Masaki Katsura, MD
Immunoglobulin G4 is the rarest of the 4 subclasses of serum IgG. Little attention had been paid to this minor component of IgG until Hamano et al1 found that patients with autoimmune pancreatitis (AIP) display high serum concentrations of IgG4. The same group also reported that pancreatic tissues in patients with AIP were histologically characterized by sclerosing inflammation with infiltration of numerous plasma cells that show strong immunoreactivity to IgG4.2 Several subgroups of extrapancreatic lesions that share similar pathologic features with AIP have been discovered in various organs and sites, such as the biliary duct,3 retroperitoneum,4, and lung.5 In the head and neck region, these types of lesions have been found most frequently in the lacrimal, parotid, submandibular, and pituitary glands,6–8 but nasal/paranasal9 and parapharyngeal10 lesions have also been reported. Given the similar pathologic features and frequent association with multiple lesions in different organs, a new designation, systemic IgG4-related sclerosing disease, has been established as a distinct clinicopathologic entity.11
Our report describes a case of IgG4-related inflammatory pseudotumor involving the skull base along the second and third divisions of the left trigeminal nerve. MR imaging demonstrated a well-defined, T2-hypointense mass in the left Meckel cave, extending to the left pterygopalatine fossa via the left foramen rotundum and further to the infraorbital canal. The mass also extended to the left masticator space via the left foramen ovale. The mass accompanied expansion of neural foramina along its course, notably the foramen rotundum and the infraorbital canal, with no signs of bone destruction. The pathologic specimen taken from the pterygopalatine fossa showed a dense inflammatory infiltrate with abundant IgG4+ plasma cell infiltration along the trigeminal nerve fibers, indicating perineural spread as the underlying pathology.
Differentiating IgG4-related disease from primary/metastatic neoplasms can be challenging. Particularly in our case, the lesion showed extensive perineural spread along the trigeminal nerve, highly suggestive of a neoplastic process. Although a definitive diagnosis requires pathologic confirmation, IgG4-related disease can be suspected on imaging by recognizing typical radiologic features,12–14 such as well-defined lesion borders, homogeneous attenuation/signal intensity, T2 hypointensity (possibly explained by the combination of fibrosis and attenuated cellularity), homogeneous and gradual enhancement pattern, absence of vascular occlusion or compression, and presence of bone remodeling (erosion or sclerosis) without destruction (probably due to the slow-growing nature and to the long-standing mechanical stress/pressure induced by the lesions within the neural foramina). Patients with IgG4-related disease frequently have lesions in areas other than the head and neck (either simultaneously or in the past), such as in the pancreas, biliary duct, retroperitoneum, kidney, and lung. Recognition of these preferential sites for IgG4-related disease can be of help in the differential diagnosis. If serologic studies show markedly increased serum IgG4 levels, the diagnosis of IgG4-related disease may be further supported. We believe