3 Myasthenia gravis in a dog
The dog had become progressively less exercise tolerant over the 2 months prior to presentation. He had also initially begun to regurgitate his food and then began to regurgitate water. The regurgitation usually occurred within a few minutes after eating, but occasionally he brought up foam without having eaten anything. The food that he regurgitated was formed and undigested. He was still interested in eating and the owners had started feeding him very small amounts, as any larger amounts were regurgitated.
He had become progressively less able to go for walks and would sit or lie down after several minutes of walking. He remained conscious during these times and he would usually be willing to walk again after a short rest. He had begun to cough during the previous week and his breathing was laboured. His faeces were formed and were passed normally and his urination was normal.
The dog was quiet but responsive and preferred to lie on the consulting room floor during the examination. He was reluctant to walk when taken outside and his gait was abnormal (Fig 3.1). His body condition score was 4/9. Mucus membranes were pink and capillary refill time was less than 2 seconds. Pulse quality was good.
Thoracic auscultation revealed normal heart sounds but crackles over the left lung fields. Heart rate was 140 beats per minute (bpm) and the respiratory rate was 60 breaths per minute with some inspiratory effort.
Weakness and exercise intolerance can by caused by many systemic disorders, including (but not limited to) cardiopulmonary diseases, metabolic disorders such as hypothyroidism or hypoadrenocorticism, neurological disorders such as polyneuropathies or myasthenia gravis, myopathies or pain due to other disorders.
Coughing can be due to upper respiratory disorders, lower respiratory disorders or cardiac disorders. As this dog had crackles on auscultation, a lower respiratory (lung) disorder was suspected. Crackles are discontinuous, non-musical sounds produced when airway pressures fluctuate or when air rushes through fluid- or mucus-filled alveoli, but can sometimes also be heard when there is fluid in the oesophagus.
Haematology, serum chemistry and routine urinalysis were performed. The haematology results showed an increase in neutrophils with a slight left shift, consistent with the presence of inflammation. The serum chemistry results were unremarkable, as was the urinalysis. Basal serum cortisol was 132 nmol/l (reference range 20–230 nmol/l).
Clinical tip on use of basal serum cortisol
A serum basal cortisol concentration of 70 nmol/l or greater effectively rules out hypoadrenocorticism. If the value is less than 70 nmol/l, an ACTH stimulation test should be performed to definitively rule out or diagnose this disorder.