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NEUROMUSCULAR CASE OF THE MONTH - FEBRUARY 2002
Tensilon responsive weakness in a 2 year old FS Silky Terrier
Contributed by Dr. Scott Schatzberg
Cornell University Hospital for Animals
Ithaca, NY
Clinical History
One month prior to clinical presentation the owner noticed decreased appetite, decreased drinking, twitching, tremors and lateral recumbency that was worse in
the morning and seemed to improve throughout the day. The dog would try to right herself from lateral recumbency but was too weak to do so. At times she would walk
outside normally, then collapse during her walk, especially in the morning and in cold weather. At other times, she would perk up and walk sometimes for hours without
trembling. Prior to presentation, she developed epiphora, ptyalism and coughing that was invariably worse in the morning, like her weakness. No abnormalities were identified
on blood work (including serum creatine kinase) performed by the referring veterinarian.
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Physical and Neurological Examination
On presentation the dog was recumbent, severely weak and trembling (limp dish rag); however, she was bright, alert
and responsive. Neurological examination revealed absent palpebral reflexes (Fig. 1) and menace response, decreased gag reflex,
weak withdrawal reflexes in all four limbs, absent hopping and general proprioceptive responses even when supported. A neuroanatomic
localization of diffuse neuromuscular disease was made. |
Figure 1. |
Diagnostic Testing
Thoracic Radiographs - Megaesophagus and mild aspiration pneumonia of the left caudal lung lobe
Acetylcholine receptor antibody titer - 0.03 nmol/l (canine reference <0.6 nmol/l). The antibody titer was repeated with a new serum
sample 2 weeks later and was still within the reference range at 0.03 nmol/l.
Edrophonium chloride challenge (Tensilon test) - Following the injection of 0.4 mg intravenously, the dog was immediately responsive and stood up, ran around, and
urinated. Palpebral reflexes were positive and rapid, hopping was normal, and general proprioception was normal and rapid. After approximately two minutes, the dog
became weak and recumbent again, returning to her prior condition. (See video clip)
Diagnosis
A presumptive diagnosis of seronegative myasthenia gravis was made. This would be consistent with the clinical presentation, the dramatic
positive response to edrophonium chloride challenge, and the negative acetylcholine receptor antibody titer on two separate occasions.
An atypical finding was the presence of weakness that was worse in the morning and normalized throughout the day.
Treatment
There was some difficulty initially in arriving at an appropriate dosage of pyridostigmine as the dog would be responsive for a few
days then become refractory. The dog is currently receiving pyridostigmine bromide (Mestinon) liquid 0.6 ml PO QID, azathioprine (8
mg capsules) PO every other day, cefadroxil 100 mg PO BID and eye ointments QID.
Outcome
The dog is doing significantly better than prior to treatment although still not normal. The owners indicated at least a 75%
improvement. A continuing problem is episodes of morning weakness and occasional weakness after playing and eating. Periodic updates
will be given on this interesting case.
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