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Presentation type	E-poster
Title	Steroid-Responsive Orbital Apex Neuropathy With Multifocal Meningeal Thickening: A Diagnostic and Therapeutic Dilemma

Purpose	To highlight the medical pitfalls of a steroid-responsive orbital apex syndrome with multifocal pachymeningitis, in which inflammatory disease and primary Central Nervous Sytem (CNS) lymphoma remained competing diagnoses.
Methods	A 27-year-old woman presented right eye (RE) painless visual loss. RE visual acuity was hand movements. RE relative afferent pupillary defect, papilledema and extinguished visual field confirmed severe right optic neuropathy. Urgent multidisciplinary assessment was initiated.
Results	MRI showed plaque-like pachymeningeal lesions of the right orbital apex, cavernous sinus and optic nerve, plus frontal lesion. Serum IgG4 was elevated, but PET-CT and biopsy did not support IgG4-related disease. Serum Borrelia IgM was positive. Open frontal biopsy after systemic steroids initiation revealed the presence of a lymph node of atypical brain location. It showed preserved follicles and reactive B-cell expansion; Other clonalities were negative. Expert review remained inconclusive for diffuse large B-cell lymphoma, possibly steroid-masked. Steroids restored RE vision to 20/20 and induced MRI regression. A month later, RE vision deteriorated with new visual field defects despite radiological improvement. The diagnosis evolved from atypical optic neuritis to meningiomatosis, hypertrophic pachymeningitis and CNS lymphoma.
Conclusion	Urgent visual rescue may compromise tissue diagnosis. Steroid response neither proves inflammation nor excludes lymphoma. Clinicoradiological discordance and equivocal histology can delay immunochemotherapy despite a lymphoma-like course and can delay immunochemotherapy. Multidisciplinary reviews are essential before commiting to potential inadapted management or delayed adequate treatment.
Conflict of interest	No

Authors 1

Last name	EL KORTOBI
Initials	S
Department	CUSL
City	Brussels

Authors 2

Last name	GUESSOUS
Initials	Y
Department	CUSL
City	Brussels

Authors 3

Last name	GUAGNINI
Initials	AP
Department	CUSL
City	Brussels