Database searching retrieved 888 manuscripts, of which 327 were duplicates. After title and abstract screening, 51 manuscripts were eligible for full-text reading. Two manuscripts could not be retrieved. Next, we excluded six cases because IgG4-RD could not be diagnosed after careful revision of the diagnostic criteria, seven manuscripts due to the absence of individual patient data, and one paper to prevent bias due to duplicated information. Screening for the references of the eligible articles yielded another 3 eligible papers. At last, we included 38 articles in our systematic review, yielding a total of 42 cases of PNS disease in IgG4-RD. Figure 1 displays the flow diagram.
Peripheral nerve, neuromuscular junction and muscle involvement of IgG4-RD were summarized in Fig. 2, with a highlight to mononeuritis multiplex and to focal myositis. Kidney and lymph node involvement were the most relevant systemic manifestations in patients with PNS involvement of IgG4-RD (Table 1). Almost half of cases of PNS manifestations in patients with IgG4-RD was attributed to secondary mechanism, particularly autoimmune comorbidities.
Table 1
Systemic involvement in peripheral nervous system manifestations of IgG4-RD.
| Affected organ | Total (n = 42) | Nerve (n = 25) | Muscle (n = 13) | NMJ (n = 4) |
| Kidney | 11 (26%) | 11 (44%) | 0 (0%) | 0 (0%) |
| Lymph Node | 11 (26%) | 8 (32%) | 1 (8%) | 2 (50%) |
| Salivary Glands | 8 (19%) | 6 (24%) | 2 (15%) | 0 (0%) |
| Lung/Pleura | 8 (19%) | 4 (16%) | 3 (23%) | 1 (25%) |
| Aorta | 7 (17%) | 5 (20%) | 0 (0%) | 2 (50%) |
| Retroperitoneum | 6 (14%) | 4 (16%) | 0 (0%) | 2 (50%) |
| Lacrimal Gland | 4 (10%) | 4 (16%) | 0 (0%) | 0 (0%) |
| Liver | 3 (7%) | 3 (12%) | 0 (0%) | 0 (0%) |
| Pancreas | 3 (7%) | 2 (12%) | 1 (8%) | 0 (0%) |
| Eye | 2 (5%) | 1 (4%) | 1 (8%) | 0 (0%) |
| Skin | 2 (5%) | 1 (4%) | 1 (8%) | 0 (0%) |
| Adrenal | 1 (2%) | 1 (4%) | 0 (0%) | 0 (0%) |
| Bone Marrow | 1 (2%) | 1 (4%) | 0 (0%) | 0 (0%) |
| Bowel | 1 (2%) | 1 (4%) | 0 (0%) | 0 (0%) |
| Orbit | 1 (2%) | 1 (4%) | 0 (0%) | 0 (0%) |
| Pacchymeninges | 1 (2%) | 0 (0%) | 1 (8%) | 0 (0%) |
| Paranasal sinus | 1 (2%) | 1 (4%) | 0 (0%) | 0 (0%) |
| NMJ: Neuromuscular Junction. |
Peripheral nerve
We included 25 patients with peripheral neuropathy and IgG4-RD of which 76% were male and the median age was 65 (55, 73) years. The most common presentation was mononeuritis multiplex[15–25] (12/25, 48%), followed by polyneuropathy[26–30] (5/25, 20%), radiculopathy/multirradiculopathy[8, 25] (4/25, 16%), polyradiculoneuropathy[31, 32] (2/25, 8%), mononeuropathy[8] (1/25, 4%) and radiculoplexhopathy[33] (1/25. 4%). Median duration of the neurological presentation was 5 (1.5,10) months. All patients presented with electrodiagnostic signs of axonal neuropathy, except for two cases of predominantly demyelinating polyradiculoneuropathy [31, 32]. Nerve biopsy was reported in 13/25 cases and had a normal result in 2/13 (15%) of cases. The most common findings were loss of myelinated fibers (6/13) and axonal degeneration (5/13), epineural vessel occlusion/recanalization (6/13) and nerve inflammatory infiltrates, which most commonly occurred in the epineurum (4/13) and perineurum (4/13). Nerve and vessel infiltrates were abundant in IgG4 positive cells in 6/13 cases. Other classical IgG4-RD findings were fibrosis (3/13 cases), though no cases of storiform fibrosis of the nerve were described, and obliterative phlebitis (1/13). Pathological evidence of vasculitis was observed in 6/13 reported cases. Positive autoantibodies included ANCA, ANA, RF and Anti-SSA, Anti-GQ1b/Anti-VGCC. There were no reports of antibodies associated with IgG4-related autoimmune neuropathy such as contactin or neurofascin.
