Normal CrC is dependent on patient age and, to a lesser degree, on gender (women have a lower CrC compared with men because of a lower overall body muscle mass). Race also affects the result. Patients older than 50 years have a lower CrC because of age-related decreases in muscle mass. The test is highly dependent on accurate collection of a 24-hour urine sample. CrC is also affected by dietary restrictions (vegetarian or vegan) and use of certain systemic medication (trimethoprim/sulfamethoxazole, cimetidine, etc.).
GFR can be estimated using serum Cr without a 24-urine collection by including the patient’s age, gender, and ethnicity, and is reported as an estimated GFR (eGFR).13,14 Many laboratories will use this formula to report eGFR in every patient who undergoes serum Cr testing.
Despite some limitations, eGFR appears to correlate well with GFR testing by CrC in most adults and is especially useful in detecting early renal failure. However, eGFR should be used with caution in certain populations (children, older adults, pregnant women, patients with paraplegia, etc.).
Clinical Significance
Renal function affects a variety of aspects in oculoplastic and orbital disease management, most importantly antibiotic dosing. The issue of antibiotic dosing is especially significant in critically ill patients with concomitant acute renal insufficiency.15–17 One of the major impacts of renal function in the evaluation of orbital pathology is with the use of intravenous contrast agents for computed tomography (CT) and magnetic resonance imaging (MRI) and is discussed in Chapter 5. The safety of intravenous contrast (IVC) agents is highly dependent on renal function. IVC is the most common cause of iatrogenic acute kidney injury.18 Contrast-induced nephrotoxicity (CIN) is uncommon in patients with normal kidney function, with an incidence of 1% to 2%. However, the incidence rises dramatically to as high as 25% in patients with chronic kidney disease.18 Prevention of CIN mainly involves adequate precontrast hydration, either with intravenous isotonic saline or with unrestricted oral fluid intake.19
Tests for Inflammatory Disease
Thyroid Eye Disease (TED)
Introduction
The use of serologic testing in diagnosing thyroid disease, using serum thyroid stimulating hormone, triiodothyronine (T3), and free thyroxine (tetraiodothyronine,T4), measured with chemiluminescent microparticle immunoassays, is well established.20 More controversial, however, is the use of thyroid function testing in the monitoring of TED treatment and progression. Activity of systemic thyroid disease and severity of TED are known to be discordant.21 Newer tests of autoimmune function, including thyroid receptor stimulating and inhibitory antibodies, have been proposed as markers of TED activity.
Thyroid-Stimulating Hormone (TSH)
Serum TSH has the highest sensitivity and specificity of any single blood test used to evaluate hyperthyroidism and should be used as an initial screening test for suspected thyroid dysfunction.22 Barring a TSH-producing pituitary adenoma, thyroid hormone resistance, and rare thyroid hormone-binding protein disorders, normal serum TSH is almost never associated with hyperthyroidism.22
Triiodothyronine (T3)
T3 is the active form of thyroid hormone. It mediates cellular actions by binding to nuclear receptor proteins functioning as transcription factors to regulate gene expression.22,23 Elevated levels of T3 may suggest increased severity of hyperthyroidism and, at least in pediatric populations, may be associated with the presence of TED.24
Thyroxine (T4)
T4 is the thyroid hormone precursor, which is deiodinated in the thyroid gland to form T3.23 Diagnostic accuracy for suspected hyperthyroidism improves when serum free T4 is added to TSH as an initial workup.22 As long as the pituitary–thyroid axis is intact, free T4 and TSH share an inverse logarithmic relationship, in which small changes in free T4 have large effects on TSH levels.22 Dosing of thyroid hormone replacement therapy is based on assessment of free T4.22 Unfortunately, the free T4 level has not been shown to have any significant association with the course of TED.25
Thyroid-Stimulating Hormone Receptor (TSHR) Antibody Testing
TED is, at least in part, caused by antibodies to the thyroid stimulating hormone receptor (TSHR) that activates thyrocytes.23,26 TSHR messenger RNA (mRNA) and functional protein have been demonstrated in orbital fibroblasts, with higher levels in patients with TED than in normal persons.26 Therefore, laboratory testing to measure these antibodies theoretically assists in the diagnosis and management of TED. There are currently two methods to assess TSHR antibodies.20 The first is to measure all immunoglobulins targeting the TSHR, as in the thyrotropin-binding inhibitory immunoglobulin (TBII) test. Although methods measuring the binding of antibodies in patient sera to TSHR on the surface of test tubes, plates, or beads display high sensitivity and specificity, they do not measure the functional activity of the immunoglobulins.27
The second method distinguishes stimulating, binding, and blocking antibodies by measuring their downstream effects, as in the thyroid-stimulating immunoglobulin (TSI) assay.
