Myasthenia Gravis
Myasthenia gravis is an autoimmune disorder of the neuromuscular junction. It is relevant in ophthalmology because it affects the eyelid/eye muscles.
Pathology
Anti-acetylcholine antibodies target the postsynaptic acetylcholine receptors at the neuromuscular junction, causing fatigability.
- Repetitive movements worsen fatigue and weakness.
- Because the eyelid muscles are used extensively throughout the day, ptosis and diplopia towards the end of the day is a classic sign.
Lambert-Eaton syndrome is another disease of the neuromuscular junction, but it affects the presynaptic calcium channels. Fatigue improves with muscle use.
Diagnostics
Presentation
- Ptosis which is worse at the end of the day
- Diplopia is worse after extended reading or TV watching
- Respiratory depression (Myasthenic crisis)
Investigations
- Autoantibodies: Anti-AChR and Anti-MUSK
- Repetitive nerve stimulation shows decrease in action potential amplitude
A CT thorax is indicated in these patients because a thymoma can cause myasthenia gravis. In these cases, thymectomy can cure the disease
Management
Acetylcholinesterase inhibitors such as pyridostigmine are used in long term management
- Steroids are used in myasthenic crisis
Myotonic Dystrophy
An autosomal dominant disorder is characterised by the failure of muscle relaxation after contraction.
Pathology
Caused by trinucleotide expansion of CTG on the DMPK gene (chromosome 19)
Diagnostics
Presentation
- Presents by the 2nd decade of life
- Bilateral ptosis, cataracts(Christmas tree) and ophthalmoplegia.
- Muscle weakness with delayed relaxation (myotonic grip)
- Frontal baldness, testicular atrophy and cardiomyopathies
Investigation
- Diagnosis made by DNA testing
Kearns-Sayre Syndrome
A rare cause of chronic progressive external ophthalmoplegia of mitochondrial inheritance. This is ultra-rare and has only been included for completeness, and to demonstrate that general myopathies often affect the extraocular muscles
Diagnostics
Presentation
- Can present at birth, most cases manifest well before the 2nd decade of life
- Bilateral ptosis and ophthalmoplegia
- Pigmentary salt and pepper retinopathy
- Cardiac conduction defects
Neurofibromatosis vs Tuberous Sclerosis
Both of these diseases are inherited in an autosomal dominant fashion.
Characteristic Signs |
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Neurofibromatosis Type 1 |
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Neurofibromatosis Type 2 |
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Tuberous Sclerosis |
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Neurofibromatosis has 2 types, reflecting 2 different mutations. Type 1 is a mutation of the neurofibromin 1 gene on chromosome 17. Type 2 is a mutation of the neurofibromin 2 gene on chromosome 22. Because they are different mutations, the disease phenotypes are also different.
