Giant Pituitary Adenoma Presenting With Disinhibition: A Case Report of a 25-Year-Old Male

Background: Giant growth hormone secreting pituitary adenomas (defined by a diameter >4cm) are rare and difficult to treat (1). These typically invade surrounding structures, making surgery challenging or impossible (1). This report highlights a giant mammosomatotrophinoma with an unusual clinical presentation. Clinical Case: A 25-year-old male presented with personality changes, disinhibition and executive dysfunction progressing over a 3 to 4-year period. Further enquiry elicited a history of increasing headaches and significant visual loss, on the background of unilateral childhood visual loss in the contralateral eye. He was noted to have clinical features of acromegaly and complete temporal visual field loss in the left eye. On initial testing plasma IGF-1 was 137 nmol/L (reference interval [RI] 13-43nmol/L), GH >100 ug/L and prolactin 23,900 mIU/L (RI 0-400mIU/L). GH remained >100ug/L at 120 minutes after a 75g oral glucose load. He has hypogonadotropic hypogonadism, however thyrotropic and corticotropic function remained normal. MRI of the brain with contrast revealed an 8.0 x 7.1 x 7.4 cm mass arising from the pituitary fossa, extending into the suprasellar region and displacing both frontal lobes. The mass encased the basilar and right internal carotid arteries. It was deemed inoperable due to its exceptionally large size and encasement of vascular structures. Medical treatment was initiated with cabergoline (3.5mg per week in divided doses) and octreotide LAR, initially 30mg then increased to 60mg 4-weekly. A weaning course of steroids was initiated for oedema of his optic nerve. Within one month of treatment there was improvement of clinical symptoms of headaches, sweating, irritable mood and disinhibition with associated modest biochemical improvement (reduction of IGF-1 to 129 nmol/L and prolactin reduction to 2119 mIU/L). MRI of the brain revealed a reduction in the size of the adenoma to 7.8 x 7.1 x 6.3 cm, with a reduction in the mass effect on the frontal lobes. The patient continues medical management and close clinical monitoring with the aim of ongoing tumour shrinkage to allow reassessment for possible surgical debulking. Conclusion: This unique case of a GH and prolactin co-secreting giant pituitary adenoma posed a therapeutic challenge due to the significant surgical risk, limiting treatment to aggressive medical therapy. The cognitive and behavioral changes experienced due to tumour size and location added to the management complexity. Although early on in the treatment course, there is improvement of symptoms and tumour size on cabergoline and octreotide LAR injections. (1) Iglesias P, Rodríguez Berrocal V, Díez JJ. Giant pituitary adenoma: histological types, clinical features and therapeutic approaches. Endocrine. 2018 Sep;61(3):407-421.