Controversy: Although thyroid autoantibodies may be helpful in delineating the etiology of thyroid disease and in identifying patients at high risk of TED, they are not a substitute for thorough history taking and physical examination, and for measurements of TSH, T4, and T3.28 Results should be interpreted with caution, as there has been high intermethod variability between different antibody assays.29 Also, comparison of clinical studies is limited by wide variations in TED scoring methods, sensitivity, and specificity of antibody testing, and length of patient follow-up.
Thyrotropin-Binding Inhibitory Immunoglobulin (TBII).
TBII testing measures the ability of antibodies to inhibit TSH binding to its receptor. This reflects the presence of antibodies but does not distinguish between stimulatory and inhibitory immunoglobulin classes. Although the TBII assay displays a high sensitivity and specificity for TSHR autoantibodies, it does not measure their functional activity.20 TBII is measured using a radioreceptor assay, and results are presented as a percent inhibition of the TSHR.
Presently, there is insufficient evidence for TBII alone to be used as a marker of TED severity or as an independent guide for treatment.
Thyroid-Stimulating Immunoglobulin (TSI).
TSI bioassay distinguishes antibodies that stimulate, bind, and block TSHR through their downstream effect on cyclic adenosine monophosphate (cAMP) production.20 TSI values are calculated as a percent increase in cAMP over control normal human serum. The normal reference range is under 140% of baseline.
Current evidence suggests that TSI activation of TSHR found in orbital tissue may upregulate inflammatory cytokines, causing the proliferation of preadipocytes and orbital fibroblasts seen in TED.30 It follows that TSI may be an independent predictor of TED activity and may be regarded as a functional biomarker.20,27,31 TSI levels strongly correlate with TED clinical activity and clinical severity scores.27 A positive association has been demonstrated in both pediatric and adult populations with elevated initial TSI levels and the development and progression of TED.30,32,33
A definitive TSI metric, however, has not been established. A retrospective study in a small cohort of patients has suggested that a TSI level greater than 400% of basal activity may be a predictor of patients with hyperthyroidism at high risk for the development of orbitopathy and likely to benefit from early referral to an ophthalmologist.30 In a large cross-sectional retrospective study in children, serum TSI elevation was shown to be a sensitive, specific, and reproducible biomarker for TED, present in 100% of children with both Graves disease and TED, and in 100% of children with euthyroid TED.34
Thyroid Peroxidase Antibody (TPOAb)
Thyroid peroxidase is an enzyme that catalyzes iodination in thyroid hormone biosynthesis. It is a major microsomal antigen against which antibodies are directed in autoimmune thyroid disease and can aid in the diagnosis of Hashimoto thyroiditis, Graves disease, and postpartum thyroiditis. TPOAb is measured by radioimmunoassay (RSR, Cardiff). It has replaced antimicrosomal antibody measurement in modern laboratory testing.
The presence of TPOAb does not correlate with TED activity, severity, or TSI levels.27 In fact, TPOAb negativity may be more prevalent in patients with TED, although the mechanism is not known.31 TPOAb negativity in combination with elevated TSI may be associated with a markedly increased risk of clinically evident TED, with an odds ratio of nearly 37.31
Thyroglobulin Antibodies (TgAb)
Thyroglobulin is a protein produced by the follicular cells of the thyroid gland and used in the production of T3 and T4 within the thyroid gland. TgAb have been found in patients with thyroid carcinoma, Hashimoto thyroiditis, and Graves disease, as well as in normal euthyroid individuals. TgAbs are measured using enzyme immunoassay. Unfortunately, there is no known correlation between TgAb and TED.31
Insulin-Like Growth Factor-1 Antibodies
Antibodies to insulin-like growth factor (IGF-1) appear to stimulate orbital inflammation and tissue swelling through the recruitment and activation of T cells and the stimulation of hyaluronin production.26 IGF-1 is also known to be a stimulator of adipogenesis. Therefore, autoantibodies directed against IGF-1 may act on a primed population of orbital fibroblasts to increase orbital adiposity and enlarge extraocular muscles.
