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Neuroimmunol Neuroinflammation 2014;1:141-6.10.4103/2347-8659.143671
Open AccessOriginal Article

Assessment of cognitive function in patients with myasthenia gravis

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1Department of Neurology and Psychiatry, Assiut University Hospital, Assiut 71516, Egypt.

2Department of ENT, Audiology Unit, Assiut University Hospital, Assiut 71516, Egypt.

Correspondence Address: Dr. Sherifa A. Hamed, Department of Neurology and Psychiatry, Floor No. 7, Room No. 4, Hospital of Neurology and Psychiatry, Assiut University Hospital, Assiut 71516, Assiut, Egypt. E-mail: hamed_sherifa@yahoo.com

    ...

    This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License (http://creativecommons.org/licenses/by-nc-sa/3.0/), which allows others to remix, tweak and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

    Abstract

    Aim: During the past decade, there has been an increasing interest in the evaluation of cognitive function in myasthenia gravis (MG), neuromuscular transmission disorder caused by acetylcholine receptor auto-antibodies. However, the results of previous studies on cognition and MG are inconsistent and controversial. This study aimed to evaluate cognition in patients with mild/moderate grades of MG.

    Methods: This study included 20 patients with MG with a mean age of 28.45 ± 8.89 years and duration of illness of 3.52 ± 1.15 years. Cognition was tested using a sensitive battery of psychometric testing (Mini-mental State Examination [MMSE], Stanford-Binet Intelligence Scale 4th edition [SBIS] and Wechsler Memory Scale-Revised [WMS-R]) and by recording P300 component of event-related potentials (ERPs), a neurophysiological analog for cognitive function.

    Results: Compared with healthy subjects (n = 20), patients had lower total scores of cognitive testing (MMSE, SBIS and WMS-R) (P = 0.001), higher Beck Depression Inventory 2nd edition scores (P = 0.0001) and prolonged latencies (P = 0.01) and reduced amplitudes (P = 0.001) of P300 component of ERPs. Correlations were identified between total scores of cognitive testing and age (r = -0.470, P = 0.010), duration of illness (r = -0.788, P = 0.001) and depression scores (r = -0.323, P = 0.045). Using linear regression analysis and after controlling for age and depression scores, a significant correlation was identified between total scores of cognitive testing and duration of illness (β = -0.305, P = 0.045).

    Conclusion: Patients with mild/moderate MG may have cognitive dysfunction. This is important to determine prognosis and managing patients.

    Introduction

    Myasthenia gravis (MG) is an autoimmune disease caused mainly by auto-antibodies against skeletal muscle nicotinic acetylcholine receptors (nAChRs) at the postsynaptic membrane resulting in depletion of ACh at the neuromuscular junction.[1] MG is uncommon with a prevalence of (25-125)/106. The disease tends to affect women more often than men (3:2) in their second and third decades.[2] The cardinal symptoms of MG are fatigue and weakness of skeletal muscles with repeated or sustained exertion in the course of the day but improved by rest. Ocular muscles are initially involved in about 2/3 of patients then spread to the bulbar and limb muscles. Approximately, 85% of patients develop generalized weakness. Many patients progress from mild to severe disease, and if weakness of respiratory muscles becomes severe enough to require mechanical ventilation, the patient is said to be in crisis.[3] Spontaneous remissions are very rare and last for varying periods that mostly occur during the first 3 years of the disease.[4] In adults, the thymus gland is abnormal in up to 90% of people with MG with approximately 70% of them have thymic hyperplasia while 10-20% have benign thymic tumors or thymoma.[5] The currently used treatment modalities for MG include acetyl choline esterase inhibitors (AChE-Is) (as pyridostigmine),[6] immunopharmacologic drugs (as prednisone,[7] azathioprine,[8] cyclosporine,[9] mycophenolate mofetil,[10] cyclophosphamide,[11] tacrolimus[12] and rituximab[13]) plasmapharesis,[14] intravenous immunoglobulins (IVIGs)[15] and thymectomy.[16]

