False positivity of Anti AChR Ab titers due to cross
False positivity of Anti-AChR Ab titers, due to cross reactivity with bungarotoxin, has previously been demonstrated. One such study reported that patients with amyotrophic lateral sclerosis, who had received long term snake venom therapy, had developed antibodies to bungarotoxin and hence demonstrated high levels of Anti-AChR Ab in their sera erroneously (Mittag and Caroscio, 1980). However subsequent studies showed that patients with ALS may have antibodies against ACh R despite not being exposed to snake venom therapy (Abbott et al., 1986, Ashizwa, 1986).
Introduction Antibodies to muscle-specific tyrosine kinase (MuSK Abs) are detected in approximately 40% of generalized spectinomycin synthesis receptor (AChR) antibody-negative myasthenia gravis (MG). MuSK-MG is a distinct clinical entity and its phenotype differs from AChR-MG. Occurrence of AChR antibodies (Abs) and MuSK Abs in the same individual is rare. The limited references concern double seropositivity detected at the onset of the disease (Zouvelou et al., 2013), years after the onset (Suhail et al., 2010) and “seroconversion” from AChR Abs to MuSK Abs taking place after thymectomy (Saulat et al., 2007, Sanders and Juel, 2008, Kostera-Pruszczyk and Kwiecinski, 2009). Routine testing for double seropositivity is not recommended in clinical practice, unless there is clinical indication. We report a female patient with AChR-MG which became double seropositive-MG many years after the onset. The reason for the re-assessment of antibody status was the biphasic clinical course which clearly indicated an immunological peculiarity.
Case report A 36-year old Caucasian female with AChR-MG first came to our attention on January 2013. The onset of the disease was 17 years ago (1996), at the age of 19, when the presenting symptoms were diplopia and unilateral eyelid ptosis. Three months later MG generalized with upper and lower limbs involvement. Serum AChR Abs were positive at a high titer of 270nM (positive≥0.6nM). She received only pyridostigmine for the first 5 years and in 2001 prednisolone was added to the regimen. On December 2001, she underwent thymectomy and the histological examination revealed follicular hyperplasia. In 2003 and in 2008 the patient gave birth to two offsprings suffering both from transient neonatal MG. At those periods the patient's titer of AChR Abs was 370nM and 587nM, respectively. From 1996 until 2012, the myasthenic weakness was restricted to ocular and limb muscles, well controlled with pyridostigmine and prednisolone and there was no episode of myasthenic crisis or requirement for short term immunotherapy i.e. plasma exchange (PE) or intravenous immunoglobulin. On May 2012, 16 years after the onset of MG, an unprovoked and severe clinical deterioration supervened which was characterized by prominent bulbar, facial, neck and respiratory muscle involvement with dramatic response to PE. The above-mentioned muscles had never been previously involved during the disease course. The patient received high dose prednisolone and cyclosporine (200mg/d) and she remained on monthly courses of PE until the end of November 2012. At our first evaluation (early January 2013) the patient was in minimal manifestations, as she reported no symptoms and the neurological examination revealed only mild fatigable lower limbs. MRI of the mediastinum did not show any residual thymic tissue. We considered that the last reported pattern of myasthenic weakness was unusual for longstanding and previously stable AChR-MG and highly indicative of MuSK-MG. For that reason we asked for re-assessment of antibody status. AChR Abs were positive at a titer of 366nM, i.e. similar to her previous titers, while testing for MuSK Abs by a commercial radioimmunoprecipitation assay (RSR Ltd, Cardiff) and by an in house radioimmunoprecipitation assay (Trakas et al., 2011) was negative. However, a live Cell Based Assay (CBA) for MuSK Abs was then performed (according to Leite et al., 2008). Interestingly, the recent serum was found anti-MuSK positive by the CBA (at both test dilutions: 1:10 and 1:20), whereas sera derived during 2008, were found negative even at 1:10 dilution. At the end of January 2013, the patient reported for the first time asymmetric distal weakness of the hand, especially of the finger extensors, interfering with daily activities. This new pattern of weakness was directly attributed to MG, based on the findings of appropriate electrophysiological testing. Prednisolone was increased up to 60mg/d with mild improvement of the distal weakness. While she was on high-dose prednisolone an exacerbation came up with moderate to severe bulbar and respiratory symptoms which responded to courses of PE. However, the distal weakness remained unaffected to the PE and it was the prominent disabling symptom until the last evaluation on September 2013.