (e.g. in periodic limb movements of sleep, findings will be seen in these channels)
4. ECG-to simultaneously assess cardiac functions (e.g. to see the effect of desaturation in a patient of OSA on cardiac rhythm)


References :
1. Mc Gregor PA. Updates in polysomongraphic recording techniques used for the diagnosis of sleep disorders. Am J EEG Technol 1989;29:107
2. Antonio Culebras, Clinical Handbook of Sleep Disorders: Butterworth-Heinmann

INTRODUCTION TO EVOKED POTENTIALS

EVOKED POTENTIALS
There are three main types of evoked potentials being used in clinical practice- visual, auditory and somatosensory.

The principle on which these evoked potentials are measured is that brain is the final destination of all the sensations. If we stimulate sensory, auditory or visual pathway, and record tiny electrical potentials so generated in brain by placing electrodes over scalp, we can demonstrate the integrity of these pathways. Further abnormalities suggest various types of diseases.

In its simplest form, visual evoked potential is performed by asking patient to look at TV screen where a visual stimulus is provided (usually one eye at a time) and recording is made by electrodes placed over the occiput.

Auditory evoked potential is recorded by providing (one by one) auditory click stimuli via headphones and responses are recorded by electrodes placed over mastoids.

Somatosensory potentials are recorded by electrically stimulating a peripheral nerve like median or posterior tibial nerves and responses are recorded over scalp (laterally for median and midline for posterior tibial, corresponding to arm and leg areas of homunculus).

MEDIAN SENSORY CONDUCTION STUDY

Median sensory conduction study (orthodromic) is performed by placing active electrode over the median nerve at wrist, (midline), reference electrode about 3 cm proximal to active electrode and ground electrode b/w stimulator and active electrode. Median nerve is stimulated at the index finger. (See figure)

Usually, a distal latency in excess of 3.5 ms is taken as abnormal. Similarly, SNAP (sensory nerve action potential) amplitude of less than 5 µv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.

It is difficult to get SNAP response by single stimulation. Thus many responses are averaged to get final response. SNAP parameters are best judged by comparing values from contralateral sides.

Reference:

1. Anatomy of Median Nerve
2. Routine Nerve Conduction Study
3. SNCS – Parameters
4. Recording Electrodes-For Sensory Studies
5. Nerve Conduction Studies – Introduction
6. Recording Procedure-Nerve Stimulation

PERONEAL MOTOR CONDUCTION STUDY

Peroneal motor conduction study is performed by placing active electrode at extensor digitorum brevis (EDB) muscle, reference electrode at metatarsophalangeal joint of little toe and ground electrode b/w stimulator and active electrode.

Peroneal nerve is stimulated at the ankle, below fibular head and in popliteal fossa. Stimulation of nerve in popliteal fossa is difficult because nerve is lying deep in the fossa. Peroneal nerve is frequently damaged near fibular head.

Usually, a distal latency in excess of 5.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 2 mv or conduction velocity of less than 40 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.

Reference:

1. Anatomy of Peroneal Nerve


2. Routine Nerve Conduction Study


3. MNCS – Parameters


4. Recording Electrodes-For Motor Studies


5. Nerve Conduction Studies – Introduction


6. Recording Procedure-Nerve Stimulation
   

TIBIAL MOTOR CONDUCTION STUDY

Tibial motor conduction study is performed by placing active electrode at abductor hallucis brevis (AH) muscle, reference electrode at metatarsophalangeal joint of great toe and ground electrode b/w stimulator and active electrode.
Tibial nerve is stimulated behind medial malleolus and in popliteal fossa. Stimulation of nerve in popliteal fossa is difficult because nerve is lying deep in the fossa.

Usually, a distal latency in excess of 5.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 40 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.




