Use of radiographic measurements in distinguishing myasthenia gravis from other causes of canine megaoesophagus

OBJECTIVES: To evaluate whether dogs with megaoesophagus due to myasthenia gravis display less oesophageal dilatation radiographically than dogs with other causes of megaoesophagus. METHODS: Thoracic radiographs of 66 dogs with megaoesophagus in which concurrent acetylcholine receptor antibody titre was known were analysed retrospectively. Maximum oesophageal diameter was transformed to a "relative oesophageal diameter" using a ratio with thoracic inlet diameter. Dogs were divided into two groups according to "MG" or "non-MG" antibody status and median relative oesophageal diameter values were compared between groups and with age, weight and sex. A receiver operating characteristic plot was used to evaluate a suitable relative oesophageal diameter cut-off. RESULTS: Twenty dogs were diagnosed with myasthenia gravis and 46 with other causes of megaoesophagus. Thoracic inlet size correlated significantly with bodyweight and surface area (r(2)=0.627 and 0.669, respectively). Median values of relative oesophageal diameter for the MG group and non-MG group were 0.58 and 0.66, respectively, and these showed a small, but significant, difference (P=0.029), although there was complete overlap in the range of relative oesophageal diameter values between groups. There was no significant association between relative oesophageal diameter and sex, age or weight or significant difference in age, sex or weight between the two groups. An increased odds ratio for myasthenia gravis existed in golden retrievers and German shepherd dogs. CLINICAL SIGNIFICANCE: Relative oesophageal diameter appears to be of limited diagnostic utility in distinguishing dogs with megaoesophagus due to myasthenia gravis from those with megaoesophagus due to other causes.