    Subjective impairments of memory and other cognitive functions are very frequent in patients with MG, however, previous studies, which investigated cognitive function in such patients showed contradictory results. Some reported memory difficulties and other cognitive dysfunction[17-21] and electroencephalographic (EEG) abnormalities.[22-24] In contrast, others reported lack of neuropsychological impairments, normal intelligence, attention, memory and motor performance withMG.[25-27]

    The exact mechanisms of the co-morbid cognitive dysfunction in patients with MG are unknown. The most likely suggested mechanism is central cholinergic deficiency due to the involvement of central neuronal nAChRs and other cholinergic nervous systems and pathways by the immune-mediated processes of MG.[20,28-30] However, controversial views suggest that the co-morbid nervous system manifestations with MG may result from nonspecific mechanisms as complications of MG, which include respiratory impairment, sleep apnea and hypoxia,[31-33] mental fatigue,[26,27,34] adverse effects from medications used for treatment of MG and mood disorder.[35,36]

    This study aimed to investigate cognitive function in adults with mild/moderate MG. Cognitive functions were assessed using a battery of sensitive psychometric testing in addition to recording event-related potentials (ERPs), a neurophysiological analog of cognitive function.

    Methods

    Subjects

    This study included 20 patients (males = 6, females = 14) diagnosed clinically and electrophysiologically as MG. Their age ranged from 16 to 50 years, and duration of illness ranged from 1 to 4 years. Clinical grading of the patients was done based on the medical, scientific advisory board of MG Foundation of America classification.[37] Patients grading was based on their histories and diagnoses shown in their medical records. Patients reported histories of weakness of ocular muscles (ptosis) (class I), of mild and predominant weakness of the limb muscles (class II a) or oropharyngeal muscles (class II b), or with moderate and predominant weakness of the limb muscles (class III a). Before the presentation, all patients were treated with AChE-Is (pyridostigmine bromide or mestinon in a dose of 60 mg/4 h during the daytime and 60 mg at night time), immunotherapy with prednisolone and/or azathioprine (imuran) or plasmapharesis. Thymectomies were performed to the seven patients with thymoma. Table 1 shows the demographic and clinical characteristics of the studied group. Patients were recruited from the Out-patient Clinic of the Department of Neurology, Assiut University Hospital, Assiut, Egypt during their follow-up visits in which they were free of clinical manifestations (i.e. after resolution of active stage of the disease for at least 3 months) and were on maintenance treatment with low doses of AChE-Is and/or steroids. Twenty healthy subjects matched for age, sex and socioeconomic status were included in this study for statistical comparisons. Control subjects were recruited from the general population. This study was accepted by the regional Ethical Committee. Detailed information on the study was given to all patients, and control subjects, and all gave their written consent to attend the study.

    Table 1

    Demographic, clinical and laboratory characteristics of the studied groups

    Demographic and clinical characteristicsPatients (n = 20) (%)Control subjects (n = 20) (%)P
    Male/female4/1610/10-
    Age (years)16-5020-50-
    28.45 ± 8.8930.22 ± 5.760.380
    Duration of illness (years)1-4--
    3.52 ± 1.15
    Clinical grade
     I0--
     IIa/IIb2/10--
     IIIa/IIIb8/0--
     IVa/IVb0--
     V0--
    Thymic pathology
     Normal5 (25)--
     Hyperplasia8 (40)--
     Thymoma7 (35)--
    Previous treatment (single or combination of the followings)
     Acetyl choline esterase inhibitors20 (100)--
     Prednisolone20 (100)--
     Azathioprine8 (40)--
     Plasmapharesis9 (45)--
     Thymectomy7 (25)--

    We excluded subjects (patients and controls) with: (1) respiratory involvement or in crisis (i.e. severe stages of the disease); (2) history of other primary neurological (e.g. transient ischemic attacks, cerebrovascular stroke or epilepsy), psychiatric (e.g. major depression) or medical (e.g. diabetes mellitus) diseases which are known to affect cognition; (3) previous serious head injury; (4) any sensory or motor disorder that would preclude psychological testing (as blindness or deafness); and (5) regular treatment with medications (other than those used for treatment of MG) which may alter cognitive testing (e.g. as benzodiazepines, beta-adrenoceptor antagonists, major tranquillizers and antidepressants).

