Anti-signal Recognition Particle Necrotizing Autoimmune Myopathy: An Atypical Presentation

Necrotizing autoimmune myopathy (NAM), also known as necrotizing autoimmune myositis, is a heterogeneous group of diseases characterized by the presence of necrotic muscle fibers on biopsy, elevated creatine kinase (CK) levels, an abnormal electromyogram (EMG) result, and the associated antibodies. The anti-signal recognition particle (anti-SRP) and the anti-3-hydroxyl-3-methylglutaryl-coenzyme A reductase (anti-HMGCR) antibodies are the two most prevalent antibodies identified with NAM. NAM is a rare disease that typically affects middle-aged Caucasian women. In this case report, we present the diagnosis of anti-SRP NAM in a middle-aged African American male. This case report displays the atypical presentation of NAM outside of the typical patient population as well as the need for additional research to determine the pathogenesis and the precise role of anti-SRP antibodies in NAM.


Case Presentation
A 58-year-old African American male with autoimmune myositis diagnosed within the past year, and required tracheostomy and percutaneous endoscopic gastrostomy (PEG), was brought to the hospital by paramedics, from a nursing home, due to difficulty in breathing. He had minimal speech capabilities due to the tracheostomy in November 2017 and was subsequently 1 1 1 placed on a ventilator. The patient was accompanied by his brother who stated that the patient was disconnected from the ventilator at the nursing home and developed difficulty in breathing. The brother also endorsed that the patient denied chest pain, dizziness, lightheadedness, headaches, palpitations, nausea, vomiting, vision changes, auditory changes, cough, congestion, back pain, abdominal pain, fevers, chills, diarrhea, constipation, or any international travel. The brother also mentioned that the patient had insulin-independent diabetes mellitus, hypertension, hyperlipidemia, chronic obstructive pulmonary disease, and a myocardial infarction status post-percutaneous coronary intervention. The brother denied any family history of malignancy and stated that the patient had no known allergies to medication or food. The patient used to be a basketball referee and had progressively mild weakness in the lower extremities for over a decade. Prior to admission, the patient was taking lisinoprilhydrochlorothiazide 20 mg/25 mg daily, sotalol 80 mg daily, apixaban 5 mg daily, atorvastatin 80 mg daily, metoprolol 100 mg twice a day, prednisone 20 mg daily, mirtazapine 15 mg, Protonix 40 mg daily, and Lantus and Novolog for diabetes mellitus.
Upon arrival at the emergency department, the patient was connected to the ventilator and had stable vital signs otherwise. Physical exam findings were positive for bilateral lower lobe rhonchi, 1 + pitting edema in the lower extremities, weak neck flexor muscles, and severely weak upper-and lower-extremity proximal and distal muscle groups with atrophy of the quadriceps muscles. His ventilation settings were pressure-regulated volume control (PRVC) with the fraction of inspired oxygen (FIO 2 ) being 40% and positive end-expiratory pressure (PEEP) of 5 mmHg. His complete blood count (CBC) showed a white cell count of 13.7 K/cm with a left shift. Serum CK and troponin levels were both elevated to 1509 U/L and 0.69 ng/ml, respectively. C-reactive protein levels were also elevated to 9.35 mg/L. Venous blood gas (VBG) showed pH of 7.51 and pCO 2 of 46 mmHg. On imaging, chest X-ray was positive for a bilateral patchy infiltrate in the lower lobes.
The patient's medical records were obtained from another institution that showed that the patient had been worked up for NAM, which included autoimmune antibodies and muscle biopsy. As shown in Figure 1, the muscle biopsy revealed "necrotizing myopathic process without any evidence of significant inflammatory process", a classic pathological finding for NAM. During the current hospital course, an autoimmune panel, including anti-nuclear antibodies (ANA), serum CK, anti-histidyl transfer ribonucleic acid synthetase (anti-Jo-1), antiribonucleoprotein (anti-RNP), anti-smooth muscle (anti-SM), anti-3-hydroxy-3-methylglutarylcoenzyme A reductase (anti-HMGCR), and anti-signal recognition particle (anti-SRP) antibodies, was ordered for further clarification of the serotype of NAM. The patient was solely positive for anti-SRP antibody, with negative ANA and anti-HMGCR antibodies, represented in Table 1.

TABLE 1: Antibodies tested throughout the course of hospital stay
A computed tomography (CT) scan of the chest and the abdominopelvic region was performed to screen for any possible malignancy leading to NAM. The result of CT scan was negative for any malignancy, ruling out a malignant etiology leading to NAM. In regards to the treatment, the patient received 50 mg of oral prednisone once daily that was tapered down to 5 mg over a five-day course. Due to the refractory nature of the disease, the patient was treated twice with 1000 mg of rituximab given two weeks apart, with each dose supplemented with 100 mg of Solu-Medrol. The patient was discharged and scheduled for a follow-up to monitor the treatment response.

Discussion
NAM occurs in about 0.0000016% of the American population annually [1]. Like most autoimmune diseases, NAM is typically seen in middle-aged females [1][2][3][5][6][7][8]. This case emphasizes the outlying population, in terms of the sex and race, that this patient identifies with. Although the etiology of NAM is unknown, some of the risk factors include statin use, malignancy, a human immunodeficiency virus infection, or connective tissue disease [2,4,9]. The patient only had a history of using high doses of Atorvastatin and hence, the diagnosis of NAM was not immediate.
NAM with anti-SRP antibodies is characterized by rapidly progressive proximal muscle weakness, markedly elevated CK levels, and poor responsiveness to corticosteroid therapy as seen in this case [1][2][4][5][6][7][8]. Intravenous immunoglobulin (IVIG) therapy, plasmapheresis, or immunosuppression with methotrexate, azathioprine, rituximab, cyclophosphamide, or mycophenolate mofetil seem to be alternative options [1]. Early administration of therapy of corticosteroids with an immunosuppressant, within three months, was found to be beneficial [2,6]. Although, according to Kassardjian et al., the early initiation of IVIG was seen to be advantageous [8]. Suzuki et al. found that oral corticosteroids and IVIG were adequate as primary treatment and plasmapheresis was useful as the secondary treatment [6]. However, there has not been an official regimen for treating anti-SRP NAM [7][8]. A case series of eight patients demonstrated a beneficial clinical response to the B-cell depletion therapy with rituximab after they had failed on other immunosuppressive therapies [10]. Similarly, this patient received two doses of rituximab, two weeks apart, with the pretreatment with Solu-Medrol in order to obtain the effectiveness of treatment [10]. There seems to be a high rate of relapse when completing or tapering treatment [2,5,8].

Conclusions
We conclude that this case requires to be monitored longitudinally to observe for the course of symptoms, adverse events, and outcomes as this patient is an outlier to the typical population that is diagnosed with anti-SRP NAM. Further studies are also required to elucidate the pathogenesis and the precise role of anti-SRP antibodies in NAM.

Additional Information Disclosures
Human subjects: Consent was obtained by all participants in this study.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.