Extraocular Muscle Antibodies
Antibodies directed against certain muscle antigens (G2s and Fp) may be sensitive, but not specific, markers of eye muscle damage in patients with TED.26 In a small observational series, antibody titers to the G2s and Fp proteins found in extraocular muscles were higher in patients with active TED than in those with inactive TED, although these findings have not been substantiated by larger studies.35
Sarcoidosis
Introduction
Sarcoidosis is defined as abnormal collections of inflammatory cells manifesting as noncaseating granulomas that can form in any organ, most commonly the lungs.
Angiotensin-Converting Enzyme (ACE)
ACE is an important component of the renin cascade, which acts in response to hypovolemia and hypotension. ACE cleaves the larger angiotensinogen I molecule into angiotensin II, which is a potent vasoconstrictor. Angiotensin II changes renal tubular reabsorption or excretion, stimulates antidiuretic hormone (ADH) secretion, and stimulates the adrenal gland. These lead to increased water retention via the renal tubules, vasoconstriction, and increased sympathetic activity.
Normally, ACE is found in many cells and fluids but is particularly plentiful in the pulmonary endothelial capillary cells and in the epithelioid cells of the renal proximal tubules. In certain pathologic conditions, primarily sarcoidosis, ACE is produced by the epithelioid cells and macrophages found in granulomas; serum ACE levels, therefore, reflect the mass of granulomas in the body.36,37 Levels of ACE can be elevated in a variety of conditions, including sarcoidosis, diabetes mellitus, osteoarthritis, and hyperthyroidism.38 Less commonly, ACE levels may be decreased in other pathologies (hypothyroidism, obstructive pulmonary disease, and renal disease), as well as in those using therapeutic ACE inhibitors.38 Measurement of ACE levels is primarily used for the diagnosis and monitoring of sarcoidosis. The ACE level is a reasonable reflection of the severity of sarcoidosis.39 Elevated ACE levels are found in 60% to 90% of patients with active sarcoidosis.40,41
The specificity of the ACE test is also related to the population studied and assay methods. Importantly, serum ACE can be increased in a variety of nonsarcoidal conditions and in other orbital inflammatory conditions, including orbital lymphoma42 and IOIS.43 Immunoglobulin G4 (IgG4) testing has been recently evaluated for a variety of diseases, including sarcoidosis. Orbital sarcoid specimens were found to be positive for IgG4 in 42% of cases.44 It should be noted that ACE activity is normally very high in children and remains elevated into puberty.39
Because of its limited specificity and sensitivity, there has been some confusion over the clinical utility of ACE testing. Although such cautions are certainly warranted when ACE is used in isolation, it remains a valuable tool in the diagnosis of sarcoidosis when taken in context with clinical presentation and tissue biopsy.
Vasculitis
Introduction
Granulomatosis with polyangiitis (GPA, previously called Wegener granulomatosis) was first described in 1897 and was further elucidated by Frederich Wegener in 1936. GPA occurs in all age groups, but most commonly in middle age. There is a predilection for males and whites.45 It is defined as an autoimmune, multisystem disease characterized by necrotizing granulomatous inflammation and vasculitis of small- to medium-sized blood vessels, with a predilection for the respiratory and renal systems. One of the most common nonrespiratory, nonrenal systems affected is the orbit, seen in 45% to 60% of patients with GPA.45–47 The orbit can be the presenting site or the only initial site involved and is, in fact, the most common manifestation of ophthalmic involvement.45,47–49 Although the classic areas of involvement are the pulmonary and renal sites, any site in the body can be affected.
