Other diagnostic tests performed routinely in essentially all patients with myasthenia gravis include CT of the chest (for the detection of thymic enlargement or thymoma), tests of thyroid function for reasons discussed further on, and in cases of uncertain diagnosis, magnetic resonance imaging of the cranium and orbits to exclude compressive and inflammatory lesions of the cranial nerves and ocular muscles.
We have encountered the following clinical problems in myasthenia: 1. The concurrence of myasthenia gravis and thyrotoxicosis. Thyrotoxicosis may produce a characteristic ocular myopathy and there is a tentative relation to periodic paralysis as indicated in Chap. 45. There is no definite evidence that thyrotoxicosis aggravates myasthenia gravis; some clinicians have even observed an inverse relationship between the severity of the two conditions. Hypothyroidism, however, does worsen the myasthenic symptoms. The ophthalmoplegia of thyrotoxicosis can usually be distinguished by the presence of an associated exophthalmos (early in the disease, exophthalmos may be absent), lack of ptosis, and the lack of definitive response to neostigmine. Polymyositis and inclusion body myopathy are differentiated from myasthenia by lack of involvement of extraocular muscles, but they may affect oropharyngeal muscles, as does myasthenia. Finding the signs of these diseases in combination with those of myasthenia indicates a concurrence of two independent autoimmune diseases.
2. The neurasthenic or depressed patient who complains of weakness when actually referring to fatigability. There is no ptosis, strabismus, or dysphagia, though an anxious individual may complain of diplopia (usually of momentary duration when drowsy) and also of tightness in the throat (globus hystericus). A number of such patients claim improvement with neostigmine but objective reversal is always uncertain. Conversely, myasthenia is as often mistaken for hysteria or other emotional illness, mainly because the physician is unfamiliar with myasthenia (or with hysteria) and has been overly impressed with the precipitation of the illness by an emotional crisis. Furthermore, fatigability is a feature of all of these conditions, but only in the psychiatric ones does it extend to the sphere of mental endurance. Those with myasthenia do not usually complain of fatigue of the mind, whereas these are frequent complaints in psychiatric conditions. A similar problem arises frequently on our services in judging breathlessness due to anxiety or cardiopulmonary disease in a patient with presumed myasthenia. Careful appraisal of the breathing pattern and determination of the vital capacity or other spirometric measurements are helpful here.
3. Progressive external ophthalmoplegia and other restricted myopathies, including the congenital myasthenic states. These may be mistaken for long-established myasthenia gravis. It should be emphasized that the extraocular muscles and levator palpebrae may be permanently damaged by myasthenia and cease to respond to neostigmine. Another possibility is that restricted ocular myasthenia may not respond to anticholinesterase drugs from the beginning and the diagnosis of myasthenia is erroneously excluded. One must then turn to other muscles for clinical and electromyographic and serologic confirmation of the diagnosis.
4. Myasthenia with dysarthria and dysphagia, but without ptosis or obvious strabismus. These may be confused with multiple sclerosis, polymyositis, inclusion body myopathy, stroke, motor neuron disease, or some other neurologic disease. Testing with an anticholinesterase inhibitor, single-fiber and repetitive stimulation recording, and measurement of antibodies usually clarifies the matter.