Methods for diagnosing seronegative myasthenia gravis

A peculiar feature of seronegative myasthenia gravis is that it presents negative acetylcholine-receptor antibodies; determination of muscle-specific receptor tyrosine kinase (MuSK) antibodies defines a subgroup of patients with generalised myasthenia gravis with certain clinical and neurophysiological peculiarities. Its diagnosis requires the presence of weakness with fatigability, determination of positive anti-MuSK antibodies and alterations in neurophysiological testing of the neuromuscular junction. It is usually more serious and has a poorer prognosis than the seropositive forms, develops in an acute or subacute manner, and the neurological deficit predominates in the facial, bulbar and respiratory muscles. Titration of the anti-MuSK antibodies and conducting neurophysiological tests, especially jitter assessment using single-fibre electromyography in clinically deficient muscles, are not only necessary for an early diagnosis of these clinical forms, but also so as to be able to carry out an objective evaluation of the clinical progression and response to treatment.