GPA can be broadly separated into two distinct clinical presentations: (1) limited and (2) systemic or generalized. Generalized GPA is a life-threatening condition. In general, the limited form of GPA is encountered by the orbital specialist more frequently (65.5%) compared with the generalized form.50
In addition to the specific serologies listed in the following sections, patients with GPA may also exhibit anemia, leukocytosis, thrombocytosis, and an elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), with reduced C3 and C4 complement levels.45,49
Antineutrophil Cytoplasmic Antibodies (ANCAs)
ANCAs are a group of cytoplasmic autoantibodies associated with particular autoimmune disorders, especially the vasculitides. The two most important ANCA subtypes are prefixed on the basis of their immunofluorescence pattern as they react with the patient’s neutrophils: cytoplasmic (cANCA) or perinuclear/nuclear (pANCA). The pathogenic role of ANCA is controversial, but the ANCAs are strongly associated with several specific vasculitides, including GPA, eosinophilic GPA (Churg-Strauss syndrome), and microscopic polyangiitis. Of note, eosinophilic GPA may rarely involve the orbit and paranasal sinuses, whereas microscopic polyangiitis does not affect the upper respiratory system49,51 and is therefore not an entity of particular importance for orbitologists.
ANCAs attack the neutrophils, specifically targeting proteinase 3 (PR3) and myeloperoxidase (MPO) antigens. In the pathologic state, ANCA leads to upregulation of pathways that increase neutrophil activation and produce reactive oxygen species, leading to inflammation and damage to surrounding tissues.52 In the initial investigation for GPA, c-ANCA testing is performed in a two-step approach. First, an indirect fluorescent antibody test is performed to detect the presence of the antibodies, and if positive, further testing is performed to quantify a titer. In cases of a positive cANCA, an enzyme-linked immunosorbent assay (ELISA) test for antibodies against PR3 is performed (see in the following section).53 On the basis of these findings, the sensitivity of cANCA is 87% to 99%, with lower sensitivity in inactive disease. The specificity of cANCA in active GPA approaches 100%, but rarely other conditions (microscopic polyangiitis, Hodgkin disease, multiple myeloma, HIV infection) may produce a false-positive result.54 Of note, the sensitivity of cANCA testing in isolated sinoorbital GPA appears to be significantly lower than in generalized GPA; in one study, only 32% of patients with known sinoorbital GPA exhibited positive cANCA testing.50
In contrast, the pANCA target antigen (MPO) is variable, resulting in less reliability than cANCA testing; therefore, MPO confirmatory testing is essential in cases of positive pANCA.55 pANCA may be elevated in a variety of disease states, including Kawasaki disease and inflammatory bowel disease (especially ulcerative colitis). In one study, one in five patients with positive pANCA and MPO serology was found to have nonvasculitic disease (rheumatoid arthritis, sarcoidosis, systemic lupus erythematosis, etc.).55 In contrast to cANCA-positive disease, pANCA-positive disease rarely affects the eye or orbit.55. In general, cANCA-positive disease also has a higher vasculitis activity score than pANCA-positive disease.55
Controversy: The utility of ANCA titers in monitoring for disease activity, response to therapy, remission, and relapse is controversial.56,57
Proteinase 3 and Myeloperoxidase
As opposed to the immunofluorescent methods used in cANCA and pANCA testing, PR3 and MPO antibody testing are ELISA-based. The PR3 and MPO tests are recommended as confirmatory tests in patients with positive ANCA results and increase the sensitivity and specificity of ANCA testing.58 The PR3 antigen is a protein found in the cytoplasm of human neutrophils. PR3 is strongly associated with cANCA positivity, whereas MPO is associated with pANCA.55 Depending on the specific laboratory, ANCA, PR3, and MPO may be tested as a complete panel initially, or PR3 and MPO may be tested only in cases of positive ANCA results.
IgG4-Related Disease
Introduction
Immunoglobulins play a key role in the adaptive immune system orchestrated by lymphocytes. Aided by T cells, B cells differentiate into plasma cells, which then synthesize glycoproteins called antibodies or immunoglobulins. Immunoglobulins bind to particular antigens and facilitate downstream functions such as complement activation in the induction of phagocytosis.59 IgG accounts for approximately 75% of the total immunoglobulin presence in human plasma (immunoglobulins M, A, D, and E make up the remaining 25%). There are four known subclasses of IgG, with relative serum concentrations in descending order as follows: IgG1 > IgG2 > IgG3 > IgG4.60 The main differences between the IgG subtypes are found in the molecular bonds between the heavy chains and antigen binding regions (Fig. 4.1), and these differences affect their structural flexibility and biologic activity. Serum IgG at birth is mostly maternal, as all subclasses are able to cross the placenta. Endogenous IgG production does not start until approximately 6 months of life. IgG1 and IgG3 reach adult levels earlier compared with IgG2 and IgG4. In pediatric patients, serum values should be compared with reference values from the same age group. Normal values of IgG vary widely depending on genetics. There is a selective increase of IgG subclasses depending on the stimulating antigen. Repeated stimulation with T cell-dependent antigens may result in a marked IgG4 antibody response. Viruses tend to stimulate IgG1 and IgG3 responses, whereas bacteria incite a more varied response.
