Comparative Neuromuscular Laboratory

A main objective in the clinical evaluation of animals with lower motor neuron dysfunction is to determine an accurate diagnosis (myopathy, disorder of neuromuscular transmission, neuropathy) and to identify whenever possible, an underlying etiology. If the disorder is correctly characterized, a specific therapy may be initiated and a realistic long-term prognosis given. Carefully performed physical and neurological examinations, baseline and special laboratory testing, and electrodiagnostic evaluations are important to the evaluation of these disorders. However, a properly performed muscle biopsy (and in some cases peripheral nerve biopsy) is essential to the diagnosis of neuromuscular diseases as it allows direct visual examination of specific components of the motor unit.

What muscle should I biopsy?
The appropriate muscle for sampling depends on the clinical presentation and presumptive diagnosis. For example, if clinical signs are restricted to the muscles of mastication, temporalis muscle biopsies would be optimal. If clinical signs are generalized and suggestive of a myopathic disorder, proximal limb muscles including the vastus lateralis or biceps femoris from the rear limb and the triceps muscle from the forelimb are suggested. If clinical signs are generalized and suggestive of a neuropathic disorder, distal limb muscles including the extensor carpi radialis from the forelimb and cranial tibial muscle from the hindlimb are suggested. Biopsy of more than one muscle is recommended in generalized disorders. If there are any questions on which muscle to biopsy, it is best to call the laboratory prior to scheduling the biopsy procedure for clarification.

Masticatory Muscle Biopsies: common mistake
The frontalis muscle is commonly biopsied instead of the temporalis muscle. The frontalis is a thin muscle that lies directly under the skin. This muscle is not affected in masticatory muscle myositis (MMM) so if biopsied by mistake, a diagnosis of MMM may be missed. Incise and retract the frontalis muscle and the thick fascia that lies underneath the frontalis and directly over the temporalis muscle. This will expose the temporalis muscle for biopsy.

The muscle biopsy should be taken by an open biopsy procedure under general anesthesia
To ensure that the muscle specimen provided for evaluation is of optimal size, free of artifact and appropriately oriented, an open biopsy procedure under general anesthesia is preferred. Biopsies may be taken in critical patients under sedation and local anesthesia if necessary. Following routine surgical preparation of the skin, a skin incision is made. Both skin and fascia overlying the muscle are incised and retracted allowing visualization of the orientation of the muscle fibers (Fig. 1). With minimal tissue trauma, collect a specimen approximately 0.5 X 0.5 X 1.0 cm (longest length coinciding with the longitudinal length of the muscle fibers), dip tissue in saline solution just to moisten, then place in a dry water-tight container such as a 5 or 10 ml red top tube. IT IS CRITICAL THAT THIS TISSUE IS KEPT COLD. Collect a smaller piece of tissue from adjacent to the original specimen and place in 10% buffered formalin.

How do I prepare the biopsy specimens for transport to the laboratory?
For optimal results, it is of critical importance that fresh muscle biopsy specimens are received at the laboratory within 24 to 48 hours of collection. If your clinic is located close to the laboratory, the biopsy specimen can be directly transported on cold packs or on wet ice. If a courier service is to be used, the muscle specimens must be shipped under refrigeration. Wrap the tubes containing the fresh muscle tissue with paper towels so they do not come in direct contact with the cold packs. Place tissue and cold packs into a Styrofoam box. In the event that the muscle specimen has to be taken on a Friday due to unforeseen circumstances, the fresh muscle specimen may be placed in a freezer and shipped on cold packs the following Monday. There will be some loss of specimen quality, however, muscle specimens handled by this method have been diagnostic. Include the formalin fixed biopsy specimen along with the fresh specimen in the same box.

Figure 1

Figure 2.