Eye muscle antibodies in patients with ocular myasthenia gravis: possible mechanism for eye muscle inflammation in acetylcholine-receptor antibody-negative patients

[Ocular myasthenia gravis]

The article describes an urgent medical and social problem - the diagnosis and treatment of the ocular myasthenia gravis. Despite recent advances in the study of synaptic pathology, the diagnosis of the ocular form of myasthenia gravis remains a challenging problem. This is due to the poor information content of laboratory and electromyographic methods in the diagnosis of ocular myasthenia gravis, and the generalized myasthenia gravis manifests in 90% of cases by external ophthalmoplegia and ptosis. The article highlights the features of the diagnosis and differential diagnosis of ocular myasthenia gravis. Data on the course and features of the clinical presentation of the disease are presented. The importance of analyzing anamnestic data and clinical manifestations of the disease is emphasized. The article discusses modern approaches to the treatment of the ocular myasthenia gravis. The authors present clinical observations of 2 patients with ocular myasthenia gravis.

Ocular Weakness in Myasthenia Gravis: Changes in Affected Muscles are a Distinct Clinical Feature

Introduction:

In this study we quantitatively describe ocular weakness patterns in myasthenia gravis (MG) to help neurologists in making the clinical diagnosis and to investigate how the current outcome measures reflect ocular weakness in MG.

Methods:

We investigated ptosis and diplopia patterns in a retro- and prospective cohort of 306 MG patients. Diplopia was systematically examined by testing extra-ocular muscle (EOM) fatigability in two horizontal and four oblique directions for 60 seconds.

Results:

Of patients with initial symmetric ptosis, 40% developed asymmetric ptosis at the second visit. Changes in form of ptosis occurred less often in seronegative MG patients (50%) than in patients with acetylcholine receptor (AChR) antibodies (70%) or muscle-specific kinase (MuSK) antibodies (69%) (p = 0.038). Of patients with diplopia on the first visit, double vision contained both a vertical and horizontal component in 95%. At the second visit, 83% manifested diplopia in other gaze directions. The mean time (in seconds) to diplopia was 11.6±14.0 and the mean time to ptosis was 27.6±19.8. Diplopia or ptosis manifested within 30 seconds in 87% and 58%, respectively. Patients who manifested diplopia after 30 seconds, reported no limitations due to diplopia.

Discussion:

Changes in the gaze directions in which diplopia occurs or ptosis side occur frequently in MG. In diagnostically challenging cases, we recommend testing ptosis and diplopia in multiple gaze directions for 30–60 seconds during at least two follow-up visits to maximize the chance of observing changes in ocular weakness patterns.

Ocular myasthenia: diagnosis, treatment, and pathogenesis

BACKGROUND

Although myasthenia gravis (MG) is often considered the best-understood autoimmune disorder and effective treatments have controlled life-threatening complications, the pathogenesis of ocular myasthenia (OM) remains enigmatic, and its clinical consequences offer therapeutic challenges.

REVIEW SUMMARY

About half of patients with MG present with visual complaints of droopy eyelids or double vision, and many will remain with purely ocular muscle weakness without generalized weakness, defined as OM. OM may be confused with disorders of the brainstem, ocular motor nerves, and eye muscles. Frustrating for the clinician, confirmatory tests such as the edrophonium test, serum acetylcholine receptor antibodies, and standard electrodiagnostic evaluations may fail to positively identify the clinical suspicion of OM. Patients may derive relief from nonpharmacologic interventions and cholinesterase inhibitors, but most will desire better symptom control with corticosteroids or need other immunosuppression. Early corticosteroid therapy may reduce the probability of generalization of the disease. The reasons for ocular muscle involvement by OM include physiologic and cellular properties of the ocular motor system and the unique immunology of OM, which, when better understood, will lead to novel treatments.

CONCLUSIONS

OM is a challenging disorder for the clinician and scientist, with both learning from the other for the betterment of the patient. The future requires answers to why the ocular muscles are so frequently involved by MG, whether the generalization of the disease may be limited by early corticosteroid treatment, and what treatment options may be developed which will improve symptoms without long-term complications.