Antigen-specific antibodies are detected by ELISA, a biochemical assay in which a target antigen coating a microtiter plate is exposed to a liquid sample containing antibodies that bind to antigen.62 These bound antigens are subsequently linked to a secondary antibody joined to an enzyme that produces a quantifiable visible signal. IgG subclass deficiencies indicate a disturbance of the immune system and do not necessarily provide a definitive diagnosis. Subclass deficiencies, however, have been associated with predispositions to certain types of infections and are associated with specific types of disorders.
Immunoglobulin G4
Serum concentrations of IgG4 are very low (normal in adults <121 mg/dL),63 making IgG4 deficiency difficult to assess. It accounts for 3% to 6% of total serum IgG in healthy adults and is the least abundant subclass.63,64 Normal concentrations between individuals vary by a factor of over 100 but tend to be stable within individuals.63 IgG4 has been called an “odd antibody” because it behaves differently from the other IgG subclasses, binding poorly to complement receptors that activate the immune cascade.63,65 IgG4 is activated by cytokines from type 2 helper T (Th2) cells, which also control IgE production.66
IgG4-related disease (IgG4-RD) is a recently recognized fibroinflammatory condition characterized by tissue infiltration by IgG4-positive plasma cells and elevated serum levels of IgG4 (>135 mg/dL).63,67 IgG4-RD was first identified and described in the pancreas in 2001, but mounting evidence implicates IgG4 as a multiorgan disease including, but not limited to, the lymph nodes, skin, meninges, aorta, liver, breast, salivary glands, bone, and, most relevant to the reader of this text, the lacrimal gland and orbit.64,68,69 A consensus statement from 2011 provides some guidance on the diagnosis of IgG4-RD (Table 4.1).70
Table 4.1
Criteria for IgG4-RD
MINIMAL CRITERIA TO PROPOSE IgG4-RELATED DISEASE INVOLVEMENT OF A NEW ORGAN/SITE (2011 BOSTON CONSENSUS)
1. Characteristic histopathologic findings with elevated IgG4 + plasma cells and IgG4/IgG ratio (two out of three of the following*): a. Dense lymphoplasmacytic infiltrate b. Fibrosis, usually storiform in character 2. Elevated serum IgG4 concentration 3. Effective response to glucocorticoid therapy 4. Report of other organ involvement consistent with IgG4-related disease |
The clinical presentation of IgG4-related orbital disease (IgG4-ROD) mimics that of IOIS and lymphoid hyperplasia. Shared features include progressive periocular swelling and a propensity for bilateral involvement, although IgG4-ROD may be differentiated from IOIS by its longer symptom duration and association with asthma and atopy.64 One study showed that IgG4-ROD accounted for 40% to 50% and 5% to 24% of cases originally diagnosed as lymphoid hyperplasia and IOIS, respectively, based on several recent diagnostic criteria.71
Controversy: It is important not to attribute moderate elevations of serum IgG4 concentration or finding IgG4-positive plasma cells on histopathology to systemic disease without correlation with specific histopathologic patterns, immunohistochemistry and clinical presentation63; other pathologic processes, including GPA and sarcoidosis, may manifest mild elevation of IgG4. Of note, serum IgG4 elevation is not a requirement for the diagnosis of IgG4-RD if tissue specimens show classic histopathologic and immunohistochemical features, especially in patients on prior corticosteroid therapy.63
Despite the heterogeneity of published studies and varied disease presentations, an elevation of serum IgG4 greater than 140 mg/dL for the diagnosis of IgG4-related disease has 90% sensitivity, 60% specificity, 96% negative predictive value, and 34% positive predictive value, as demonstrated in a recently published study.72 Serum IgG4 may be useful as a marker for treatment response; reduction of serum IgG4 levels after glucocorticoid and rituximab treatments have been demonstrated. However, there does not seem to be a clear correlation between persistent serum IgG4 elevation and disease relapse.73
Plasmablast Count
The use of serum IgG4 levels for diagnosing IgG4-RD is limited by poor specificity and positive predictive value, as well as the prozone effect. Wallace et al. have proposed that total circulating plasmablasts may be a new biomarker for IgG4-RD diagnosis and response to therapy.74,75 Total plasmablast counts are measured by flow cytometry of peripheral blood. Median serum concentrations of plasmablasts were nearly 50-fold higher in untreated, biopsy-proven IgG4-RD patients than in healthy controls and nearly 10-fold higher than in patients with other inflammatory diseases before treatment.
