Discussion
Meningeal involvement of IgG4-RD is rare and was first described in 2009 (7, 8). It is estimated to be slightly above 2% of overall clinical manifestations, based on prior studies (9). Fewer than 80 patients Ig4 related to HP have been described in the literature (1). It can mimic a variety of inflammatory, infective, neoplastic, and hemorrhagic conditions. IgG4RD-HP related diagnoses can be a diagnostic dilemma, despite a thorough clinical, laboratory, and imaging investigation.
Tissue biopsy remains the gold standard for the final diagnosis. Early clinical and etiological diagnosis is relevant to decisions about therapy. IgG4-HP closely follows granulomatosis with polyangiitis (GPA) as a leading cause of inflammatory HP. Other diagnoses to consider are lymphoma, sarcoidosis, tuberculosis, rheumatoid arthritis, and Langerhans-cell histiocytosis. (9).
There have been two from IgG4-RD HP described in the literature (9, 11). Isolated IgG4-HP was characterized by the absence of extra meningeal organ involvement (including the pituitary gland). Mutually exclusive lesions of the brain or spinal cord defined as “single-organ pachymeningitis.” When extra-meningeal involvement was reported, IgG4-HP was defined as systemic IgG4-RD. Non-specific IgG4-HP was retained when data was lacking for classification.
IgG4-RDs occur, reported in men aged between 50 and 60 years (10), although our patient is much younger than that. The finding of an elevated serum IgG4 concentration is supportive of IgG4-RD but is not a constant finding (12). It can also increase in cancerous, allergic, autoimmune disorders, and other conditions. It should not be used as diagnostic criteria and evidence for starting empiric treatment. IgG4 levels are closely related to disease activity and the extent of organ damage (13). It is therefore essential to ascertain extra-neurologic organ involvement in patients with IgG4-HP presenting with high serum IgG4 levels and elevated plasma inflammatory markers. In such circumstances, PETCT has been shown to be of diagnostic value (9).
CSF alone is not a reliable parameter for the diagnosis of IgG4-HP. It is, however, common to find lymphocytic meningitis, and a moderate increase in CSF proteins with intrathecal synthesis of IgG with oligoclonal bands and increase in intrathecal IgG4 levels could be a surrogate marker for neurologic IgG4-RD when tissue biopsies could not be performed (13).
Intracranial hypertrophic pachymeningitis rarely accompanies subdural hygroma or hematoma. Only three cases have been reported in the literature, of which two cases involved a diagnosis of IgG4- RDs (14.15.16). Our case presented as an SDH mimic which is also rarely reported with HP. (1,17) Skull involvement concomitant with IG4RD HP has been reported in a few cases (6,18). These skull lesions are described as hyperostosis or thickening, while our case is unique in that it has lytic skull lesions.
Pulmonary involvement reportedly occurs in 12–50% of patients with IgG4-related disease and takes the form of various sizes of lung nodules, lung masses, patchy ground-glass opacities, infiltrates resembling consolidation, reticular opacities, thickened bronchovascular bundles, central airway stenosis and obstruction, bronchiectasis, pleural effusion, nodular pleural lesions, and interstitial lung disease (19).  In this case, however, there is the commitment involvement of Meningeal disease with pulmonary nodules which has been uncommon. In one report, a 48-year-old man presenting with headache was found to have HP along with incidental bilateral pulmonary nodules and had systemic features of both GPA and IgG4-related disease as well as
seropositivity for both cytoplasmic antineutrophil cytoplasmic antibodies (c-ANCA) and IgG4 (20). He was treated with prednisone and rituximab with improvement in his symptoms.
Steroids remain the first-line treatment (4). Short-term response to surgery in single-organ IgG4-HP is good and, in some cases, maybe sufficient to achieve remission (9). Although nearly all patients with IgG4-RD respond to glucocorticoids, approximately 40% of those that fail to achieve complete remission relapse within one year (21). Patients with multiorgan disease or an extremely high serum IgG4 are likely to require an agent other than glucocorticoids alone. Rituximab is preferred over other glucocorticoid sparing medications. It has been shown in I4G studies frequently as an excellent treatment for IgG4-RD and IG-HP among other immunosuppressive drugs (22, 23). It is important to follow this disease to look for new organ involvement in addition to recurrence.