Reference:

1. Anatomy of Tibial Nerve


2. Routine Nerve Conduction Study


3. MNCS – Parameters


4. Recording Electrodes-For Motor Studies


5. Nerve Conduction Studies – Introduction


6. Recording Procedure-Nerve Stimulation

ULNAR MOTOR CONDUCTION STUDY

Ulnar motor conduction study is performed by placing active electrode at abductor digiti minimi (ADM) muscle, reference electrode about 3 cm distal to active electrode and ground electrode b/w stimulator and active electrode.
Ulnar nerve is stimulated at wrist, below elbow and above elbow. This is because elbow is a common site of nerve damage, thus identifying any evidence of focal demyelination in this segment of nerve is important. Usual evidence of focal demyelination is in the form of conduction block, which is defined as -fall in CMAP amplitude on proximal stimulation in excess of 50%, as compared to distal stimulation. Other evidence is in the form of presence of focal slowing. Identifying these features at elbow, and therefore stimulating the nerve both above and below elbow is important in this study.

Usually, a distal latency in excess of 3.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.

Reference:

1. Anatomy of Ulnar Nerve


2. Routine Nerve Conduction Study


3. MNCS – Parameters


4. Recording Electrodes-For Motor Studies


5. Nerve Conduction Studies – Introduction


6. Recording Procedure-Nerve Stimulation

MEDIAN MOTOR CONDUCTION STUDY

Median motor conduction study is performed by placing active electrode at abductor pollicis brevis (APB) muscle, reference electrode about 3 cm distal to active electrode and ground electrode b/w stimulator and active electrode. Median nerve is stimulated at wrist and elbow.

Usually, a distal latency in excess of 3.8-4.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.



Reference:

1. Anatomy of Median Nerve
2. Routine Nerve Conduction Study
3. MNCS – Parameters
4. Recording Electrodes-For Motor Studies
5. Nerve Conduction Studies – Introduction
6. Recording Procedure-Nerve Stimulation

TIBIAL NERVE ANATOMY

The tibial nerve is the larger of the two major divisions of the sciatic nerve. It is derived from L5, S1 and S2 roots. It leaves the popliteal fossa between the heads of the gastrocnemius and supplies all muscles in the posterior compartment of the legs, i.e. gastrocnemius, soleus, plantaris, popliteus, flexor digitorum longus, flexor hallucis longus and tibialis posterior.

At the ankle the tibial nerve runs posterior to the medial malleolus under the flexor retinaculum (tarsal tunnel) to enter the foot. While coming out of (or within) tarsal tunnel, the nerve divides into four branches.

Two of these, the medial and lateral calcaneal nerves are purely sensory and supply sensation to the heel.

The other two branches, the medial and lateral plantar nerves innervate the intrinsic muscles of the foot and provide sensation to the medial and lateral sole respectively. Notably, medial plantar nerve supplies abductor hallucis brevis and lateral plantar nerve supplies abductor digiti quinti pedis.

Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition


2. Cimino WR. Tarsal tunnel syndrome: review of the literature. Foot Ankle 1990, 11:47.


3. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition

ROUTINE NERVE CONDUCTION STUDY

Routine nerve conduction studies are performed to screen major nerves in both upper and lower limbs to get an idea about their normal function. The study is modified depending upon the clinical diagnosis or as requested by referring doctors (which are usually physicians, orthopedic surgeons, neurosurgeons or neurologists).

A routine test includes motor conduction studies performed on median and ulnar nerves in upper limb and posterior tibial and peroneal nerves in lower limbs. Sensory conduction studies are performed on median and ulnar nerves in upper limbs and sural nerves in lower limbs.

Therefore, if patient presents with wrist drop, then radial motor and sensory conduction study may be needed in addition to the above mentioned studies. Similarly, if patient presents with foot drop, superficial sensory studies may be required.

ELEMENTS OF NORMAL SLEEP ACTIVITY - POSTERIOR OCCIPITAL SHARP TRANSIENTS (POSTs)

Shape
Mono- or biphasic triangular waves.

