16 Ocular Myopathies
S Soundari
INTRODUCTION
Myasthenia results from the dysfunction of the neuromuscular junction caused by autoimmunity. Ocular myasthenia most commonly presents with diplopia, ptosis or both that is variable and characteristically worse towards the end of the day. Serum antibodies to acetylcholine receptors are detected in 90 percent of the patients with generalized myasthenia but only 50 percent will be detected in ocular myasthenia. Neonatal forms of myasthenia gravis occur in 10 to 15 percent of children born to the mothers with myasthenia gravis, because of the placental transfer of antibodies to ach receptor.
The impairment of the neuromuscular conduction causes weakness and fatigue of the skeletal musculature, but not of cardiac and involuntary muscles. The disease affects females twice as commonly as males and may be ocular, bulbar or generalized.
CLINICAL FEATURES
•Myasthenic signs and symptoms are variable and tend to worse with fatigue and stress
•Fatigability: When testing for lid fatigue, the patient is asked to look up without blinking at the examiners hand for 1-2 min. Lid fatigue on prolonged up gaze is perhaps the most frequently elicited signs
•Peek sign: When the patient is asked to close the lids gently, one or both inadvertently open slightly or peek
•There can absence of Bell’s phenomenon
•Cogan’s lid twitch: After prolonged down gaze refixation to the primary position results in overshooting of the upperlid
•Hop of the upper lid occurs on looking to the side
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•Myasthenic ptosis, when unilateral is associated with controlateral lid retraction
•If one eye lid is elevated manually as the patient looks up, the fellow eyelid will show fine oscillatory movements
•Ice pack test: The degree of ptosis improves after the ice pack is placed on the eyelid for 2 minutes. The test is negative in non myasthenic ptosis
•Diplopia: This is very frequently vertical although any of the muscle can be involved. Pupil is not involved. A pseudointernuclear ophthalmoplegia can occur
•Saccadic abnormalities like hypometric large saccades, hypermetric small saccades, quiver movements, hyper fast saccades can occur.
Investigations
Tensilon test: Intravenous injection of edrophonium is the gold standard for the diagnosis of ocular myasthenia. Edrophonium is a short acting anticholinestrase which increases the amount of acetylcholine available at the neuromuscular junction. In myasthenia this results in transient improvement of symptoms and signs such as weakness, ptosis and diplopia. Uncommon complications include bradycardia, loss of consciousness and even death. Lacrimation, salivation and abdominal cramps are mentioned as common minor side effects. The test should be done with a resuscitation trolley in hand if in case of sudden cardiorespiratory arrest.
Objective baseline measurement of ptosis or diplopia with hess chart should be taken. Intravenous injection of atropine 0.3 mg is given to minimize muscarinic side effects. Intravenous dose of 0.2 ml
Fig. 16.1: Myasthenia
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containing 2 mg of edrophonium hydrochloride is given, if definitive improvement is noted the test can be terminated.
If no response then the remaining 0.8 ml of 8 mg is injected after 60 seconds if there is no adverse reaction. The response lasts only for 5 minutes.
Perverse reaction like worsening of the strabismus or a paradoxical response like right hypertropia becoming a left hypertropia after the injection is considered positive by some.
Neostigmine Test
•Intramuscular injection of neostigmine is useful in children. The effect lasts for 15 minutes to peak and lasts for only 30 minutes
•Presence of acetylcholine receptor antibodies is virtually diagnostic of myasthenia gravis.
Electromyography
•Repetitive stimulation and single muscle fiber will show a decremented response
•Sleep test is useful in neonates and babies. There will be improvement after sleep
•Imaging the chest with the computed tomography or magnetic resonance imaging for the presence of thymoma.
Differential diagnosis for myasthenia gravis:
•Isolated or combined third, fourth, sixth or seventh cranial nerve palsies
•Decompensated strabismus
•Thyroid disease
•Eaton lambert myasthenic syndrome
•Botulism
•Chronic progressive ophthalmoplegia
•Myotonic dystrophy.
Treatment
Treatment of myasthenia gravis can be divided into—optical treatment, medical, and surgical treatment.
Optical Treatment
•Because of the variability of signs and symptoms it is difficult to treat. For binocular diplopia occlusion of one eye, but it makes the patient to view monocularly
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•Fresnel prism can be tried if the ocular deviation is stable for weeks
•The crutch glasses are helpful in the case of ptosis.
Medical Treatment
•Anticholinergic drugs like pyridostigmine (60 mg) three times a day. One must be always aware of the cholinergic crisis if too much of pyridostigmine is given. The patient should be told to stop if bulbar symptoms or generalized weakness occurs.
•Corticosteoids is used along with pyridostigmine. The patient should be maintained on steroids for months before tapering and should be a slow tapering for months when the patient is maintained on low dose of steroids there can relapse or unmasking of generalized myasthenia.
•Immunosuppressant: Azathriopine is effective against myasthenia. It is given in the dose of 2-3 mg/kg/day
•Cyclosporine A, plasmapheresis, mycophenolate and IV gamma globulin also can be used in generalized myasthenia.
