Myasthenia gravis and cranial mediastinal mass in a 14 year-old MC cat

Clinical History

A 14 year-old MC domestic shorthaired cat was present for an acute onset of generalized weakness, decreased appetite and vomiting. Previous history included urethral obstruction. Click here to see video of tensilon test.

Physical and Neurological Examination

No specific abnormalities were identified on general physical examination. The cat was alert with normal cranial nerve examination and, with the exception of generalized weakness and reluctance to walk, the neurological examination was normal.

Diagnostic Tests

CBC, serum biochemistry panel and urinalysis - normal
Tensilon test - positive
Acetylcholine receptor antibody titer - 10.99 nmol/l (feline reference <0.3)
Total T4 - 1.0 µg/dL (feline reference 0.8-4.0)
Free T4 (Equilibrium dialysis) - 19 pmol/L (feline reference 10-50)
Serum specific fPL - 12.1 µg/L (reference < or = 3.5 µg/L)

Thoracic radiographs - A large soft tissue mass was present within the cranioventral aspect of the thorax (Figure 1A). Primary differentials for this mass include thymoma, infiltrated lymph node, ectopic thyroid tumor or branchial cyst. One week later 10 ml of clear cystic fluid was removed from the cranial mediastinal mass. Cytology was compatible with a sterile transudate without evidence of neoplastic cells. The conclusion was the mass was most likely a completely drained or resolved benign mediastinal cyst. Repeat thoracic radiographs showed no evidence of the previously reported cranial mediastinal mass (Figure 1B).

Figure 1A

Figure 1B

Abdominal ultrasonography - The mesentery in the right anterior quandrant was hyperechoic with pain elecited during the ultrasound of the right anterior quadrant. No free abdominal fluid was noted.

Clinical Course and Outcome

A diagnosis of immune-mediated myasthenia gravis was made based on the positive response to Tensilon and the elevated acetylcholine receptor antibody titer. Although cytology on the fluid aspirated from the mass did not show neoplastic cells and was suggestive of a benign mediastinal cyst, thymoma cannot be ruled out. Vomiting was an initial complaint and was likely associated with pancreatitis, diagnosed by elevated serum fPL and abdominal ultrasound. A dilated esophagus was not noted radiographically.

Treatment was initiated with Mestinon syrup (at 6mg PO TID, then adjusted to 11mg PO BID after 2 weeks), immunosuppressive dosages of prednisone (2 mg/lb SID), pepcid AC and I/D diet. The cat did well on these medications with improved strength and no recurrence of vomiting.

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