Amplitude
30-50 µv

Frequency / duration
Variable, from 1 Hz to 4-6 Hz

Distribution
Occipital

Persistence
They may last up to few seconds, and are seen mainly in stages 1, 2 & 3 of NREM sleep

Synchrony
After 2 years of age, they are bisynchronous and symmetrical


Reference:

1. Erwin, CW, Somerville, ER and Radtke, RA. A review of electroencephalographic features of normal sleep. J. Clin. Neurophysiol. 1:253-274
2. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
3. Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition

ELEMENTS OF NORMAL SLEEP ACTIVITY - SLEEP SPINDLES

Shape
Group of rhythmic waves characterized by gradually increasing and decreasing amplitude.

Amplitude
Usually less than 50 µv, may decrease with age



Frequency / Duration
12-14 Hz / Duration more than 0.5 seconds

Distribution
They are characteristically frontocentral in location

Persistence
They may last up to few seconds, and are seen in mainly stages 2 and 3 of NREM sleep

Synchrony
After 2 years of age, they are bisynchronous and symmetrical

Miscellaneous

1. Sleep spindles are usually well developed by 3-6 months of age, appearing in prolonged runs lasting 8s or longer separated by interval of less than 10 s. After that time, the duration of spindle bursts decreases.
2. Spindles are commonly asynchronous over the two hemispheres until the age of 8 months in normal infants; continuously asynchronous spindles after 2 years of age are abnormal.
3. Spindle bursts are fairly asymmetrical in normal infants, but a marked and persistent reduction on one side may suggest ipsilateral cerebral dysfunction.
   

Reference:

1. Hughes JR. Sleep spindles revisited. J. Clin. Neurophysiol. 2: 37-44.


2. Jankel, WR and Niedermeyer, E. Sleep spindles. J. Clin. Neurophysiol. 2: 1-36.


3. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition


4. Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition

COMMON PERONEAL NERVE ANATOMY

Sciatic nerve originates form the L4 thru’ S2 roots. It leaves pelvis by passing thru’ the greater sciatic foramen and enters thigh. In the upper popliteal fossa, it divides into common peroneal and tibial nerves. Within the sciatic nerve, the fibers that eventually form the common peroneal and tibial division run separately from each other.

In the upper thigh, tibial division provides innervation to all hamstring muscles except short head of biceps femoris which is supplied by the peroneal division. Thus, short head of biceps femoris is the only peroneal innervated muscle above knee joint.

Soon after separating from tibial division, the common peroneal gives off the lateral cutaneous nerve of the calf, which innervates the skin over the upper third of the lateral aspect of the leg (not highlighted in figure). Then the peroneal nerve winds around the fibular neck and divides into its terminal braches, the superficial and deep peroneal nerves.







Superficial peroneal nerve
The superficial peroneal nerve is predominantly sensory; it innervates the skin of the lower two thirds of the lateral aspect of the leg and the dorsum of the foot and sends motor branches to the peroneus longus and brevis.














Deep peroneal nerve
The deep peroneal nerve is predominantly motor; it innervates tibialis anterior, extensor hallucis, extensor digitorum longus & brevis (all ankle and toe extensors) and peroneus tertius. It sensory branches supply the skin of the web space b/w the first and second toe.


Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
   
2. Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999
   
3. Katriji MB, Wilbourn AJ. Common peroneal neuropathy: a clinical and electrophysiologic study of 116 lesions. Neurology 1988;38:1723.

ELEMENTS OF NORMAL SLEEP ACTIVITY - K COMPLEXES

SYNONYM = K WAVE
Shape
Consist of an initial sharp component, followed by a slow component that fuses with a superimposed fast component. It may or may not be followed by sleep spindles. It is easily differentiated from vertex waves by longer duration and greater complexity and variation.

Amplitude
More than 200 µv in monopolar

Duration
More than 500 ms

Distribution
Frontal and vertex region

Persistence
They are seen at irregular intervals in stages 2, 3 and 4 of NREM sleep.

Synchrony
Bisynchronous

Miscellaneous

1. V waves and K complexes appear in well developed from for the first time at the age of 5-6 months.


2. They can be elicited during sleep by sensory stimulation (particularly auditory). The positive component usually occurs 0.75 seconds after the stimulus.

