BACKGROUND: Resistance to dopamine agonists is not uncommonly seen in prolactinomas. However, development of resistance to dopamine agonists after an initial period of robust treatment response is rare, and only 39 cases have been reported in the past four decades. We describe a Chinese man with this rare condition and explored the postulated mechanisms that may explain this phenomenon. We compiled similar cases that were previously reported and compared their etiology, progress, and response to treatment. On the basis of these cases, we derived a list of differential diagnoses to consider in patients with secondary resistance to dopamine agonists.
METHODS: A 63-year-old Chinese man presented with blurred vision and was subsequently diagnosed with a macroprolactinoma. He had initial response to cabergoline but developed secondary resistance to it after 5 years. The prolactinoma continued to grow, and his serum prolactin remained markedly elevated despite adherence to escalating dosages of cabergoline up to 6 mg/week. The patient finally underwent transsphenoidal surgery and was found to have a sparsely granulated lactotroph tumor with Ki-67 index of 5%. Postoperatively, there was improvement in his serum prolactin level, although he still required treatment with cabergoline at 6 mg/week.
CONCLUSIONS: Surgery can facilitate disease control in patients with prolactinomas that develop secondary resistance to dopamine agonists. Malignant prolactinoma is an important differential diagnosis in this group of patients, especially when serum prolactin remains markedly elevated despite resolution or stability of the primary pituitary lesion, suggesting a metastatic source of prolactin secretion.

The clinical manifestations of equine pituitary pars intermedia dysfunction (PPID) in temperate climates are well described. The classic presentation is that of an older animal with hirsutism, laminitis, poor muscle tone, pendulous abdomen and weight loss. This case series highlights the additional clinical signs of anhidrosis and heat stress with secondary exercise intolerance that were seen as primary presenting problems in equids with PPID in the hot, humid conditions of a tropical climate. The clinical signs resolved with medical treatment for PPID.

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Neuropsychological and psychopathological modifications induced by dopaminergic drugs in patients with Parkinson\'s disease (PD) are invariably not taken into sufficient consideration by the neurologist. Among the former, modifications of sexual urges and behaviours are of particular importance with regard to severity and variety of clinical pictures. Although rare, such modifications may assume the connotations of an aberrant sexual behaviour with criminal implications, in line with a diagnosis of paraphilia. The authors report the case of a 51-year-old male PD patient who, after a few years of dopaminergic treatment with pergolide, developed a paraphilic disorder, consistent with DSM-IV TR diagnosis of frotteurism, and delusional jealousy. The patient presented mild motor impairment and lack of or negligible cognitive deterioration, thus providing evidence that these disorders are not typical of advanced PD. Pergolide was reduced and quetiapine, an atypical neuroleptic, was introduced with subsequent subsiding of the paraphilic disorder and improvement of delusional jealousy.

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Hypersexuality is a known, though not frequently publicized, behavioral disturbance in patients with Parkinson disease. Hypersexuality has been associated with dopaminergic drug therapy. We describe a patient with Parkinson disease who presented with compulsive hypersexual behavior. Lowering of dopaminergic drug doses was of minimal behavioral benefit but was not well tolerated because of worsening of motor function. A trial with donepezil led to significant amelioration of the compulsive behavior without adverse motor effects.

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A 74-year-old patient with idiopathic Parkinson\'s disease was evaluated for unintended sleep episodes that occurred after long-term treatment with 400 mg/day of L-dopa. Overnight sleep studies and multiple sleep latency testing were carried out under double-blind administration of either L-dopa or placebo. Mean sleep latency with L-dopa was 7 minutes, in contrast to a normal value of 19 minutes, 25 seconds with placebo. The authors\' results suggest that L-dopa may cause daytime somnolence in some patients with Parkinson\'s disease.