Incomitant Strabismus/Module 8: Myogenic Strabismus

Graves' Ophthalmopathy
Eye movements can be disrupted by factors other than a defective nerve supply to the extraocular muscles. One of these is pathology of the muscle's/muscles' structure and/or function, causing a myogenic palsy. Myogenic palsies can cause incomitant strabismus

Thyroid dysfunction: Thyroid dysfunction can result in an overactive or underactive thyroid gland. It is encountered in all age groups, but is rare under the age of 20. The aetiology is still not clear, but there is now evidence that the dysfunction is caused by a disturbance in the immune system. Dysfunction of the thyroid gland causing it to overact (Graves' disease being the most common) can cause a number of general signs, including nervousness, palpitations, heat intolerance and weight loss. In patients with thyroid dysfunction, there is no way of predicting whether ocular signs will occur. Ocular problems can occur in patients who are hyper, hypo or euthyroid. The ocular condition may progress even after a systemic thyroid balance has been achieved.

Thyroid eye disease (Graves' ophthalmopathy): The fact that ocular problems may appear in patients with no thyroid dysfunction and that the treatment of thyroid dysfunction often has no effect on the ocular signs, has resulted in much research being generated into the aetiology of thyroid dysfunction. In thyroid eye disease, there is lymphocytic infiltration within the orbit, mainly concentrated in the extraocular muscles. This causes an increase in the total orbital content, leading to proptosis and eventual mechanical restriction of the affected extraocular muscles with resultant diplopia. Some believe that the ophthalmic signs represent a separate disease. But others claim that the two conditions are closely linked, both being due to a disturbance of the immune system. It is important to recognise that research is ongoing and that consequently the discussion on aetiology is often changing. This is reflected in the myriad of names that have been given to the disease we most commonly know as thyroid eye disease or Graves' ophthalmopathy. Some/older texts will have used the following terms: thyrotoxicosis, dysthyroid eye disease, ophthalmic Grave's disease, exophthalmos ophthalmoplegia, endocrine ophthalmopathy, endocrine exophthalmos, and endocrine myopathy. Management entails the treatment and continued assessment of any thyroid dysfunction that may be present by an endocrinologist. If thyroid dysfunction (Graves' Disease) is present then this condition needs treatment and continued assessment from endocrinologist, physician etc.

For thyroid eye disease, a combination of medical, ophthalmic, surgical and orthoptic treatment may be necessary.

Exophthalmos may require: (i) artificial tears to prevent symptoms of dry eye and exposure, (ii) lateral tarsorrhaphy and/or (iii) reduction by steroids, radiation treatment or orbital decompression surgery

Strabismus: If the ocular motility is affected, the following may be necessary: (i) assessment to monitor progress, (ii) prisms to relieve diplopia or (iii) occlusion if prism therapy is difficult. All patients should have detailed visual assessment to monitor optic nerve function, including testing of visual acuity, contrast sensitivity, colour vision and visual fields.

Surgery: Strabismus surgery is only considered when the ocular motility condition becomes relatively stable and is not showing change every few months. Since operating has the main aim of freeing a restriction, there may be an over-response to a relatively minimal amount of surgery.

Botulinum toxin injection: As an alternative to surgery, the use of Botulinum toxin is often considered as a treatment to prevent muscle contracture.

Myastenia Gravis
Myasthenia gravis is a condition which affects striated muscle. The aetiology is not entirely clear but it appears that it is a disorder of neuromuscular transmission due to a reduction of acetylcholine receptor sites at the motor end-plate. The condition is characterized by abnormal fatigability of the striated muscles after repetitive contraction which improves with rest. The disease can occur at any age. Some of the general features include variability of muscle function, remissions and exacerbations, and the condition being minimal or non-existent in mornings and increasing as day progresses. Ocular signs are often the first complaint, with ptosis occurring in 90% of cases. Diplopia is the next most common symptom. About 20% of patients only show signs of ocular involvement - ocular myasthenia.

Treatment guidelines are:

General: The condition is usually manged by a neurologist using systemic medication to control the condition.

Orthoptic: Continued assessment to monitor the ocular motility imbalance is important, prescribing prisms and/or occlusion as necessary. Treatment using prisms can be difficult if the deviation is varying considerably.