Reference:

1. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition


2. Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition


3. Stern JM, Engel J. Atlas of EEG patterns, Philadelphia: Lippicott Willams & Wilkins
   
   

ELEMENTS OF NORMAL SLEEP ACTIVITY – VERTEX WAVES

SYNONYMS = V WAVES, VERTEX SHARP TRANSIENTS
Shape
Sharp contoured compounded potentials

Amplitude
Up to 250 µv

Frequency / duration
Less than 2 Hz / Duration less than 200 ms

Distribution
Maximum at vertex (C3, C4) but may have wider distribution

Persistence
They are seen at irregular intervals in stages 1 and 2 of NREM sleep

Synchrony
Bisynchronous, may be unilateral

Miscellaneous

• V waves and K complexes appear in well developed from for the first time at the age of 5-6 months.
• They are most likely secondarily to auditory evoked potentials that converge from their cortical projection areas to a region underlying the vertex electrodes.

Reference:
1. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
2. Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition
3. Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company
4. Kooi, K. A. et al. Polarity and field configuration of the vertex components of the human auditory evoked response: a reinterpretation. Electroencephalogr. Clin. Neurophysiol. 31:166-169

RADIAL NERVE ANATOMY

The radial nerve is the largest branch of the brachial plexus. The radial nerve is derived primarily from the C5, C6, C7, C8 and T1.
The radial nerve enters the arm from axilla along the medial side of the humerus to reach the spiral groove. From the axilla to the spiral groove, the radial nerve gives off motor branches to triceps and anconeus. It also receives the posterior cutaneous nerve of the arm, the posterior cutaneous nerve of the forearm and the lower lateral cutaneous nerve of the arm in this region.
At the spiral groove, the radial nerve is in contact with the humerus as the nerve travels laterally, and then it pierces the lateral intermuscular septum. Here, it is bordered medially by the brachialis (Br) muscle, and laterally (from proximal to distal) by the brachioradialis (BR), the extensor carpi radialis longus (ECRL), and the extensor carpi radialis brevis (ECRB). All these muscles receive motor supply from radial nerve. The radial nerve then crosses the elbow joint anterior to the lateral epicondyle of the humerus. At the elbow the radial nerve divides into a motor nerve, the posterior interosseus nerve; and a sensory nerve, the superficial radial nerve.

Posterior Interosseus Nerve
The posterior interosseus nerve enters and innervates the supinator (Sup) muscle. The nerve then gives motor branches to - extensor digitorum communis (EDC), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis longus (EPL), extensor pollicis brevis (EPB), and extensor indicis proprius (EIP).

Superficial Radial Nerve (SRN)
At the elbow, the superficial radial nerve stays superficial to the supinator and proceeds anterolaterally, deep to the BR muscle. Approximately at the junction of the proximal two thirds and the distal one third of the forearm (approx 8 cm from tip of radial styloid), the SRN becomes superficial and crosses over to the posterior aspect of the distal radial forearm, passing superficial to the tendons of the anatomical snuffbox (APL, EPL, and EPB) and traversing the wrist over the extensor retinaculum. It supplies cutaneous sensation to the dorsal surface of the lateral hand, as shown in the figure.

Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
2. Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999
3. http://www.wheelessonline.com/ortho/the_superficial_branch_of_the_radial_nerve_an_anatomic_study_with_surgical_implications
   

ULNAR NERVE ANATOMY

The ulnar nerve derives from C8 and T1 nerve roots. It runs on the medial aspect of upper arm, and gives off no branches in the upper arm. It passes posterior to the medial epicondyle of the humerus to enter the cubital tunnel. Near elbow, ulnar nerve gives motor branches to flexor carpi and medial portion of flexor digitorum profundus.

In forearm, it gives rise to a palmar cutaneous branch which arises near the middle of the forearm and supplies the skin on the medial part of the palm, and the dorsal cutaneous branch which arises in the distal half of the forearm and supplies cutaneous sensation on the dorsal, ulnar surface of the hand and digits 4 and 5.

