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A 5 year old Asian boy presented to the paediatrician with diplopia following ear ache. Isolated VI nerve palsy was suspected. Full blood count, ESR, magnetic resonance imaging (MRI), and ENT examination were normal. He recovered within a week, but subsequently suffered six episodes of transient convergent squint with abduction deficit. He was referred to our neurologist for further opinion.
His developmental milestones, family history, and ocular and general examination as well as investigations were normal apart from vitiligo around the lids. At subsequent consultation, a right convergent squint was noted but with apparent full ocular movements. AchR Ab titres, performed this time, were weakly positive at 25×10∧10 M (<5×10∧10 M = negative and 5–50×10∧10 M = positive).
Ophthalmic evaluation at this stage revealed visual acuities of 6/12 (right), and 6/7.5 (left), intermittent alternating convergent squint, normal ocular movements, motor fusion, and refraction. His orbicularis oculi function was weak, but no lid twitch or ptosis on sustained upgaze was elicited. Cold stimulation by ice pack test2 showed a transient improvement in orbicularis function, but no change in his strabismus. Repetitive stimulation electromyogram (EMG) of orbicularis oculi was normal, but single fibre EMG could not be done as the child became very distressed. Saccadic studies3 showed longer and slower saccades which strongly suggested myasthenia (fig 1).
The clinical features, saccadic studies, positive antibody titres, and the association of vitiligo confirmed the diagnosis of ocular myasthenia. During the follow up, his AchR Ab levels, interestingly, were negative. Benign idiopathic VI nerve palsy,1 sometimes recurrent, is a diagnosis of exclusion. Variable strabismus is a known feature of myasthenia gravis. Elevated AchR Ab is the hall mark of myasthenia; however, it may be low to normal in younger age, boys, and in ocular myasthenia. Periocular single fibre EMG is often difficult and stressful to perform in younger children. The tensilon test needs a frank clinical sign to demonstrate the improvement. The ice pack test is helpful as shown by improvement in his orbicularis strength. Strabismus is known to be resistant to cold stimulation by ice pack compared with ptosis.2 The saccadic velocity pattern of myasthenia differs from paralysis or restrictive problems.3 The myasthenic eye can reach a normal peak saccadic velocity, but cannot sustain it.
This report highlights the difficulty in diagnosing some ocular myasthenia and the value of saccadic studies, which are simple, non-invasive, and repeatable.
Eye position and velocity during 10° saccades from the midline. The traces show the averages over 10 saccades. The position and velocity of the right eye is shown by a continuous line, while that of the left eye is shown by a dashed line. After an initial rapid movement, saccades into the field of action of the lateral rectus drift slowly towards their target direction. (Courtesy of R Clements)