Acetylcholine Receptors Antibody Titers and Other Tests for Myasthenia Gravis
Introduction
Myasthenia gravis (MG) typically occurs in one of two distinct patterns. Early-onset MG, defined as onset at age less than 50 years, is classically a disease of females, with a high incidence of thymic hyperplasia; of note, the female predilection disappears with onset between 40 and 50 years.76,77 Late-onset MG, which occurs after 50 years of age, is more common in males.78 Symptom severity usually peaks within 3 years of onset.79 In approximately 10% to 15% of all cases of MG, the disease remains limited to the extraocular muscles and is called ocular MG (oMG). External ophthalmoplegia (with or without diplopia) is the presenting sign in approximately 75% to 85% of patients with MG.80 MG that is initially limited to the ocular adnexa often progresses \to generalized MG within the first 2 years of presentation (50–80%); therefore, the diagnosis of oMG should be reserved to those patients who have not “generalized” within this time window.80–86 Pediatric MG is uncommon in North America and Europe, comprising about 10% to 15% of all MG cases, but this number increases to almost 50% in Asia.87,88 Familial disease is very rare.77 MG has an estimated incidence of 4 to 11 in one million persons and a prevalence of 5 to 15 in 100,000 persons in the United States.80
The typical workup in case of suspected MG usually includes acetylcholine receptor antibody testing, single fiber electromyography (SF-EMG), and CT or MRI of the mediastinum.78,89
Serologic Testing
Acetylcholine receptor (AChR) antibodies remain the mainstay of serologic confirmation of MG and are positive in about 85% of patients with generalized MG.78,90 In patients with thymoma, positive AChR antibodies are the rule. However, patients diagnosed with MG early in their disease may initially lack AChR antibodies; the majority of such patients will develop antibodies within 1 year, and retesting in suspected cases over several months is therefore prudent. The level of AChR antibody may or may not correlate with the severity of MG.76,91,92 Negative AChR titers may be seen in up to 20% of patients with MG and in about one half of patients (40–77%) with oMG.80,86,90,93
There are three readily available AChR antibody assays: binding, blocking, and modulating, named after their presumed modes of action, although the exact mechanisms are still under debate.94–96 The sensitivity varies between each subtype of AChR antibody and the type of thymic involvement (60% positivity in thymic follicular hyperplasia, 20–25% in thymoma, and 9% in cases of thymic atrophy). Of the three subtypes, the binding AChR antibody is the most commonly found in MG and considered the most sensitive for both generalized MG and oMG. Of the 15% to 20% of MG patients with negative AChR antibodies, 40% will be positive to muscle specific kinase (MuSK) antibody testing.78,97 Overall, MuSK antibodies are found in about 5% of MG patients.98 Unlike AChR antibody-positive MG, the MuSK antibody titer definitely correlates with severity of clinical disease.99 MG with MuSK antibodies is distinctly more severe than typical AchR-positive MG.100,101 Of note, ocular symptoms are uncommon in this subgroup of MG. MuSK antibodies are unique among MG antibodies in that they are of the IgG4 subclass and do not bind complement.