    Electroencephalographic recording

    Electroencephalographic was done using the eight channels Nihon Kohden machine (4217), employing scalp electrodes placed according to the international 10-20 system with bipolar and referential montages. Hyperventilation and photic stimulation were used as provocation tests.

    Cognitive assessment

    Cognitive functions were assessed independently for each participant by two experienced psychologists and under supervision of a psychiatrist, using a set of standardized Arabic translated neuropsychological tests that are sensitive for mild cognitive impairment and covering different cognitive domains. They included: Mini-Mental State Examination (MMSE),[38,39] Stanford-Binet Intelligence Scale 4th edition (SBIS)[40,41] and Wechsler Memory Scale-Revised (WMS-R).[42] From SBIS, we selected vocabulary and comprehension for assessment of verbal reasoning, pattern analysis for assessment of visual reasoning, quantitation for quantitative reasoning, and bead memory and memory for sentences for short-term memory. From WMS-R, we tested digit forward digit backward, mental control, associate learning, logical memory, and visual reproduction.

    Event related potentials testing

    Before examining ERPs, all participants underwent basic audiological testing (Amplaid Model 720, Milan, Italy). Testing for ERPs was done on a separate day after completion of neuropsychological testing (Neuropack S1 EMG/EP measuring system, MEB-9400 (Nihon Kohden, Japan). ERPs are series of scalp waves that are extracted from the EEG by time domain analysis and averaging of EEG activity following multiple stimulus repetitions. They were elicited with an auditory discrimination task paradigm by presenting a series of biaural 1,000 Hz (standard) versus 2,000 Hz (target) tones at 70 dB with a 10 ms rise/fall and 40 ms plateau time. P300, the late component of ERPs was obtained. Latencies and amplitudes (peak to peak) of P300 component of ERPs were measured. The P300 wave is a positive deflection in the human ERPs. It is most commonly elicited in an “oddball” paradigm when a subject detects an occasional “target” stimulus in a regular train of standard stimuli. The P300 wave only occurs if the subject is actively engaged in the task of detecting the targets. Its amplitude varies with the improbability of the targets. Its latency varies with the difficulty of discriminating the target stimulus from the standard stimuli. Typical peak latency is elicited when a young adult subject makes a simple discrimination in 300 ms. In patients with decreased cognitive ability, the P300 is smaller and later than in age-matched normal subjects. The P300 have multiple intra-cerebral generators, with the hippocampus and various association areas of the neocortex contribute to the development of this potential. The P300 component of ERPs represents the transfer of information to consciousness, a process that involves many different regions of the brain.[43]

    Psychological evaluation

    Standardized psychiatric interview was done by applying the Diagnostic and Statistical Manual of Mental Health Disorders, 4th edition (DSM-IV) criteria for the diagnosis of depression.[44] A differentiation between clinical depression and depressive symptoms was made throughout clinical interview of the patient. The Arabic version[45] of the Beck Depression Inventory, 2nd edition (BDI-II)[46] was used to assess the severity of depressive symptoms. BDI-II items are in alignment with DSM-IV criteria. BDI-II consists of 21 items each corresponds to a symptom of depression summed to give a single score for the BDI-II. According to that scale, the patient may have, not having or has minimal depressive symptoms if scoring 0-13, mild symptoms if scoring 14-19, moderate symptoms if scoring 20-28 and severe symptoms if scoring 29-63.

