Background Dopamine agonists are the first line of therapy for prolactin secreting pituitary micro- and macroadenomas, even in the setting of visual field loss. Dopamine agonist therapy results in both a decrease in prolactin level and a reduction in tumor size. Prolactin is normalized and tumor disappears in approximately 60-70% of patients treated with dopamine agonists alone. Resistance to treatment is more prevalent with bromocriptine compared to the newer agonist carbergoline. When resistance occurs, dopamine agonists are unable to reduce prolactin levels or decrease tumor size. We report a unique case of prolactin secreting macroadenoma resistant to dopamine agonist treatment.
Clinical history A 57-year old female was diagnosed with prolactin secreting pituitary macroadenoma following a work up for headaches. She denied visual disturbances and visual fields were normal. MRI of the pituitary gland demonstrated a large macroadenoma with superior displacement of the optic chiasm. Carbergoline 0.5 mg po twice weekly was initiated. After 4 weeks of treatment her prolactin levels normalized However, there was no change in tumor size by repeat MRI at four months despite persistent normalization of prolactin (see table).
Discussion We describe a unique case of a prolactin secreting pituitary macroadenoma demonstrating significant reduction in hormone out put with dopamine agonist treatment without a concomitant reduction in tumor burden. According to recent literature approximately 20-30% of prolactinomas are resistant to treatment with dopamine agonists in that neither prolactin nor tumor burden decrease. A discussion of reasons for the discordant response in this patient will follow.
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