Two to 50% of AChR and MuSK antibody-negative MG (“double-seronegative MG”) patients will have positive lipoprotein receptor-related protein 4 (LRP4) titers, depending on ethnicity, race, and geography.97,102,103 In a recent study of LRP4 antibodies by cell-based assay, the overall positivity in double-seronegative MG was 19% but varied widely among different European populations (7–33%).103 A minority of AChR- and MuSK-positive patients may also exhibit LRP4 positivity.103 Of note, 27% of oMG who are AChR and MuSK antibody negative will exhibit LRP4 antibodies, as shown by cell-based assay.103
About 5% of all MG will have negative testing for classic AChR, MuSK, and LRP4 antibodies. This subgroup is known as “seronegative MG.” In about 50% to 60% of this subset, testing of AChR antibodies using a cell-based (rather than the usual radioimmune) assay may show “clustered” AChR antibodies.97,104 This is especially important to remember in oMG, where only 50% of patients are AChR antibody positive; the majority of the AChR seronegative patients will exhibit clustered AChR antibodies.97,104,105 Seronegative MG parallels AChR positive MG clinically and therapeutically.104
Nonserologic Tests of Interest
Ice Test.
Ice testing is highly specific for MG, with a sensitivity of 80% to 90%.106–108 The margin-to-reflex distance is measured first, then an ice pack is placed over the closed eyelids for 2 minutes, and the palpebral fissure is remeasured. An improvement of 2 mm or more in the patient’s ptosis is considered positive.106,108 Diplopia may also improve but not resolve with ice applied for 5 minutes.109
Electrophysiology.
Single-fiber electromyography (SF-EMG) has been reported as the most sensitive test for MG (85% for generalized MG and 50% for oMG).78,93,110 Note that SF-EMG is more sensitive but less specific for MG compared with classic EMG.76 Furthermore, it is significantly more difficult to perform, and therefore access to SF-EMG testing is usually limited to larger university hospital centers. In suspected oMG, SF-EMG of either the frontalis or the orbicularis oculi muscle is preferred to other sites.
Chest Imaging.
Chest imaging, usually with contrast-enhanced computed tomography, should be obtained in all patients with MG to rule out thymic hyperplasia or thymoma, which occurs in about 10% of MG patients overall.86
Edrophonium (Tensilon) Testing.
Edrophonium testing is still performed in many centers; the authors have abandoned this test in favor of other modalities. Although helpful if positive, a negative edrophonium test does not rule out MG. Furthermore, rare but significant cardiac side effects, including bradycardia and asystole, may occur; in older patients with a history of cardiac disease, the test should be performed with careful cardiac monitoring.111,112
Sjögren Syndrome
Introduction
Sjögren syndrome (SS), named after Swedish ophthalmologist Henrik Sjögren, is a chronic autoimmune disorder characterized by B-cell hyperactivity and lymphocytic infiltration of exocrine glands and epithelia.113 Most commonly, this leads to dry eye and dry mouth from salivary and lacrimal gland periepithelial lymphocytic infiltration and damage.114 Epithelial structures of other systems can also be involved, including skin, the peripheral nervous system, kidneys, liver, and lungs. SS is classified as primary (pSS) when in isolation and secondary (sSS) when it occurs with another connective tissue disease. The etiology of SS is unknown and, as with other autoimmune disorders, is thought to be multifactorial, but many include genetic and environmental factors.115–117
There are several classification criteria for the diagnosis of pSS, with the most recent by the American College of Rheumatology (ACR)/Sjögren International Collaborative Clinical Alliance (SICCA) in 2012 (Table 4.2).118,119 This schema uses ocular and salivary gland evaluations along with laboratory testing of antinuclear antibodies (ANAs), subtypes of ANAs, and rheumatoid factor (RF). For the purposes of this section, only these laboratory studies will be discussed.