Systemic involvement occurred in all (25/25, 100%) of reported cases. Organs most commonly involved were the kidney (11/25, 44%), lymph nodes (8/25, 32%), followed by salivary glands (6/25, 24%), aorta (5/25, 20%), lacrimal glands (4/25, 16%), and retroperitoneum (4/25, 16%). There were no reported cases of isolated neuropathy. Serum IgG4 levels were reported in 22/25 (88%) of cases, and were elevated in all patients, with a median value of 762 (257, 1400) mg/dL. Peripheral neuropathy in IgG4-RD was considered primary in 12/25 (48%) cases, with 8/12 cases caused by direct invasion/compression of the affect nerves and 4/12 cases caused by IgG4-related vasculitis. Neuropathy was considered secondary to a comorbid disease in 12/35 (48%) patients. In these cases, the inferred cause was ANCA-associated vasculitis in 7 patients, other vasculidites in 2 patientes, CIDP in 2 patients and diabtes in one patient. The cause of neuropathy could not be determined in 1/25 (4%) patients. The 2019 ACR criteria were met in 18/25 patients (72%).
Neuromuscular junction
We found four case reports with neuromuscular junction disease [10, 34–36], of which two (66%) were female and the median age was 63 (49, 76,5) years. Three patients presented with myasthenia gravis [10, 35, 36] and one patient manifested with Lambert Eaton (syndrome) [34], in a chronic presentation with a median of 70 (38, 101) months. Interestingly, no patient with myasthenia gravis presented with acetylcholine receptor antibodies and two (67%) patients presented with anti-Musk antibodies. Anti-VGCC was reported positive in the Lambert-Eaton myasthenic syndrome patient.
Only two (50%) patients fulfilled the ACR 2019 diagnostic criteria of IgG4-RD and involvement of other organs occurred in all patients, mainly in aorta, retroperitoneum and lymph node. Serum IgG4 levels were elevated in all patients, with a mean value of 264 mg/dL. No case report displayed evidence of direct neuromuscular junction invasion by IgG4-RD cells and, hence, all cases were interpreted as secondary PNS manifestations due to autoimmune comorbidity.
Muscle
We included 13 cases of muscle disease and IgG4-RD ([9, 11, 37–47]), of which seven (53%) were male and the median age was 54 (38.5, 67,5) years. Focal myositis, with localized weakness and muscle pain, was the neurological presentation in seven[9, 39, 41–43, 46, 47] (53%) cases. Proximal symmetric tetraparesis occurred in the remaining six[11, 37, 38, 40, 44, 45] patients (47%). Disease was usually subacute, with a median duration of four months (0.5, 30) at the time of the diagnosis. Creatine kinase (CK) levels were also elevated in most patients (5/7, 83%), with a median value of 1232 (294.5, 1791) mg/dL. MRI imaging was reported in 4/7 cases of focal myositis. The main findings were muscle fascia thickening, scattered contrast enhancement, central T1 hypointensity (presumably due to muscle fibrosis) and muscle edema. In all 3 cases of proximal myopathy for which MRI was described, diffuse muscle edema and fat substitution of affected muscles was reported.
Muscle biopsy was reported in 10/13 (7/7 focal myopathies and 3/6 proximal myopathies) cases. In all cases of focal myositis, muscle biopsy showed infiltration by abundant IgG4 + cells. Fibrosis was reported in 4/7 of such cases but found to have a storiform pattern in only 1/4. In proximal myopathies, muscle biopsy patterns varied, but all cases had mixed inflammatory infiltrates, including plasma cells, with abundant IgG4 + cells. Endomysial fibrosis was reported in 2/3 cases.
Only four (30%) patients fulfilled the ACR-2019 classification criteria for IgG4-RD. Isolated muscle involvement occurred in 8 (61%) cases. When systemic involvement occurred, the most affected organs were the lungs (3/5, 60%), salivary glands (2/5, 40%), and skin (2/5, 40%). Most patients presented with elevated IgG4 serum levels (11/12, 91%) with a median level of 313 (187, 507.5) mg/dL. Two [37, 38] of the cases exhibited muscle infiltration patterns commonly observed in other inflammatory diseases [48] (CD8-rich infiltrate, macrophage-rich infiltrate, histiocytes, and giant cells), yet in both instances, the ratio of IgG+/IgG4 + cells was 40%. A third case showed only nonspecific inflammatory infiltrate with an IgG+/IgG4 + proportion ranging between 20–40% [44]. Figure 1 summarizes the presentation of PNS manifestations of IgG4-related disease.