NERVE CONDUCTION VELOCITY TESTING AND DIAGNOSTIC ULTRASOUND TESTING

Nerve Conduction Velocity and Diagnostic Ultrasound Testing

Nerve Conduction Velocity (NCV)

Nerve conduction velocity study (NCV) measures basic parameters of the nerve function – strength and speed of how an electrical signal (action potential) spreads through the nerve. This data complements electromyography (EMG) in making the diagnosis.

Both nerves and muscles produce electrical signals called action potentials which are detected and measured during NCV. A nerve is actually a bundle of axons - long twigs of nerve cells conducting electrical signals from one end of the nerve to another. An NCV machine is capable of detecting and analyzing these tiny electrical signals coming from active neurons.

In motor nerves, these electrical signals travel toward the muscle causing muscle contraction.

In sensory nerves, these electrical signals are travelled toward the spinal cord, bringing signals from skin and other tissues which we feel as different sensations like temperature, pain, pressure and others.

NCV and Axons

NCV measures different characteristics of action potentials traveling along the axons, and is not significant for diagnosing diseases that primarily affect nerve function. NCV uses electrodes similar to those used in electrocardiograms placed on the skin over a nerve. Ction potential is generated by giving a mild electrical shock which is then recorded by other electrodes as it travels through the nerve.

The speed of nerve conduction is influenced by a coating around axons, called myelin sheath. Myelin sheath insulates each axon and forces action potentials to "jump" quickly along the axon. Speed of action potential is slowed down when myelin sheath is damaged. Healthy axons provide a strong action potential. If axons degenerate the action potential becomes weaker.

Different diseases preferentially either affect myelin sheathing or damage axons. This is why the type of nerve damage detected by NCV is so important in making the right diagnosis.

Though some people may find the electric shocks of the NCV or the needle pricks of the EMG uncomfortable, these methods do not leave any permanent damage and are quite tolerable. NCV and EMG have remained for decades as the gold standard test for evaluating the nerve and muscle function. Considering that there are more than two hundred different diseases affecting nerves and muscles NCV and EMG are very important and valuable tools in gathering data on the type, distribution and severity of damage. This data is useful in making an accurate diagnosis and starting an appropriate treatment earlier. Patients usually understand that EMG is a valuable tool and that the benefit of precise diagnosis outweighs discomfort of the procedure.

Ultrasound

Sonography was performed by a musculoskeletal radiologist who was blinded to the subject’s symptoms, signs, and the results of NCS, using a 12–5 MHz linear array transducer (HDI 5000; Phillips Ultrasound, Bothell, WA). Subjects sat down with the arm on a table in a position of supinated forearm, neutral-positioned wrist, and semi-flexed fingers. After identifying the ulnar artery, flexor retinaculum, and median nerve, transverse images of the median nerve were scanned at 2 levels: the carpal tunnel inlet (at the level of pisiform) and the carpal tunnel outlet (at the level of the hook of hamate) (Fig. 1A). The cross-sectional area (CSA) of the median nerve at each level was measured by directly tracing with an electronic caliper around the margin of the median nerve. The margin of the median nerve was defined as the margin outside the hypoechoic nerve fascicles and inside the hyperechoic nerve sheath (Fig. 1B).16 each measurement was performed 5 times; the highest and lowest values were eliminated, and the remaining 3 measurements were averaged.

Sonographic examination of the median nerve has been suggested as a useful alternative to electrophysiologic study in the diagnosis of carpal tunnel syndrome. To determine its usefulness and the best diagnostic criterion, sonograms of patients with the disease were compared with sonograms of healthy subjects in a case–control study.

How the Test Is Performed

An ultrasound machine creates images that allow various organs in the body to be examined. The machine sends out high-frequency sound waves, which reflect off body structures. A computer is used to receive these reflected waves which use them to create a picture.
The test is done in the ultrasound or radiology department. You will be lying down for the procedure. A clear, water-based conducting gel is applied to the skin over the area being examined to help with the transmission of the sound waves. A handheld probe called a transducer is moved over the area being examined. The radiologist may ask you to change your position so that other areas can be examined.
For specific information about ultrasound examinations, please refer to the following topics:

• Abdominal ultrasound
• Breast ultrasound
• Doppler ultrasound of an arm or a leg
• Doppler/ultrasound of the heart (echocardiogram)
• Duplex ultrasound
• Pregnancy ultrasound
• Testicle ultrasound
• Thyroid ultrasound
• Transvaginal ultrasound
• Vascular ultrasound

Preparation for the procedure will depend on the body region being examined.

How the Test Will Feel

There is generally little discomfort with ultrasound procedures. The conducting gel may feel slightly cold and wet.
The reason for the examination will depend on your symptoms.

Normal Results

Results are considered normal if the organs and structures in the region being examined are normal in appearance.

What Abnormal Results Mean

The significance of abnormal results will depend on the body region being examined and the nature of the problem. Consult your health care provider with any questions and concerns.
References

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• Zanette G, Marani S, Tamburin S. Extra-median spread of sensory symptoms in carpal tunnel syndrome suggests the presence of pain-related mechanisms. Pain 2006; 122: 264–70.
• Nathan PA, Keniston RC, Meadows KD, Lockwood RS. Predictive value of nerve conduction measurements at the carpal tunnel. Muscle Nerve 1993; 16: 1377–82.
• Atroshi I, Gummesson C, Johnsson R, Ornstein E. Diagnostic properties of nerve conduction tests in population-based carpal tunnel syndrome. BMC Musculoskelet Disord 2003; 4: 9.
• Lew HL, Date ES, Pan SS, Wu P, Ware PF, Kingery WS. Sensitivity, specificity, and variability of nerve conduction velocity measurements in carpal tunnel syndrome. Arch Phys Med Rehabil 2005; 86: 12–6.
• Beekman R, Visser LH. Sonography in the diagnosis of carpal tunnel syndrome: a critical review of the literature. Muscle Nerve 2003; 27: 26–33
• Uchiyama S, Itsubo T, Yasutomi T, Nakagawa H, Kamimura M, Kato H. Quantitative MRI of the wrist and nerve conduction studies in patients with idiopathic carpal tunnel syndrome. J Neurol Neurosurg Psychiatry 2005; 76: 1103–8.
• American Academy of Neurology. Practice parameter for carpal tunnel syndrome (summary statement): report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology 1993; 43: 2406–9.
• Cosgrove DO, Meire HB, Lim A, Eckersley RJ. Ultrasound: general principles. In: Adam A, Dixon AK, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging . 5th ed. New York, NY: Churchill Livingstone; 2008:chap 3.