Background Diaphragmatic weakness leading to respiratory insufficiency is a life threatening consequence of myasthenia gravis (MG). Thymectomy forms an integral part of the management of MG. Although transsternal thymectomy is generally associated with minimal morbidity and mortality, the phrenic nerve is vulnerable to damage during surgery and this can also cause diaphragmatic weakness. It can be difficult to define the cause of such diaphragmatic weakness in the post operative setting but there are important implications for treatment.

Aim We describe three cases of diaphragmatic paralysis in patients with MG post thymectomy.

Cases All three patients underwent transsternal thymectomy. There was no suggestion of any involvement of the phrenic nerve during surgery in any of the three cases. However, all three patients suffered respiratory failure due to diaphragm weakness post thymectomy and required non-invasive positive pressure ventilation for up to 10 years. Neurophysiological studies undertaken in all three patients did not reveal any evidence of phrenic nerve dysfunction suggesting that diaphragmatic paralysis was myasthenic in origin. Indeed all three patients improved after prolonged treatment of their MG

Conclusion We present these cases to highlight that diaphragmatic weakness can be seen in MG following thymectomy in the absence of phrenic nerve damage. Such weakness can respond to treatment and improve even after many years.