At the wrist, the nerve enters Guyon's canal and divides into a superficial sensory and deep motor branch. The superficial branch supplies sensation to the palmar surface of the ring and the little fingers. The deep motor branch supplies abductor digit minimi flexor digit minimi muscles, opponens digiti minimi, third and fourth lumbricals, the palmar and dorsal interossei, the flexor pollicis brevis and adductor pollicis brevis and first dorsal interosseous. In essence, ulnar nerve supplies all small muscles of hands except abductor pollicis brevis, flexor pollicis brevis, opponens pollicis and 1st and 2nd lumbricals (which are supplied by median nerve).

Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
2. Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999
3. http://depts.washington.edu/anesth/regional/ulnarnerve.html

SENSORY CONDUCTION STUDIES - PARAMETERS

Sensory axons are evaluated by stimulating a nerve while recording the transmitted potential (known as sensory nerve action potential or SNAP) from the same nerve at a different site. (See orthodromic and antidromic studies)
SNAPs are of much lower amplitude (measured in millivolts) than compound muscle action potentials, and they often require averaging of multiple responses. Three main parameters recorded with sensory nerve conduction studies are – latencies, amplitudes and conduction velocity.

LATENCY

Latencies reflect time taken (in milliseconds) for an impulse to travel from the point of stimulation to the recording electrode. Two types of sensory distal latencies are used – peak latency and onset latency.

SNAP AMPLITUDE
This is a semiquantitative measure of the number of sensory axons that conduct between the stimulation and recording sites. It is expressed in microvolts.

CONDUCTION VELOCITY
This requires stimulation at a single site only because unlike CMAP, SNAP is true nerve action potential. (See motor conduction studies – parameters).

Reference:

1. Aminoff, MJ. Electrodiagnosis in Clinical Neurology: Nerve conduction studies, New York: Churchill Livingston, 4th edition
2. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
   
   
   

MOTOR CONDUCTION STUDIES - PARAMETERS

When a motor or mixed nerve is stimulated and recording is made by placing electrodes over a muscle supplied by that nerve, the recorded potential is known as compound muscle action potential or CMAP. There are three main parameters of CMAP, which are routinely evaluated during motor nerve conduction studies. They are latency, amplitude and conduction velocity.

LATENCY
This is the time in milliseconds between nerve stimulation and initial deflection from baseline. It reflects the time required for action potential to travel along the fastest-conducting axons to activate the muscle fibers.

• The latency includes not only the time taken for impulse to travel along the nerve till it reaches nerve terminal, but also the time taken for neuromuscular junction transmission and muscle activation.
  
• Whenever possible, the nerve is stimulated at two points: a distal point near the recording site (distal latency) and a more proximal point (proximal latency).
  
• Prolonged latencies are usually taken as evidence of demyelination.

AMPLITUDE
This is usually measured as height in millivolts of CMAP, from the baseline to the negative peak.

• CMAP amplitude is a semiquantitative measure of the number of axons conducting between the stimulating and the recording points.
  
• Decreased CMAP amplitudes usually suggest either axon loss or conduction block from demyelination located b/w the stimulation site and recorded muscle. But it can be due to reasons other than motor nerve dysfunction (e.g. neuromuscular junction, muscle fiber etc).

CONDUCTION VELOCITY

Measurement of differences in distance and latency b/w proximal and distal stimulation sites allows calculation of conduction velocity in the segment of nerve b/w the site of stimulation and is expressed in meters per second.

• Normal conduction velocities are from 40-50 m/sec in the legs and from 50-70 m/sec in the arms.
  
• Motor conduction velocity can not be calculated by performing a single stimulation. This is because, the latency of compound muscle action potential reflects transmission across nerve, junction and muscle, measurement of true conduction velocity across the nerve will necessarily require stimulation at two points.
  
• Decrease in conduction velocities is usually taken as sign of demyelination.
  
  

Reference:

1. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
   
2. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders, Boston: Butterworth-Heinemann