A Plea to Reconsider the Diagnosis

Aseptic meningitis is defined as meningitis with negative bacterial cultures from CSF and is habitually equated with viral meningitis.² This erroneous equivalence may curb critical thinking about alternative diagnoses, as aseptic meningitis may also be associated with a wide range of both infectious and noninfectious etiologies (Table). A thorough history and physical examination are the essential first steps in determining the etiology of aseptic meningitis, as many of the listed etiologies can be effectively eliminated through the evaluation of risk factors and exposures. Laboratory evaluation of CSF including cell count with differential, glucose, and protein levels is required. Gram stain and culture should be obtained to evaluate for bacterial meningitis even if suspicion is low. Multiplex and dedicated PCRs to viral agents as well as a serologic test for arboviruses, are widely available. Multiple episodes of aseptic meningitis with HSV, known as Mollaret’s meningitis, or enterovirus, which is more common in males with X-linked agammaglobulinemia, should be considered in patients with recurrent disease. Imaging is not indicated for every patient with aseptic meningitis; however, if anatomic abnormalities or malignancy are suspected, or in the evaluation of recurrent disease, then an MRI of the brain should be considered.

This case highlights how the analysis of CSF pleocytosis is not always predictive of a specific underlying etiology. The classic teaching that viral meningitis is associated with lymphocytic pleocytosis is based on studies of mumps meningitis.³ It is important to recognize that a neutrophilic pleocytosis is also described in viral infections including those caused by an enterovirus, herpes simplex, and arboviruses.^4,5 Moreover, while the magnitude of the neutrophilic pleocytosis should always raise suspicion of bacterial meningitis, it should also be associated with hypoglycorrhachia and elevated CSF protein levels. Antibiotic pretreatment of bacterial meningitis can alter CSF chemistries (raise CSF glucose levels and lower CSF protein levels), but chemistries are unlikely to return completely to normal.⁶ In this case, one clue that hinted toward a noninfectious etiology was the recurrence of relatively normal CSF glucose and protein levels in the setting of such a highly inflammatory pleocytosis on multiple occasions.

There is a wide spectrum of CNS mass lesions known for causing chemical meningitis including epidermoid, dermoid, craniopharyngiomas, and RCCs. While imaging can be suggestive, histological examination is often required to make a specific diagnosis. In this patient, the diagnosis of chemical meningitis secondary to a ruptured brain tumor was confirmed by MRI. CNS tumors that may cause aseptic meningitis are typically slow-growing lesions that cause symptoms due both to local growth and regional neurovascular compression. These masses can rupture and disseminate inflammatory contents into the subarachnoid space giving rise to chemical aseptic meningitis. Their contents may include materials rich in keratin, cholesterol, and lipids, which cause an intense sterile inflammatory reaction when discharged, possibly via cholesterol activation of the inflammasome.^7,8 The subsequent inflammatory response produces a neutrophilic pleocytosis, often suggestive of bacterial meningitis, while simultaneously maintaining the near normalcy of the CSF glucose and protein levels. Elevated levels of CSF cholesterol may raise suspicion of the diagnosis. Not all discharging tumors result in purely chemical meningitis, as secondary bacterial meningitis with S. pneumoniae or other respiratory flora can coexist if cysts communicate with the respiratory tract.⁹

Rathke’s cleft is formed during the development of the pituitary gland by the evagination of oral ectoderm through the precursor of the oral cavity.¹⁰ The cleft gives rise to the endocrine cells of the anterior pituitary. It subsequently disconnects from the oral cavity and develops into the pars intermedia between the anterior and posterior pituitary. Cystic enlargement of Rathke’s cleft through epithelial proliferation and secondary secretions leads to the development of an RCC. RCCs are nonneoplastic lesions, and the majority are diagnosed incidentally. Asymptomatic RCCs are common and are detected in 13%-22% of routine autopsies.¹¹ Symptomatic lesions may present with headaches due to mechanical effects on pain-sensitive dura or cranial nerves. Severe acute onset headaches may arise secondary to pituitary hemorrhage. RCCs can also cause ophthalmic or endocrinological impairment due to sellar compression. As in the present case, rarely cystic rupture and subarachnoid extravasation of epithelial-derived contents lead to a chemical aseptic meningitis.¹²

Surgical resection is indicated for symptomatic RCCs that exert neurologic, ophthalmic, or endocrinological symptoms.¹³ The surgical goal is the removal of the lesion and complete excision of the capsule unless it is extremely adherent to neurovascular structures. Surgical morbidity is related to the risk of hypopituitarism, visual decline, incomplete resection with lesion regrowth, and aseptic meningitis. Surgical approaches to this region are potentially complicated by proximity to optic nerves, pituitary glands, major arteries, and perforating vessels belonging to the circle of Willis. In addition, potential dehiscence of the skull base floor due to progressive cyst growth can give rise to a delayed risk of CSF leak and complicate surgical recovery. Surgery was indicated for this patient because of the parasellar location of his cyst placing him at risk for visual decline due to compression of the optic chiasm as well as pituitary dysfunction or obstructive hydrocephalus from ventricular compression.

This case is illustrative for learning because, at the outset, there were many possibilities to explore in an unimmunized child with meningitis. This patient’s neutrophilic cell count and partial antibiotic treatment only compounded the certainty of a bacterial etiology. However, further scrutiny of the history and laboratory parameters revealed the true underlying diagnosis of RCC. Ultimately, the plea to reconsider the pleocytosis was heard.