Antibodies targeting the calcium binding skeletal muscle protein calsequestrin are specific markers of ophthalmopathy and sensitive indicators of ocular myopathy in patients with Graves' disease

We have identified several eye muscle antigens and studied the significance of the corresponding serum autoantibodies in patients with Graves' disease. Of these antigens, only calsequestrin is expressed more in eye muscle than other skeletal muscles, which could explain at least partly the specific involvement of eye muscle in patients with Graves' disease. Earlier, we found a modest relationship between anti-calsequestrin antibodies and ophthalmopathy, but in that study we used calsequestrin prepared from rabbit heart muscle and measured antibodies by immunoblotting. We have reinvestigated the prevalences of anti-calsequestrin antibodies in larger groups of well-characterized patients with thyroid autoimmunity with and without ophthalmopathy and control patients and healthy subjects, using standard enzyme-linked immunosorbent assay incorporating highly purified rabbit skeletal muscle calsequestrin, which has a 97% homology with human calsequestrin, as antigen. Anti-calsequestrin antibodies were detected in 78% of patients with active congestive ophthalmopathy, in 92% of those with active inflammation and eye muscle involvement, but in only 22% of patients with chronic, 'burnt out' disease. Tests were also positive in 5% of patients with Graves' hyperthyroidism without evident ophthalmopathy (two patients) and one patient with 'watery eyes' but no other clear signs of congestive ophthalmopathy and IgA nephropathy and no known thyroid disease, but in no patient with Hashimoto's thyroiditis, toxic nodular goitre, non-toxic multi-nodular goitre or diabetes, or age- and sex-matched healthy subjects. In serial studies of all 11 patients with Graves' hyperthyroidism who had active ophthalmopathy at the time of the first clinic visit, or developed eye signs during the first 6 months, and positive anti-calsequestrin antibodies in at least one sample, anti-calsequestrin antibodies correlated with the onset of ocular myopathy in six patients. Antibodies targeting calsequestrin appear to be specific markers for ophthalmopathy and sensitive indicators of the ocular myopathy subtype of ophthalmopathy in patients with thyroid autoimmunity. However, these results must be considered preliminary until a large prospective study of patients with newly diagnosed Graves' hyperthyroidism, in which serum levels of calsequestrin antibodies are correlated with clinical changes and orbital eye muscle and connective tissue/fat volumes, has been carried out.

The clinicopathologic basis of Graves' ophthalmopathy: a review

PURPOSE

Graves' ophthalmopathy (GO) is a controversial disease, with disagreement within the medical community regarding its pathogenesis, diagnosis, and treatment.

METHODS

We reviewed recent literature on clinical and pathological aspects of GO from both the endocrinologist's and ophthalmologist's perspective.

RESULTS

Investigations into the pathogenesis of GO have included possible antigenic targets, orbital cell types, and development of animal models. Diagnosis has been improved recently with new tools and grading systems, but can be complicated by conditions that may simulate one or more of the findings of GO. The new findings of clinical studies also compel practitioners to reassess commonly used GO treatments such as orbital irradiation.

CONCLUSIONS

Improved understanding of the pathogenic mechanisms of GO should hopefully lead to new diagnostic and therapeutic approaches to this problematic condition.

Thyroid-associated ophthalmopathy: a practical guide to classification, natural history and management

Thyroid-associated ophthalmopathy (TAO) is an autoimmune disorder that can be divided into three clinical subtypes: congestive, myopathic and mixed ophthalmopathy. It is probably caused by immune cross-reactivity between orbital and thyroid antigens. The best candidate antigens are the thyrotropin receptor and the novel protein, G2s, which is now identified as a fragment of the winged helix transcription factor, FOXP1. The relationship between radioiodine therapy and TAO is controversial, with two randomised controlled trials showing a transient worsening of the eye disease after treatment. The diagnosis of TAO is a clinical one, based on the presence of specific symptoms and signs. Orbital imaging, preferably magnetic resonance imaging, is useful when the diagnosis is in doubt and in patients with suspected optic neuropathy who may benefit from early intervention. Despite their lack of specificity, orbital antibodies may add weight to the diagnosis and may potentially be a useful tool in classifying the different subtypes of TAO and in monitoring disease activity. While antibodies against G2s and the thyrotropin receptor are seen in all subtypes, those against Fp and collagen XIII may be associated with the myopathic and congestive subtypes, respectively, where Fp is the flavoprotein subunit of the mitochondrial enzyme, succinate dehydrogenase. In most patients, TAO is self-limiting and no specific treatment is required. When treatment is indicated, glucocorticoids are the mainstay of therapy. Orbital radiotherapy improves the efficacy of glucocorticoids, but is probably less beneficial as monotherapy. Orbital surgery is best reserved for patients with 'burnt out' inactive disease, but urgent orbital decompression may be required for optic neuropathy. The severity and clinical activity of TAO are important in determining the need for specific treatment and the likelihood of success with medical therapy, respectively.