    Statistical analysis

    Calculations were done with the statistical package SPSS, version 12.0 (SPSS Inc. Chicago, IL, USA). Data were presented as mean ± standard deviation. Student’s t-test was used for comparison of means. Correlations between score of cognitive testing and demographic and clinical characteristics and depression scores were assessed using Pearson’s test. Linear regressions analyses were done using the total score of cognition testing as the dependent variable and age, duration of illness and depression scores as independent variables. For all tests, P < 0.05 was considered as significant.

    Results

    This study included 20 patients with MG. They had a mean age of 28.45 ± 8.89 years and duration of illness of 3.52 ± 1.15 years. Patients reported normal EEG records. All patients had depressive symptoms of mild (n = 15, 75%) and moderate (n = 5, 25%) severities. Each patient had a different combination of abnormalities in various cognitive testing subsets particularly WMS-R (n = 16, 80%). Patients had significantly lower scores of MMSE, different subsets of SBIS, WMS-R and total scores of cognitive testing (MMSE, SBIS and WMS-R) (P = 0.0001) and higher scores of BDI-II (P = 0.0001) [Table 2]. The majority of patients had abnormalities in latency and/or amplitude of P300 component of ERPs (n = 14, 70%). Patients had significantly prolonged latencies (P = 0.01) and reduced amplitudes (P = 0.001) of P300 component of ERPs [Table 3]. Significant correlations were identified between total scores of cognitive testing and P300 latency, P300 amplitude, age, duration of illness and depression scores [Table 4] Using linear regression analysis and after controlling for age and depression scores, significant correlation was identified between total scores of cognitive testing (MMSE, SBIS and WMS-R) and duration of illness (β = -0.305, P = 0.045).

    Table 2

    Comparison between patients and controls in scores of cognitive functions and depression

    Variable Patients (n = 20)Controls (n = 20)P
    MMSE23.25 ± 2.3527.56 ± 1.450.036
    SBST
     Vocabulary36.33 ± 5.4550.45 ± 3.880.042
     Comprehension35.46 ± 9.0749.76 ± 7.560.007
     Total verbal reasoning score75.32 ± 8.8596.82 ± 16.250.0001
     Visual reasoning36.63 ± 4.6448.68 ± 5.040.0001
     Total visual reasoning score68.43 ± 8.0988.33 ± 14.700.0001
     Quantitative test36.57 ± 6.5445.30 ± 5.430.0001
     Total quantitative reasoning score75.53 ± 8.6796.65 ± 9.570.0001
     Bead memory45.30 ± 7.2860.50 ± 10.080.0001
     Memory for sentences44.72 ± 6.3465.56 ± 8.570.0001
     Total score for short-term memory85.65 ± 9.66150.25 ± 25.260.0001
     Total score of SBST289.56 ± 55.48360.34 ± 50.040.0001
     IQ78.53 ± 6.4695.35 ± 8.730.0001
    WMS-R
     Digit forward4.56 ± 1.016.64 ± 0.880.035
     Digit backward2.23 ± 0.255.58 ± 0.450.010
     Mental control3.57 ± 1.455.89 ± 1.060.042
     Logical memory10.65 ± 1.3014.83 ± 2.450.007
     Associate learning8.52 ± 2.0412.06 ± 2.240.005
     Total scores of cognitive testing (MMSE, SBST and WMS-R)76.54 ± 8.3596.54 ± 6.280.0001
     Depression scores20.64 ± 6.248.65 ± 3.550.0001
    Table 3

    Comparison between patients and controls in event‑related potentials

    VariablePatients (n = 20)Controls (n = 20)P
    P300 latency (ms)
    Right sided250.00-450.00285.00-353.00-
    350.80 ± 35.88320.88 ± 25.750.010
    Left sided270.00-450.00250.00-350.00-
    355.60 ± 33.08325.45 ± 20.450.010
    P300 amplitude (mv)
    Right sided2.20-20.256.88-20.54-
    7.55 ± 2.4512.45 ± 2.840.001
    Left sided2.55-18.096.80-22.25-
    6.67 ± 3.2312.63 ± 2.560.001
    Table 4

