Importance of clinical tests in ocular myasthenia

Abstract

A 24-year-old otherwise healthy male presented to us with unilateral ptosis and contralateral lid retraction with limitation of extraocular movements; the disease had a gradual chronic course, which raised a suspicion of ocular myasthenia. Ice pack test was performed, which improved the ptosis; further investigations confirmed the diagnosis of ocular myasthenia. Patient was started on pyridostigmine and oral prednisolone which improved the extraocular movements and ptosis.

Background

Ocular myasthenia can be a mystery to solve especially when the presentation is unusual. Ocular myasthenia can mimic any ophthalmoplegia, and diplopia may be absent in chronic cases. The case reminded us of this important clinical lesson that myasthenia must be suspected if the limitation of extraocular movements cannot be explained by any particular cranial nerve palsy. The suspicion can further be confirmed by simple bedside tests like ice pack test, sleep test and Cogan’s lid twitch sign.

Case presentation

A 24-year- old healthy man presented to the strabismus clinic of our institute with drooping of left eyelid, along with limitation of extraocular movements noticed by the patient since 3 years, and further worsening was noticed since 5 months. Patient gave a history of gradual deterioration of symptoms, there was no history of diplopia, dysphagia, muscle weakness, trauma, surgery or fever. On examination, patient had a visual acuity of 20/20 both eye, left eye had severe ptosis with a compensatory lid retraction in the right eye (figure 1), with severe limitation of extraocular movements in both eyes (figure 2) and a positive Cogan’s lid twitch sign (video 1).

Figure 1.

Clinical presentation of the patient. Note the unilateral ptosis and contralateral lid retraction.

Figure 2.

Note the exo-deviation and limitation of extraocular movements.

Video 1.

Download video file ^{(2.6MB, mp4)}

DOI: 10.1136/bcr-2019-231296.video01

Primary and secondary deviations were measured by modified Krimsky’s test in all nine gazes and were found to be incomitant, with right eye fixing the deviation was found to be 45 prism dioptre and with left eye fixing exodeviation was found to be 60 prism dioptre in primary position, and in downgaze the deviation with right eye fixing was 45 prism dioptre and with left eye fixing it was >70 prism dioptre. Other anterior segment and fundus examination findings were within normal limits.

Investigations

The gradual onset and progressive deterioration of symptoms raised a possibility of ocular myasthenia gravis, for which ice pack test and sleep test were done, both the tests were positive (figure 3), ice pack was applied simultaneously on both eye lids. For further confirmation neostigmine test was performed, which was also positive (figure 4). In both ice pack and neostigmine test, the vertical palpebral aperture height of both the eyes was measured before and after the test after negating the frontalis action, so as to avoid any false positive results. The height in the right eye improved from 2 to 4 mm in ice pack test and in neostigmine test the height improved from 2 to 7 mm. Repetitive stimulation of orbicularis oculi muscle showed a decremental response. Acetyl choline receptor antibodies (AchR-Ab) and muscle specific kinase antibody titers were negative, chest X-ray and CT chest were advised to rule out thymus enlargement, but were normal. MRI of brain was normal, which ruled out supra nuclear causes for the extraocular movement limitation. Thyroid function tests were normal and forced duction test was found to be free for inferior rectus.

Figure 3.

Ice pack test.

Figure 4.

Neostigmine test. Note the improvement of ptosis after the administration of neostigmine.

Differential diagnosis

The differential diagnosis in this case included, chronic progressive external ophthalmoplegia, but was ruled out as it has an onset in first to decade, along with symmetrical ophthalmoplegia and ptosis.

Thyroid ophthalmopathy was excluded as thyroid function test was within normal limits and forced duction test revealed free inferior rectus.

Lambert Eaton syndrome was ruled out as patient had chiefly ocular symptoms, more over sleep test was positive and sensory examination and motor examination were within normal limits. Contrast enhanced computed tomography (CECT) chest was also normal.

Treatment

Thus, a diagnosis of ocular myasthenia was made and patient was started on 60 mg pyridostigmine two times per day along with oral prednisolone 50 mg once daily, following treatment symptoms of the patient improved.

Outcome and follow-up

The exodeviation also reduced to 30 prism diopter (pd)within 2 weeks of starting the treatment and strabismus surgery will be planned once the patient has stable deviation for at least 6 months.¹

Discussion

Myasthenia gravis is an autoimmune disease, affecting the neuromuscular junction and is characterised by the formation of auto-antibodies against the acetyl choline receptors. Generalised myasthenia affects the muscles of limb and respiratory muscles; on the contrary, ocular myasthenia affects the levator palpebrae superioris (LPS) and orbicularis oculi most commonly,² although any extraocular muscle can be involved. The characteristic features of extraocular muscles, like rapid contraction and high firing frequency of twitch fibres, make them vulnerable to the disease. Ptosis results from the involvement of LPS complex, which can also be explained in our patient, the contralateral lid retraction was the result of Hering's law of equal innervation to yoke muscles.

Lack of diplopia in our patient can be explained by the chronic disease course of 3 years.

Sleep and rest usually improves symptoms in myasthenia patients. Cooling and neostigmine reduces the activity of anticholinesterase enzyme, thus increasing the levels of acetyl choline in the neuromuscular junction and improving the symptoms. Normal AchR antibody titer does not exclude the diagnosis of ocular myasthenia, as they are present only in 30%–77% patients of ocular myasthenia.³ Cogan’s lid twitch sign has been reported to be 99% specific and 75% sensitive.⁴

Fifty per cent of patients with ocular myasthenia progress to generalised myasthenia to prevent this; steroids were added along with pyridostigmine. Steroids have been reported to prevent the progression of ocular myasthenia to generalised disease to 7%–17%, especially if steroids are added early in the disease course.^{5 6}

Learning points.

• Myasthenia gravis can mimic any pattern.

• Suspicion should be raised when there is gradual worsening of symptoms and limitation of extraocular movements cannot be explained by the involvement of a particular cranial nerve.

• Ice pack test can be easily performed bedside and has a high sensitivity and specificity of 96% and 88%, respectively.⁷