Eye muscle antibodies in Graves' ophthalmopathy: pathogenic or secondary epiphenomenon?

The extra ocular (eye) muscles are one of the principal tissues involved in the autoimmune-mediated inflammation of Graves' ophthalmopathy (GO). Several eye muscle proteins are targeted by autoantibodies or sensitized T lymphocytes, or both, and include: G2s, which is now identified as the terminal 141 amino acids of the winged-helix transcription factor FOXP1, the flavoprotein (Fp) subunit of the mitochondrial enzyme succinate dehydrogenase, the so-called "64kDa protein", a non-tissue specific membrane protein called 1D and the calcium binding protein calsequestrin. Of these, antibodies against G2s and Fp are the most sensitive markers of eye muscle damage in patients with thyroid autoimmunity even though neither antigen is specific to eye muscle and neither antibody is specific to GO. However, the recent finding that the calsequestrin gene is 4.7 times more expressed in eye muscles than other skeletal muscles suggests that we should reconsider the possible role of anti-calsequestrin autoantibodies in ophthalmopathy. GO may comprise two main subtypes with different pathogenetic mechanisms, namely ocular myopathy in which eye muscle inflammation predominates and congestive ophthalmopathy where inflammatory changes occur in the periorbital connective tissues in the absence of eye muscle dysfunction. Anti-G2s and anti-Fp antibodies are closely associated with the ocular myopathy subtype of GO while antibodies targeting type XIII collagen, the only member of the collagen family to have a transmembrane domain, are closely linked to congestive ophthalmopathy. Since both G2s and Fp are intracellular antigens it is unlikely that either antibody causes eye muscle fiber damage in GO, although a role in the later stages of the disease when the fiber has released its cellular contents has not been excluded. Eye muscle antibodies that are cytotoxic to eye muscle cells in antibody-dependent cell-mediated cytotoxicity (ADCC) are more likely to play a role in eye muscle fiber damage since they target a putative eye muscle cell membrane antigen, the identity of which is currently being investigated. While anti-G2s and anti-Fp antibodies are probably secondary to an underlying reaction, such as cytotoxic T lymphocyte targeting of an eye muscle membrane antigen that has yet to be identified, they are reliable markers of immunologically mediated eye muscle fiber damage in patients with Graves' hyperthyroidism. In conclusion, while a pathogenic role for eye muscle antibodies has not been excluded, they are most likely secondary to cytotoxic T cell reactions in GO and, as such, good markers of this autoimmune disease.

Specific stimulation of peripheral blood mononuclear cells from patients with acute myocarditis by peptide-bound flavin adenine dinucleotide (FAD), a naturally occurring autologous hapten

The tryptic FAD-peptide carrying the flavin in 8alpha-(N3)histidyl linkage as natural hapten was isolated by HPLC from the bacterial enzyme 6-hydroxy-d-nicotine oxidase. The same flavin protein linkage is found in the mitochondrial succinate dehydrogenase flavoprotein subunit, the predominant flavoprotein with covalently bound FAD in mitochondria of cardiomyocytes. Peripheral blood mononuclear cells (PBMC) were isolated from four patients with acute myocarditis, seven patients with dilated cardiomyopathy (DCM) and from four healthy control individuals. The response of PBMC to the FAD-peptide was evaluated by measuring proliferation ([3H]-dThd incorporation) and cytokine secretion [interferon (IFN)-gamma]. PBMC from all patients with acute myocarditis showed positive responses to the FAD-peptide, in contrast to PBMC from patients with DCM or control individuals. Following the recovery of the patients from the acute inflammation of the heart, PBMC no longer exhibited a proliferation response to the FAD-peptide. A chemically synthesized FAD-free peptide with identical amino acid sequence induced no response of PBMC. The results are consistent with a recall response by activated T cells, specific for the normally cryptic mitochondrial flavin-hapten, which may be liberated following cardiomyocyte destruction during the inflammation of the heart.

