NEUROMUSCULAR CASE OF THE MONTH - February 2005

Is this orthopedic or neuromuscular?
Contributed by Dr. Rebecca Packer
University of Missouri
Columbia, MO

Clinical History
An 8 month-old female spayed Labrador retriever (Fig. 1) was evaluated for a 6 week history of sudden lameness and stiff gait in both thoracic limbs. The dog was previously normal and healthy. It “bunny-hopped” when climbing stairs or running and seemed painful to the owner.  Activity level was reported as decreased since the onset of this problem. There was no history of trauma or other inciting cause, although the owners were out of town at the time of onset. During this time the dog was either crated or under observation by the pet-sitter. The condition seemed fairly static since the onset without progression.

Fig. 1

Fig. 2

Physical and Neurological Examination
There was decreased mass of the pectoral and scapular muscles bilaterally (Fig. 2) with the distal limb musculature considered normal.  (Note: The owner reported that the muscles did not appear to atrophy, but rather just failed to develop appropriately.  A littermate was very well-muscled with no abnormalities).  All other aspects of the general physical examination were normal.  Neurological examination yielded an abnormal gait [Go to video clip], with the thoracic limbs appearing adducted at the shoulders or elbows.  Hyperesthesia was not noted during the examination or during paraspinal and limb palpation.  Postural reactions, myotatic reflexes, withdrawal reflexes, and cranial nerves were normal. Laxity of the scapulohumeral joints with equivocal laxity of the scapular attachment to the thoracic wall was suggested on orthopedic evaluation.

Diagnostic Testing
A complete blood count was within reference range.  Creatine kinase was mildly elevated [336 U/L, reference range 10-274 U/L]; all other parameters on the serum biochemical profile were within reference range.  Serology performed prior to referral was negative for heartworm disease, Lyme disease, and Ehrlichia.

Electrophysiologic Studies
Electromyography (EMG) was performed under general anesthesia, and did not show any spontaneous muscle activity.  All muscle groups were evaluated, including limb musculature, paraspinal muscles, and temporal-mandibular muscles.  Motor nerve conduction velocity was evaluated for the median and radial nerves, and was within reference range.

Muscle biopsy
Pectoral muscle biopsy was performed to definitively rule out a primary myopathic condition, and was unremarkable.

Conclusion and further diagnostics
A precise diagnosis has not yet been reached; however, there was no conclusive evidence that this is a neuromuscular disease. The next step in the investigation of a possible orthopedic problem would be arthroscopy of the scapulohumeral joints bilaterally, and evaluation of the ligaments for laxity.  Should ligament laxity be identified, surgical correction could be performed arthroscopically.  An alternative diagnostic option would be to perform an MRI of the cervico-thoracic spinal cord to screen for central spinal cord disease; however, abnormalities would be expected upon EMG evaluation of the forelimbs and paraspinal musculature. Updates will be provided as further diagnostics are performed.

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