Surgical Treatment
•Thymectomy is very effective for ocular myasthenia. The result of thymectomy for generalized myasthenia are very favorable with about 35 percent entering complete remission and 50 percent improving.
•Eyelid surgery or ptosis and eye muscle surgery for diplopia are considered only if it is stable for few months and as a last resort.
CHRONIC PROGRESSIVE EXTERNAL OPHTHALMOPLEGIA
The clinical features are the involvement of the upgaze and then the lateral movements and may later be affected in all gaze resulting in a fixed globe. Because the muscle involvement is symmetrical, diplopia does not usually occur. There is also slowly progressive bilateral ptosis.
Kearns-Sayre Syndrome
•This is a mitochondrial cytopathy, inherited from the mother.
•It is characterized by pigmentary retinopathy with coarse granularity.
•Conduction defects of the heart can occur. Heart block may result in sudden death.
•Other features are short stature, muscle weakness, cerebellar ataxia, neurosensory deafness, mental handicap and delayed puberty.
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Treatment
Treat the associated conditions. Lubricants for the exposure keratopathy, base down prisms within reading glasses for reading if the down gaze is restricted.pacemaker may be required for the cardiac condition. In ocular pharyngeal dystrophy, for the dysphagia and aspiration cricopharyngeal surgery. Genetic counseling.
MYOTONIC DYSTROPHY
This is dominantly inherited in the gene of chromosome 19q. Usually manifest in the third decade. Peripheral muscle
involvement which makes the release of grip difficult which can be tested with the Hand Shake. A mournful expression caused by bilateral facial muscle wasting. Slurred speech because of the involvement of tongue muscles and pharyngeal muscles hypogonadism, frontal baldness, intellectual detoriation, pulmonary and cardia complication can occur.
Other ocular features are presenile cataract with polychromatic luster, ptosis, pigmentary retinopathy, light near dissociation, external ophthalmoplegia.
ESSENTIAL BLEPHAROSPASM
Blepharospasm can be a very disabling condition in terms of vision and social life.
More commonly affects female in the older age group. This is a type of facial dystonia in which there idiopathic tonic contraction of orbicularis oculi. If it is secondary to any ocular pathology (corneal or conjuctival foreign body, trichiasis, blepharitis, dry eyes) then it is called secondary blepharospasm.
Clinical Features
•There is a bilateral involuntary lid closure which may be precipitated by stress, fatigue, social interactions. This is always bilateral. Disappears during sleep
•Secondary ocular changes like ptosis or entropion can occur
•This can be differentiated from hemifacial spasm which does not disappear during sleep.
Treatment: Botulinum toxin given as multiple injections on the upper and lower lid. The effect generally last for 3 months time. In cases of secondary blepharospasm treat the underlying cause which is precipitating the blepharospasm.
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Fig. 16.2: Blepharospasm
Other treatment option are medical like benzodiazepine or surgical like myectomy.
Meige’s Syndrome
This is a blepharospasm with midfacial spasm. It may lead on to compromise of speech , eating and drinking.
Breughel Syndrome
This is associated with severe mandibular and cervical muscle involvement.
Hemifacial spasm:
This is a tonic clonic spasm of the musculature which occurs even during sleep. Usually affects the younger age group. It is thought to be caused by the irritation of the root of seventh cranial nerve by a compressive lesion. MRI of the cerebellopontine angle should be obtained to rule out tumor.
Treatment
It includes observation, botulinum toxin injection or neurosurgical decompression of the seventh nerve (Janetta procedure)
Tourette’s Syndrome
This includes multiple compulsive muscle spasms associated with utterances of bizarre sounds.
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Tic Douloureux
Acute episodes of pain in the areas of distribution of the trigeminal nerve.
Tardive Dyskinesia
This is a orofacial dyskinesia, often with restlessness and dystonic movements of the trunk and the limbs. Ususlly it is associated with the long-term use of antipsychotic medication.
Facial Myokymia
Fleeting movements of the facial musculature which may be associated with stress, multiple sclerosis, caffeine or rarely tumors of the brain stem.
Lid Apraxia
•In lid opening apraxia there is total inhibition of the LPS with no activation of orbicularis oculi. This results in the lid closure with difficulty in initiating the lid opening. It is associated with parkinson’s disease, progressive supranuclear palsy, Huntington’s disease and Wilson’s disease.
•Lid retraction and poor closure of the lids can occur in Parkinson’s, Parinaud’s and progressive supranuclear palsy.
BIBLIOGRAPHY
1.Amar Agarwal. Handbook of Ophthalmology; Slack USA 2005.
2.American Academy of Ophthalmologysection 5- Neuro-ophthal 2004-2005.
3.Jack J Kanski (6th ed) Clinical ophthalmology, Butterworth Heinmann 2007.
4.Parson’s Diseases of The Eye (18th ed) Butterworth-Heinemann International editions.
5.Sunita Agarwal, Athiya Agarwal, et al. Textbook of Ophthalmology 4th vol.; Jaypee, India 2003.