Trip Answers

Question answered:04/12/06 Warning! this question is over two years old.

We found an emedicine article (1) on Spinal Muscular Atrophy which details the variants of the condition and lab tests:

 

Lab studies:

Laboratory testing

The creatine kinase (CK) level is typically normal in SMA type I and normal or slightly elevated in the other types.

CSF findings are normal.

Genetic testing

Both prenatal and postnatal tests are now commercially available.

Tests for chromosome arm 5q should be performed.

The 1992 ISMAC found that the accuracy of prenatal prediction by means of chorionic villi sampling and amniocentesis was 88-99%.

Caution should be exercised when prenatal prediction is done in the presence of atypical features (see SMA variants) because these clinical variations may represent other pathogenic processes.

Other Tests:

Most cases spare the cardiac system, and ECGs are normal.

Electrophysiologic studies are useful in differentiating the SMAs from other neurogenic and myopathic diseases (Hausmanowa-Petrusewicz, 1986; Krivickas, 1998).

With the exception of Kennedy and Davidenkow syndromes, sensory nerve conduction is normal in SMA.

Compound motor action potentials (CMAPs) are low normal or reduced, depending on the severity of disease. In chronically weak muscles, CMAPs may be in the near-normal because of reinnervation and collateral sprouting. Motor velocities are normal. Modest slowing of motor conduction, when present, may accompany severe motor axon loss because of the loss of the fastest-conducting motor fibers.

In affected muscles, needle-electrode examination reveals widespread broad and polyphasic motor unit potentials (MUPs) firing in a reduced or rapid neurogenic recruitment pattern.

Superimposed low-amplitude, short-duration, and polyphasic MUPs may be present. These resemble myopathic motor units but are not due to a primary muscle disease. Instead, axon loss and resprouting results in fragmented motor units. Therefore, the appearance of MUP is a result of early and ongoing motor-unit reinnervation.

Fibrillation potentials can be seen in limb and paraspinal muscles and are most striking in early or progressive SMA. In late-juvenile and adult-onset forms, ongoing denervation may be sparse.

Fasciculation potentials are uncommon, but spontaneously firing motor unit action potentials (MUAPs) at 5-15 Hz have been described as a unique feature of SMA I and II.

Mild pseudomyotonic discharges have been observed in patients older than 6 years. However, these discharges are not specific for etiology and may be seen in chronic neurogenic disorders.

Procedures:

Muscle biopsy is often necessary to differentiate SMAs from other neuromuscular disorders. Muscle selection should be centered on clinically affected muscles but not to such a degree that degeneration renders the tissue unrecognizable.

Adequate results can be obtained with open or needle biopsy as long as the physician has adequate experience in the procedure and in processing of the tissue.

Electron microscopy can be used to evaluate for storage diseases.

References

 

http://www.emedicine.com/neuro/topic631.htm

 

---

DISCLAIMER: TRIPanswers is a collection of clinical questions and answers.  Each provider will have their own methodology in answering questions and these are likely not to be as rigorous as systematic review.  If you have any doubt as to the implications of this contact the Q&A Service Provider for further information. This document is presented for information purposes only. This document cannot and should not be used as a basis of diagnosis or choice of treatment, and is in no way intended to replace professional medical care or attention by a qualified practitioner. TRIPanswers and TRIP Database Ltd are not responsible or liable for, directly or indirectly, ANY form of damage whatsoever resulting from the use/misuse of information contained in or implied by this document.  Also, ensure you have read the terms and conditions for using the site.