Case Presentation: A 2-year-old female presented to our Gastroenterology service with persistent failure to thrive (FTT). She was born at term and remained in the 10th percentile in weight for the first nine months. She continued to grow in length but did not gain weight. Initial inpatient workup at 13 months revealed a CBC, TSH, CMP, erythrocyte sedimentation rate, and celiac workup within normal limits. She weighed 6.34 kg (0.1 %ile, Z= -3.10) and was diagnosed with insufficient caloric intake. Readmission at 15 months for new weight loss yielded an unrevealing endocrine and genetic workup and neurological exam was unremarkable. The diagnosis of insufficient caloric intake was unchanged.For the next nine months, various high calorie foods and cyproheptadine for appetite stimulation were trialed and monitored at monthly clinic visits and by her mother, a dietician, with little improvement. At 2-years-old, the patient weighed 7.9 kg (< 0.01 %ile, Z= -4.73) and was admitted to our Gastroenterology service to monitor for refeeding syndrome after a planned nasogastric tube placement. There was suspicion for diencephalic syndrome due to characteristic poor weight gain despite a good appetite and preserved linear growth. A brain MRI identified a low-grade neoplasm suggestive of pilocytic astrocytoma (4.3 x 2.3 x 3.1 cm) and an MRI of the spine was concerning for drop metastases. Diencephalic syndrome was diagnosed 15 months after onset of FTT, and chemotherapy with vincristine and carboplatin was initiated. Currently, there is evidence of spinal metastases and no decrease in mass size. She receives feeds through a gastrostomy tube and her current weight is 11.6 kg (2.71 %ile, Z= -1.93), 14 months after her diagnosis.

Discussion: Diencephalic Syndrome (DS) is a rare but potentially deadly cause of failure to thrive in infants. It is associated with neoplasms, most commonly astrocytomas in the hypothalamic-optic chiasmatic region [1]. While the pathogenesis of DS is not fully understood, it is hypothesized that inappropriate growth hormone (GH) release, partial GH resistance, and excessive β-lipotropin secretion leads to increased lipolysis and loss of adipose tissue [2,3]. Clinically, DS is characterized by severe emaciation with preserved linear growth. These children consistently fail to thrive despite normal caloric intake [4]. Minor features include pallor, hypotension, and hypoglycemia. Affected children may appear happy and mentally alert, in stark contrast to their striking physical appearance. The non-specific presentation makes it difficult to make an early diagnosis. Previous reports of DS have shown that most children undergo extensive medical workup with several specialists before suspicion for neurological etiology arises, as neurological symptoms such as nystagmus, strabismus, and visual field defects tend to have late onset [5]. Its rarity and subsequent lack of awareness among clinicians further contributes to diagnostic delay [6]. Like most of the documented cases in the literature, our patient was diagnosed with DS via brain MRI 15 months after the onset of symptoms.

Conclusions: Misdiagnosis and diagnostic delays are common for patients with DS. Our case aims to increase clinical suspicion for DS in a child with severe emaciation despite retained linear growth and adequate caloric intake. In a failure to thrive patient exhibiting these characteristic features, a detailed neurological exam and brain imaging should be considered.