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myasthenia gravis ( mg ) is an autoimmune syndrome caused by a failure of neuromuscular transmission , which results in variable muscle weakness and is exacerbated by exercise ( 1 ) .
autoantibodies against the muscle nicotinic acetylcholine receptor ( achr ) can be detected in approximately 80 - 85% of all patients with generalized mg ( 2,3 ) and in approximately 40% of anti - achr seronegative mg patients who have antibodies against muscle - specific kinase ( musk ) ( 3,4 ) .
recently , the novel antigen , the low - density lipoprotein receptor - related protein 4 ( lrp4 ) , has been identified as a target for the autoantibodies in mg ( 5 ) , and anti - lrp4 antibodies were detected in the serum of approximately 2 - 46% of double seronegative mg ( dsmg ) patients ( 3,5 - 7 ) . in this report , we present a case of undiagnosed mg with an acute onset of respiratory insufficiency without either ocular or limb involvement .
our study highlights the fact that making an early diagnosis of such cases can be very difficult .
a 72-year - old japanese man was admitted to our department presenting with an 18-month history of dropped head .
a neurological examination revealed mild neck extensor weakness ( mmt 4/5 ) , but there were no signs of weakness in the ocular , tongue , facial or limb muscles .
the results of routine blood tests , including those for thyroid function and muscle enzymes , were normal .
an arterial blood gas analysis of room air was paco2 55.9 mmhg and % vc ( vital capacity ) was 63.7% in spirometry , and he needed noninvasive positive - pressure ventilation ( nppv ) .
a chest computer tomography ( ct ) scan showed no definite evidence of either pneumonia or pulmonary thromboembolism .
no abnormalities were observed on magnetic resonance imaging ( mri ) of the brain and cervical spine and cerebrospinal fluid examination
. repetitive nerve stimulation ( rns ) studies of the trapezius muscle showed about a 20% decrease in his compound muscle action potential , although rns studies in the orbicularis oculi and abductor digiti minimi muscles were normal .
needle electromyography of the sternocleidomastoid and cervical paraspinal muscles showed a mild , low amplitude of the motor unit potentials . a muscle biopsy from the left biceps brachii muscles showed no significant lesions .
the administration of an oral cholinesterase inhibitor ( pyridostigmine bromide , 60 mg three times daily ) or an intravenous immunoglobulin ( ivig , 400 mg / kg 5 days ) did not alleviate his respiratory symptoms .
two months after admission , steroid pulse therapy ( methyl prednisolone 1,000 mg 3 days ) was started , and his respiratory and neck symptoms and hypercapnia thereafter markedly improved ( figure ) .
we performed the steroid pulse therapy every week for three times , which allowed him to gradually withdraw from the nppv .
subsequently , he was administered tacrolimus ( 3.0 mg / day ) , and he has since remained symptom - free for over 8 months . the patient s clinical course is shown
we present the case of a dsmg patient with anti - lrp4 antibodies without a common phenotype . to the best of our knowledge ,
this is the first clinical report of mg with seropositivity for only anti - lrp4 antibodies showing predominantly respiratory symptoms .
several reports have described the clinical features of mg patients with anti - lrp4 antibodies .
one patient presented with isolated neck extensor weakness and the other showed ocular , bulbar and cervical symptoms .
reported the clinical presentation of three dsmg patients with anti - lrp4 antibodies who showed isolated ocular symptoms and a good response to treatment with cholinesterase inhibitor and/or corticosteroids ( 9 ) .
also described a case of mg presenting with bulbar symptoms and double seropositivity for anti - achr and anti - lrp4 antibodies ; this patient responded to cholinesterase inhibitor and corticosteroids ( 7 ) .
all of these reported cases showed mild symptoms and a good response to the conventional treatments of mg .
however , the typical clinical symptoms of mg patients with anti - lrp4 antibodies remain unclear because the number of detailed clinical reports is too small .
our case suggests that mg patients with anti - lrp4 antibodies could show a dropped head as an initial symptom at presentation .
studied the clinical data of over 800 mg patients and reported that the majority of dsmg patients with anti - lrp4 antibodies presented with ocular or mild generalized symptoms .
they showed a positive response to cholinesterase inhibitors or corticosteroids , which was similar to the clinical features of mg patients with anti - achr antibodies ( 10 ) .
it was also reported that 6.6% of mg patients with anti - lrp4 antibodies demonstrated myasthenic crisis , and that double seropositive mg patients ( especially anti - musk and anti - lrp4 antibodies ) had more severe symptoms , such as myasthenic crisis ( 10 ) .
our case suggests that seropositivity for anti - lrp4 antibodies alone ( without anti - achr and/or anti - musk antibodies ) can induce respiratory symptoms . in our case ,
corticosteroid therapy elicited a good response , which was similar to that of the previously reported cases ( 7,9 ) . on the other hand ,
. indicated , 9.6% of mg patients with anti - lrp4 antibodies did not respond to treatment with cholinesterase inhibitors ( 10 ) .
moreover , in our case , ivig was ineffective , whereas tacrolimus was effective and prevented the recurrence of myasthenic symptoms .
. showed that 10 out of 32 mg patients with anti - lrp4 antibodies were treated with ivig , and 12 out of 28 patients were treated with azathioprine ; however , the outcome of these therapies was not described in the paper ( 10 ) .
further accumulation of clinical data is therefore necessary to understand the effect of ivig and immunosuppressants on mg patients with anti - lrp4 antibodies .
lrp4 is a member of the low - density lipoprotein receptor family and is known to bind directly to neural agrin ( 11 ) , which is necessary for the nmj ( neuromuscular junction ) formation and neural agrin - induced activation of musk and achr clustering ( 11 - 13 ) .
higuchi et al . showed that anti - lrp4 sera from mg inhibited the interaction between lrp4 and neural agrin ( 5 ) .
shen et al . reported that anti - lrp4 sera decreased cell surface lrp4 levels and inhibited both agrin - induced musk activation and achr clustering ( 14 ) .
they also demonstrated that immunization with the extracellular domain of lrp4 causes muscle weakness in mice ( 14 ) .
moreover , recent studies in mice show that the loss of muscle lrp4 alone is sufficient to cause myasthenic symptoms ( 15 ) . on the other hand , because it has been also recently reported that anti - lrp4 antibodies are present in the serum and cerebrospinal fluid of patients with amyotrophic lateral sclerosis ( als ) ( 16 ) , the specificity of the anti - lrp4 antibodies for mg remains controversial ( 17 ) . acute neuromuscular respiratory failure is caused not only by mg , but also by other diseases , such as guillain - barr syndrome , myopathies and als ( 18 ) .
it is often challenging to diagnose mg at an early phase because it rarely presents as isolated respiratory muscle weakness ( 19 ) .
this presentation suggests that neurologists should consider mg with anti - lrp4 antibodies in the differential diagnosis of patients presenting with unexplained dyspnea .
we herein report the case of a 72-year - old man demonstrating myasthenia gravis ( mg ) with a dropped head and acute respiratory insufficiency . 
 there was no ocular , bulbar , or limb involvement . 
 the patient was seronegative for anti - acetylcholine receptor ( achr ) antibodies and anti - muscle - specific tyrosine kinase ( musk ) antibodies . 
 subsequent tests showed seropositivity for anti - low - density lipoprotein receptor - related protein 4 ( lrp4 ) antibodies . 
 the addition of steroid pulse therapy resulted in a full remission of his respiratory symptoms . 
 this presentation suggests that lrp4-positive mg should be considered in the differential diagnosis of patients presenting with acute respiratory insufficiency without either cranial or limb involvement .