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