Introduction: Medical treatment of macroprolactinomas with dopamine ag
onists decreases tumor mass and improves visual defects. We report an
unusual complication of a macroprolactinoma responding to bromocriptin
e: a visual field defect caused by downward herniation of the optic ch
iasm. Materials and Methods: A 64-year-old woman was found to have a 4
.5 cm macroprolactinoma with superior displacement of the optic chiasm
, bitemporal hemianopia, and serum prolactin concentration (P) of 17,0
60 mu g/L. Bromocriptine was initiated at 2.5 mg/day and increased to
7.5 mg/day over 2 months. Results: After 2 months, visual fields impro
ved significantly and tumor height decreased to 3 cm with resolution o
f the optic chiasm displacement. P decreased to 1,180 mu g/L. After 5
months of therapy, visual fields were normal, and P was 8 mu g/L. Afte
r 8 months of therapy, new bilateral visual defects were observed. Mag
netic resonance imaging (MRI) revealed further decrease of the tumor h
eight to 1.5 cm, and inferior and leftward traction of the optic chias
m as the probable mechanism for the new visual field deficit. P was <1
mu g/L. Bromocriptine was decreased to 5 mg/day to allow reduced trac
tion on the optic chiasm and its blood supply. Over the next 4 months,
visual field abnormalities resolved. Conclusions: We report the devel
opment of a visual field abnormally that is explained by chiasmal hern
iation caused by a shrinking macroprolactinoma. This complication reso
lved with a decrease in the bromocriptine dose. We suggest that patien
ts undergoing bromocriptine therapy for macroprolactinomas be followed
for this potential complication.