In the setting of a challenging diagnosis, brain-biopsy plays an important role in directing management when diagnostic tests are equivocal.

23-year-old male presented with a progressive visual decline in his left eye for 4 months, 40-pound weight-loss and recently diagnosed hypothyroidism. His vision was 20/50 OD and 20/80 OS with bilateral optic atrophy noted.

MRI Brain showed T2 FLAIR hyperintensity in both optic nerves, optic chiasm, optic radiation, pituitary stalk, bi-frontal periventricular regions, thalamus, and hypothalamus. CSF revealed WBC54; 72% lymphocytes, elevated protein (121 mg/dL) and glucose (110 mg/dL). Cytology showed numerous lymphocytes, without malignancy.

The differential diagnoses favored inflammatory etiologies such as NMO, primary lymphocytic hypophysitis or sarcoidosis.

Despite IV steroids, vision declined (20/70 right eye and hand motion left eye). Endocrine work-up revealed panhypopituitarism and diabetes insipidus. Serum NMO-IgG-antibody was negative.

Biopsy of anterior horn of lateral ventricle showed a small cluster of atypical cells with large irregular nuclei and eosinophilic cytoplasm positive for Oct3/4, Sall-4, c-KIT, and PLAP. Testicular ultrasound did not show primary testicular tumor.

The final diagnosis was primary intracranial germinoma. The patient underwent external beam radiation therapy and his recent visual exam revealed 20/40 vision OD and hand motion OS.

Primary Intracranial Germinoma is often misdiagnosed due to variable neuroradiologic features and non-diagnostic CSF. It should be suspected in younger individuals with vision symptoms and pituitary dysfunction. Our case was atypical because there was no mass but diffuse hyperintensities of the visual pathways on MRI favoring an inflammatory etiology. A negative CSF cytology and non-response to corticosteroids made the diagnosis challenging. Threshold to do a brain-biopsy should be low in cases where there is progressive bilateral vision loss.