    Pearson's correlation (r and P value) between total scores of cognitive testing and clinical variables, lab variables, depression scores and ERPs variables

    VariablesTotal scores of cognitive testing (MMSE, SBST and WMS-R)
    rP
    P300 latency−0.6500.001
    P300 amplitude0.5570.001
    Age−0.4700.010
    Duration of illness−0.7880.0001
    Depression scores−0.3230.045

    Discussion

    The results of this study indicate that patients with mild/moderate MG may experience significant manifestations of cognitive impairment in the absence of disease activity and despite the short duration of illness. Patients with MG may experience poor performance in different cognitive tasks indicating central or brain involvement. These included deficits in verbal relations, comprehension, visual reasoning, pattern analysis, quantitation, bead memory, short-term memory and memory for sentences, digit forward, digit backward, mental control, logical memory, and associate learning. In the agreement with our findings, patients with MG commonly reported subjective cognitive complaints. In patients with MG, several previous studies reported memory difficulties[17,18] and impaired performance on varieties of cognitive tests as MMSE and memory tests,[19] the Boston Naming Test, the Logical Memory and Design Reproduction portions of the WMS, Rey Auditory Verbal Learning Test,[17] and measures of response fluency, information processing and verbal and visual learning.[20,21,47] In additions, the detected abnormalities in P300 component of ERPs also suggest the central or brain involvement in MG. In fact, abnormal evoked potential responses were noted in patients with MG.[48-50] In contrast, several studies reported normal IQ, memory, attention and motor performance and normal ERPs in MG.[25-27] We believe that such discrepancies could be explained by differences in methodologies, small sample size, different lists of inclusion and exclusion criteria and lack of control for potential confounding variables.

    Several mechanisms have been hypothesized as etiologies of cognitive impairment in patients with MG. The central cholinergic deficiency due to the involvement of the central nAChRs and central cholinergic pathways by the disease process of MG have been suggested as the high likely mechanisms.[20,28-30] This hypothesis is based on the fact that there are structural identities between different muscle and neuronal nAChRs subunits with the possibility of cross-reactivity between different nAChRs antibodies.[51-53] The hippocampus, a cerebral structure highly involved in learning and memory, has abundant cholinergic innervation and enriched in nAChRs that modulate synaptic plasticity via mechanisms involved in long-term potentiation.[54] Few suggested that cognitive dysfunction co-morbidity may be due to the immune responses driven by muscle and neuronal nAChRs antibodies expressed by cancer (e.g. thymoma) (i.e. paraneoplastic syndrome).[55,56] Others suggested that it might be a nonspecific autoimmune response in presence or absence of tumor.[57] This it further supported by an association of MG with other nonnervous system medical immune-mediated disorders as diabetes mellitus,[58] thyroiditis[59] and systemic lupus erythematosis.[60]

    Despite the strength of our findings, this study had some limitations which include: (1) a relatively small sample size that is explained by the fact that we included only a homogenous group of adults with mild/moderate MG in their remission stages and on maintenance treatment; (2) due to the cross-sectional nature of the study, the temporal relation between the disease stage and occurrence of cognitive deficits is difficult to be determined. Future research should include longitudinal studies that prospectively assess the relation of the disease process to cognition over time; (3) CSF analysis for determination of immune markers of the disease is missing as our ethical committee did not allow CSF sampling for uncomplicated patients. It has been realized that CSF examination better detects immune involvement of CNS, and (4) our investigation did not include testing for mental fatigue or sleep pattern. These factors cannot be excluded as causes or potentials for cognitive dysfunction with MG.

    In conclusion, adults with mild/moderate MG may experience prominent cognitive deficits in different domains regardless to the presence of depression. This is important to determine prognosis and managing patients.

    Financial support and sponsorship

    Nil.

    Conflicts of interest

    There are no conflicts of interest.

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