Isolated bilateral triceps muscle weakness as a presenting complaint in myasthenia gravis: A review
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.258004
Source of Support: None, Conflict of Interest: None
Weakness in myasthenia gravis (MG) has a characteristic predilection for ocular, bulbar, and proximal muscles; although, in unusual and atypical manifestations like limb girdle MG, predominant distal weakness presenting as wrist drop and finger drop, isolated foot drop, and head drop do exist with a prevalence rate of 4.4% among MG patients. Here, we report a case who presented clinically with isolated triceps muscle weakness, along with mild ocular manifestations at a later date. Knowing the variations existing in the presentation of the disease improves the approach towards establishing the diagnosis, decreases unnecessary investigations which are a financial burden to the patient, and also has therapeutic implications.
A 47-year old gentleman, working as a security guard, presented with a 6-month history of bilateral proximal upper limb weakness near the elbow joint. Weakness was insidious in onset, noticed in the form of inability to lift the objects from the shelf, with difficulty in straightening at elbow joint. There was a significant diurnal variation with worsening of the motor power during the evening hours. This used to improve after sleep and rest. The weakness was progressive, with impairment in the working abilities characterized by inability to do pushups in the security practice sessions, and inability to push the door/gates by extension at the elbow joint in the working place. However, there was no history suggestive of other proximal and distal muscle weakness apart from weakness at elbow for extension. The patient recently noticed a mild drooping of bilateral eyelids (left more than right side) of 2-week duration, with the double vision improving after rest and sleep. There was no history of altered higher mental functions, other cranial nerve dysfunction, and bowel and bladder symptoms.
Neurological examination demonstrated normal higher mental functions, mild fatigable bilateral ptosis (left > right), and improvement with rest and on ice pack test. No weakness and fatigability of proximal (deltoid and trapezius) muscle group was noted; however, bilateral triceps muscles showed wasting and flaccidity with a power of 2/5 on the Medical Research Council (MRC) grading [Figure 1]. Clinical examinations of other muscle groups of the upper limb and lower limb were normal. Sensory and cerebellar examination were normal.
After clinical evaluation, the possibility of MG was considered. Differential diagnosis of cervical compressive myeloradiculopathy was kept in mind, in view of isolated weakness with wasting of bilateral triceps muscle.
Repetitive nerve stimulation (RNS) showed a decremental responses in bilateral deltoid [Figure 2] and triceps muscles. On the other hand, the amplitude decrement in other muscles tested including the trapezius, orbicularis oculi, and nasalis, were within normal limits (<10%) bilaterally. Magnetic resonance imaging (MRI) of the cervical spine was suggestive of moderate central disc prolapse, cervical canal stenosis with cord compression at the C5–C6 level with mild signal changes in the cord [Figure 3]. As the above test further increased the diagnostic dilemma, further evaluation was done with neostigmine challenge test, which showed significant triceps power improvement (from MRC 2/5 to 4/5) [Figure 3], and additionally, radioimmunoassay for serum acetylcholine receptor (AChR) antibodies revealed positivity. Computed tomography (CT) of the chest showed no evidence of thymic hyperplasia or the presence of a thymoma.
A definitive diagnosis of MG was made after clinical examination and diagnostic evaluation in spite of the atypical group of muscle involvement, giving due weightage to the characteristic fatigable weakness, positive RNS, AChR antibody positivity, and significant improvement of bilateral triceps power with neostigmine challenge test. The diagnosis of compressive myelopathy was excluded as there were no upper motor neuron signs below the level of compression and no abnormalities were noted on sensory examination. Moreover, the neostigmine challenge test showing significant improvement in triceps power, which is unlikely with compressive myelopathy. Surgery was deferred. Diagnosis was further justified as there was significant improvement in the triceps power on follow-up at 3 months following immunomodulation with steroids and symptomatic treatment with pyridostigmine.
MG is a disease of neuromuscular junction due to autoimmune destruction/defect or congenital defect in the acetylcholine receptors (AChRs). MG is differentiated from other diseases by its special character of “fatigability.” MG is usually a generalized disorder. Ocular myasthenia is the only well-defined focal subtype. The clinical variation in the form of an atypical presentation of MG is a well-reported entity; however, the pathological basis for selective muscle involvement in MG is poorly understood. The high predilection of specific muscle involvement in MG is well recognized. These include the levator palpebrae superioris, extraocular muscles (medial and superior rectus), neck flexors, and deltoid muscles. In patients with predominant involvement of selected muscles like the distal predominant weakness, hand and finger extensor muscles (causing wrist drop and finger drop) are more frequently involved than distal leg muscles (foot drop). Neck extensor weakness is less common and presents as the “dropped head syndrome.”, Predominant and isolated involvement of these muscles at the onset of symptoms mimics myopathy, radiculopathy, and myelopathy, causing a diagnostic dilemma. Although we were not able to exclude superimposed myopathies by doing a muscle biopsy, muscle pathology was not considered as clinical, serological (antibody against AChR), and electrophysiological (RNS) evidence were strongly suggestive of the presence of MG.
The patient evaluated in our study presented with the development of bilateral focal triceps weakness with wasting initially, with recent mild involvement of ocular muscles. Clinically isolated or predominant triceps muscle weakness in MG is a known but uncommon entity, which has been reported commonly among the African-American population., To the best of our knowledge, our case is the first reported case from the Indian/Asian population and the second case of Asian ethnicity. On comparing the clinical variants of our patient to the previously reported case series and case reports [Table 1], we have observed similarities in male preponderance and the mean age of diagnosis (mean = 48 years, our patient's age 47 years).
Although muscular atrophy is not frequent in MG, it has been reported in a significant number (10% of study population) of patients and was ascribed to denervation as result of chronic depletion of acetylcholine at the involved motor end plates., A recent case report from India described significant proximal muscle wasting and shoulder girdle weakness as manifestations of an atypical presentation in myasthenia gravis.
The features of compressive myelopathy on MRI of the cervical spine was an incidental finding in our case, as there was considerable improvement in the triceps power (2/5 to 4/5) on performing the neostigmine challenge test, and significant improvement in the triceps power (4+/5) was also noted on the follow-up visit following treatment with immunomodulation in association with steroids.
This report along with the previously published literature highlights the importance of considering atypical presentations of MG for a timely diagnosis and giving a due weightage to the correlation of the clinical history/examination with the various investigations. Clinical neurology still has relevance, and one needs to treat the patient and not the investigation.
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The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Dr. Prof (Brig) S PGorthi, Professor and Head of Department, Department of Neurology, Kasturba Hospital, Manipal Academy of Higher Education, Manipal.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3]