Analysis of mitochondrial antigens reveals inner membrane succinate dehydrogenase flavoprotein subunit as autoantigen to antibodies in anti-M7 sera

The role of mitochondrial proteins as antigens to antibodies of anti-M7 sera was analysed by flavin fluorescence, one- and two-dimensional Western blots and blue native gel electrophoresis. Flavin fluorescence of succinate dehydrogenase (SucDH, complex II of the respiratory chain) of rat liver inner mitochondrial membranes correlated with the immunoreactivity of a representative anti-M7 myocarditis serum. Antigens of isolated bovine heart mitochondria reacting with antibodies of myocarditis serum on two-dimensional Western blots were identified by MALDI-TOF and NanoESI mass spectrometry as myosin heavy chain beta and as dihydrolipoamide dehydrogenase of the mitochondrial 2-oxoacid dehydrogenase complexes. The SucDH-flavoprotein was not resolved as a discrete protein spot on two-dimensional polyacrylamide gels. However, separation of the rat liver inner mitochondrial membrane complexes by blue native gel electrophoresis followed by Western blotting, and Western blots of purified Escherichia coli SucDH complex revealed that anti-M7 sera contained antibodies directed against the SucDH-flavoprotein subunit.

Immunosuppressive treatment of ocular myasthenia gravis

Myasthenia gravis (MG) is caused by autoantibodies against proteins at the neuromuscular junction. This autoimmune process leads to abnormal fatiguability and weakness of striated muscle. Ptosis and diplopia are among the most common manifestations of MG. The term "ocular MG" (OMG) as opposed to "generalised MG" (GMG) is used to define the clinical subtype of MG with isolated eye muscle weakness. Although OMG may appear to cause only moderate disability, it can significantly impair the patient's activities of daily living and progress to generalised myasthenia. Therefore, a clear management plan should be installed early in these patients. Since prospective treatment trials have not been performed, basic management strategies for OMG have to be deduced from retrospective studies, trials in GMG, and generally accepted clinical experience. Cholinesterase inhibitors are used in all types of MG, but are often less helpful in OMG. In the absence of thymoma, thymectomy is usually not considered in OMG, although a few studies have described histological abnormalities in thymuses from patients with OMG. Corticosteroids are of great short term benefit in most patients with OMG but potential adverse effects limit their long term use. Azathioprine is needed to reduce long term corticosteroid adverse effects, but this agent requires about 6 months to be effective. In summary, OMG has a good prognosis in most patients, with corticosteroids and azathioprine being the major treatment options. The challenges for the clinician are to recognise the condition despite the large number of differential diagnoses, to minimise the patient's symptoms using the therapies available and to carefully limit potentially hazardous therapeutic efforts, especially in mild or even uncertain cases.

Myasthenia gravis with ocular involvement in older patients

BACKGROUND

There has been little previous study reporting the eye findings and presentation of elderly patients with myasthenia gravis. The purpose of this study was to review the findings and course of myasthenia gravis after the sixth decade of life.

METHODS

Retrospective observational case series. The authors reviewed the clinical records of 27 patients with onset of myasthenia gravis at age 60 years or more who were seen at a tertiary care academic ophthalmology centre in Houston between January 1992 and March 1999. The diagnosis of myasthenia gravis was based on conventional clinical and laboratory criteria.

RESULTS

Twenty patients (74%) were men. Of the 16 patients who underwent testing for anti-acetylcholine receptor antibodies, 11 (69%) were seropositive. Concurrent thyroid disease was found in seven patients (26%), including five (71%) of the seven women. No patient had thymoma. Sixteen patients (59%) manifested generalized symptoms during follow-up; 12 did so within 1 year of disease onset. Patients responded well to both anticholinesterase and corticosteroid therapy. At the most recent follow-up visit 18 patients (67%) were clinically improved, and no patient was clinically worse.

INTERPRETATION

Myasthenia gravis in this study was characterized by a male predominance, high rate of concurrent thyroid disease, high rate of progression to mild generalized symptoms, absence of thymoma, good response to medical therapy and minimal life-threatening complications. Clinicians should consider the diagnosis of myasthenia gravis in an older patient presenting with diplopia or ptosis.