Table 4.2
Diagnostic Criteria for Sjögren Syndrome (SS) by the American-European Consensus Group and Sjögren International Collaborative Clinical Alliance
| Symptom/Test | American-European Consensus Group Criteria* | American College of Rheumatology/Sjögren International Collaborative Clinical Alliance Criteria† |
| Ocular symptoms | √ | |
| Oral symptoms | √ | |
| Ocular signs | √ | √ |
| Histopathology of salivary gland | √ | √ |
| Salivary function | √ | |
| Serum antibodies | √ | √ |
Since dry eye and dry mouth are the only disease manifestations in 31% of patients with SS, it is important for the oculoplastic surgeon to be aware of the appropriate testing.120
Antinuclear Antibodies
ANAs, or antinuclear factors (ANFs), are autoantibodies that bind to nuclear contents. There are many subtypes of ANAs, each binding to a different nuclear protein or protein complex. The ANA test evaluates blood serum for the presence of autoantibodies, generally using indirect immunofluorescence and ELISA techniques. The level is reported as a titer with immunofluorescence and describes the highest dilution of the serum at which detection of autoantibody is possible; generally, a titer of 1 : 160 or greater is significant. The most common immunofluorescence pattern is speckled. ANAs are present in 59% to 85% of patients with SS.121 However, a positive ANA in isolation is not diagnostic for SS, as it can be positive in systemic lupus erythematosus (SLE) and in up to 23% of healthy adults.122
Antinuclear Antibody Subtypes
Specific autoantibodies are more helpful in the diagnosis of SS compared with ANA titer. Antibodies against anti-SS extractable nuclear antigens are the most important, classified as either anti-Ro and anti-La (named for the donor who provided the prototype serum) or, more commonly, SS-A and SS-B.123 Data correlating these antibody titers and disease activity are conflicted, but in general, higher titers are associated with greater extraglandular manifestations.124
Anti-Ro/SS-A Antibodies.
There are two autoantibodies in the SS-A category that react with different proteins with molecular weights of approximately 52 kD and 60 kD. Antibodies against SS-A are found in up to 50% to 75% of patients with pSS and in 15% with sSS, but can also be found in other autoimmune disorders, particularly SLE, and even in healthy individuals.125,126 The sensitivity of SS-A for pSS is about 97%.125
Anti-La/SS-B Antibody.
SSB/La antibodies are positive in 40% to 50% with pSS and in 15% of patients with SLE.
Rheumatoid Factor
RF is an antibody against the Fc portion of IgG and is the most common autoantibody found in SS.127 High RF (generally over the 95th percentile) can occur in a variety of disorders, including rheumatoid arthritis, SLE, and SS. Although elevated levels of RF can also be found in 5% to 10% of healthy individuals, they are present in 36% to 74% of patients with SS.128,129 RF is not specific to SS, but the sensitivity in SS is 75% to 95%.130 Additionally, there is an association of younger age of onset and extraglandular involvement with elevated RF.117,124
Nonserologic Tests of Interest
Tear Composition Analysis and Ocular Surface.
In addition to the ocular surface evaluation and tear function, tear composition can be evaluated in patients with suspected SS. The analysis of tears for anti-SSA and anti-SSB autoantibodies may be positive even when serum is not.131 Other tear film analyses can be conducted and include concentration of the tear enzymes lysozyme and lactoferrin, which show good sensitivity (85%) and specificity (92%) for aqueous tear deficiency, which is often seen with SS.132 Tear analysis may also show elevated interleukin-6 and tumor necrosis factor-α in SS patients with dry eye symptoms, although this is not specific, since they are elevated in patients with non-SS dry eye .133
Salivary Gland Ultrasonography.
Prior analysis of the salivary gland with scintigraphy has mostly fallen by the wayside. Studies looking at salivary gland ultrasonography, evaluating for heterogeneity of the parenchyma, have been found to have good specificity and fair sensitivity (95% and 63%, respectively) for pSS.134,135
Giant Cell Arteritis (Temporal Arteritis)
Introduction
In addition to myriad systemic and neurologic sequelae, giant cell arteritis (GCA) is a common cause of ischemic optic neuropathy and other cranial neuropathies in the older adult population.136 Systemic manifestations include headache, jaw claudication, scalp tenderness or necrosis, and constitutional symptoms such as fatigue, fever, and weight loss.
The mainstay of GCA testing remains the Westergren ESR, CRP, and platelet count. Unfortunately, these tests tend to be nonspecific, and clinical correlation is of paramount importance. Because of the nonspecific nature of these tests, temporal artery biopsy is typically necessary to confirm or refute any suspected GCA diagnosis.137
Westergren Erythrocyte Sedimentation Rate and C-Reactive Protein
ESR and CRP are among the oldest laboratory tests still in use.138–140 Both bloods tests are used to detect inflammation in the body.141–143 During an inflammatory process, fibrinogen enters blood in large amounts and causes the RBCs to stick to each other, raising the ESR.140 The normal range of ESR is 0 to 23mm/hr for males and 0 to 29 mm/hr for females, but varies depending on age. A widely accepted estimate of normal ESR follows two straightforward formulas:
For males, normal ESR is<age in years/2
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