Intracranial hypertrophic pachymeningitis may involve the skull bones, the pericranial soft tissue and, rarely, the cerebral parenchyma (when inflammatory cells infiltrate through the Virchow-Robin spaces) (9). Pachymeningeal thickening may be diffuse or focal/nodular. The nodular type may be divided into two main groups according to their location (4, 13). The first group includes cases located in the area from the parasellar region and cavernous sinus to the superior orbital fissure, and often affects the IInd to VIth cranial nerves and the supraclinoid carotid segment. The second group shows falcotentorial and clival involvement, and can affect the Vth and VIIth–Xth cranial nerves (13).
Intracranial hypertrophic pachymeningitis of the cerebral convexity, which may mimic convexity meningioma, occurs in about 15 % of the focal conditions (2). Cranial nerve impairment is due to their compression by thickened pachymeninges at the skull-base foramina (14). Such external stenosis at the base of the skull may also lead to occlusion of the internal carotid artery, resulting in cortical deficits (15). These lesions have a mass effect, but the inflammatory perivascular infiltration also plays an important part in the cortical irritative symptomatology (9, 16). Intracranial hypertrophic pachymeningitis may occasionally also lead to occlusion of the venous sinus and obstructive hydrocephalus (15). Spinal hypertrophic pachymeningitis is rare. The cervical and thoracic regions are probably most affected, and the meningeal thickening causes radiculomyelopathy (17).
The pathogenesis of hypertrophic pachymeningitis remains unclear. The pathologic and laboratory data indicate a close association with autoimmune diseases such as Wegener’s granulomatosis, rheumatoid arthritis and connective tissue diseases, and there has been discussion as to whether the condition is caused by systemic autoimmune inflammation (18). Blood tests usually show a high sedimentation rate (3, 5, 8, 18) and a study from Japan revealed slight to moderately elevated CRP in about half of the cases (18). Several cases have been described of positive antinuclear antibodies (ANA), rheumatoid factor (RF) and p-ANCA (18). Samples of cerebrospinal fluid reveal pleocytosis in 30 – 70 % of patients (5, 8, 18) and elevated total protein in about 80 % (18). The lumbar puncture opening pressure is usually normal, but may be elevated (18).
A neuroradiological examination plays a key role in the evaluation of the disease. However, it can take two years before there are significant diagnostic imaging findings (19). CT images reveal hyperintense dural lesions with homogeneous contrast enhancement, but in some cases, for example with slight dural thickening, there are no abnormal findings (18). MRI is the best means of identifying the lesions and excluding other disease processes. MRI findings are characteristic, and show various degrees of the lesion’s inflammatory pattern (5, 8). T1-weighted images show iso- or hypointense thickened dura (6, 8, 9, 13) with intense enhancement after injection of paramagnetic contrast medium due to inflammatory reaction in the pachymeninges. The dura mater appears hypointense in T2-weighted images, in some cases surrounded by a hyperintense rim of the lesion (8, 9, 13, 19). The central hypodense area is attributed to a fibrous process, while the peripheral hyperintensity is due to an active inflammatory reaction (9, 13). The use of gadolinium is important for assessing the meningeal enhancement, which can be used to distinguish between pachymeningitis and leptomeningitis. MRI findings may also provide an indication of the clinical course. Diffuse and homogeneous thickening of the dura and well defined T2-weighted borders are often associated with a short clinical course and a good prognosis. A hypointense hypertrophic meninx on T2-weighted series, without a hyperintense rim and with dishomogeneous contrast enhancement, is associated with chronic conditions and an unfavourable prognosis (8).
The examination should include meningeal biopsy in order to exclude secondary intracranial hypertrophic pachymeningitis (2, 18, 20). According to Bang et al. and Goyal et al. (14), it should be possible to make a diagnosis on a clinical basis of diffuse or skull-base located intracranial hypertrophic pachymeningitis (positive MRI and effective corticosteroid therapy), while in cases with nodular intracranial hypertrophic pachymeningitis with a convexity site and doubts about the diagnosis, a biopsy is essential in order to exclude meningioma and other neoplasms. Kupersmith et al. (3) recommend invasive diagnostics in the event of clinical or radiological deterioration during ongoing treatment. Histopathological examination of thickened dura shows non-specific inflammatory reaction in various phases of evolution, with fibroblastic infiltration of neutrophils, lymphocytes and plasma cells (4, 16). Vasculitis and granulomatous changes have been described (17, 21).
There is no consensus on the ideal therapeutic approach to intracranial hypertrophic pachymeningitis. Spontaneous resolution of symptoms, findings and meningeal thickening has been reported (22). Corticosteroid therapy is often effective for ameliorating the symptoms and findings and in halting the progression of the disease. In case of treatment resistance or relapse when tapering the dosage, other immune suppressants such as cyclophosphamide, methotrexate or azathioprine can be added (13). Radiotherapy and surgical removal of the affected tissue have been used (3). Because of the risk of irreversible damage to the nervous system due to compressive myelopathy, early surgical intervention appears to be essential when steroid therapy does not prevent the progression of symptoms of spinal hypertrophic pachymeningitis (17).