Amr Mousa
Introduction
Myasthenia gravis (MG) is a chronic autoimmune disorder affecting the neuromuscular junction, characterized by fluctuating muscle weakness. Ocular manifestations are often the first and most common presentation of MG, making early recognition by ophthalmologists crucial in preventing systemic progression and improving patient outcomes (1).
Epidemiology
MG has an incidence of 7–23 per million annually and a prevalence of 70–320 per million globally (2). Ocular MG (OMG) accounts for 15–20% of all MG cases and often progresses to generalized MG in approximately 50% of patients within two years (3). It affects both sexes and all age groups but demonstrates a slight female predominance in younger patients and male predominance in older populations (2, 4).
Pathophysiology
MG results from autoantibodies targeting the acetylcholine receptors (AChR) or associated proteins, such as muscle-specific kinase (MuSK), at the neuromuscular junction. This blockade reduces signal transmission, leading to muscle weakness (1). In OMG, this primarily affects the extraocular muscles, levator palpebrae superioris, and orbicularis oculi (5).
Clinical Presentation
The hallmark features of OMG include:
- Ptosis: Often asymmetric and variable in severity, worsening with fatigue (1).
- Diplopia: Due to extraocular muscle weakness causing ocular misalignment (2).
- Cogan’s Lid Twitch: A brief overshoot of the upper eyelid when shifting gaze from downward to primary position (5).
Symptoms are typically exacerbated by fatigue, stress, or prolonged use of affected muscles (3).
Diagnosis
Diagnosis of OMG involves clinical evaluation, laboratory tests, and imaging:
- Ice Pack Test: Improvement in ptosis after cooling the eyelid for 2–5 minutes (1).
- Serologic Tests: Detection of AChR or MuSK antibodies confirms autoimmune MG and it is important to test for Anti-titin antibodies as it can indicate the presence of thymoma, a severe condition that is often associated with MG (2).
- Electrophysiological Studies: Repetitive nerve stimulation or single-fiber electromyography shows impaired neuromuscular transmission (3).
- Imaging: CT or MRI of the chest to exclude thymoma, which is associated with MG (4).
Management
Treatment of OMG aims to relieve symptoms and prevent progression:
- Cholinesterase Inhibitors: Pyridostigmine provides symptomatic relief by enhancing neuromuscular transmission (2).
- Immunosuppressive Therapy: Corticosteroids, azathioprine, or mycophenolate mofetil may be used in refractory cases or generalized MG (4).
- Thymectomy: Indicated in patients with thymoma or generalized MG, potentially improving outcomes (3).
- Supportive Measures: Prism glasses for diplopia and eyelid crutches for severe ptosis (5).
Prognosis
With appropriate treatment, most patients achieve good control of symptoms. However, monitoring for progression to generalised MG is essential. Early diagnosis and management significantly improve quality of life and visual outcomes (1, 4).
References
- Grob D, Brunner N, Namba T, et al. The course of myasthenia gravis and therapies affecting outcome. Annals of the New York Academy of Sciences. 1981;377:652-669.
- Benatar M. A systematic review of diagnostic studies in myasthenia gravis. Neuromuscular Disorders. 2006;16(7):459-467.
- Jaretzki A 3rd, Barohn RJ, Ernstoff RM, et al. Myasthenia gravis: Recommendations for clinical research standards. Annals of Thoracic Surgery. 2000;70(1):327-334.
- Evoli A, Batocchi AP, Tonali P. Ocular myasthenia: Diagnostic and therapeutic aspects. Current Opinion in Neurology. 1999;12(1):17-21.
- Chan JW. Ocular myasthenia gravis: Diagnosis and treatment. Current Opinion in Ophthalmology. 2008;19(6):493-499.