Glucagonoma

Glucagonoma is a rare, usually malignant neuroendocrine tumor of the pancreatic alpha cells that secretes excessive glucagon, leading to a characteristic paraneoplastic syndrome. In veterinary medicine, glucagonoma is most commonly reported in dogs, with a few cases in cats. The tumor is typically located in the pancreas, often in the body or left lobe, and may metastasize to the liver, regional lymph nodes, or other organs. The clinical syndrome is dominated by the effects of hyperglucagonemia, which include severe hyperglycemia, hypoaminoacidemia, and a distinctive dermatopathy known as superficial necrolytic dermatitis (SND) or hepatocutaneous syndrome. The disease is progressive and often diagnosed at an advanced stage due to nonspecific early signs.

The exact etiology of glucagonoma is unknown, but it is believed to arise from neoplastic transformation of pancreatic alpha cells. No specific viral, bacterial, or environmental causes have been identified. Genetic mutations, such as alterations in the MEN1 gene (multiple endocrine neoplasia type 1), have been implicated in human glucagonomas, but similar genetic predispositions have not been confirmed in dogs or cats. Chronic hyperglucagonemia from any cause may contribute to the metabolic derangements, but the tumor itself is the primary driver. In dogs, there is no established breed or sex predilection, though some reports suggest a higher incidence in older animals, typically over 10 years of age.

Glucagonoma is extremely rare in veterinary medicine. It is most commonly reported in dogs, with fewer cases in cats. In dogs, there is no strong breed predisposition, but cases have been described in mixed-breed dogs, Golden Retrievers, and other breeds. The median age at diagnosis is around 10-12 years, with a range of 6-15 years. No sex predilection has been consistently identified. In cats, the disease is even rarer, with only isolated case reports. Geographic distribution is not well characterized, but the condition is likely underdiagnosed due to its nonspecific clinical signs and the need for specialized diagnostic testing. The incidence is unknown, but it is considered a rare differential for diabetes mellitus and skin disease in older dogs.

Glucagonoma arises from pancreatic alpha cells, which normally secrete glucagon in response to hypoglycemia. The tumor autonomously secretes excessive glucagon, leading to a state of hyperglucagonemia. Glucagon promotes hepatic glycogenolysis and gluconeogenesis, resulting in hyperglycemia and insulin resistance. Chronic hyperglucagonemia also inhibits hepatic amino acid uptake and promotes gluconeogenesis from amino acids, leading to hypoaminoacidemia. The hypoaminoacidemia is thought to be a key driver of the characteristic skin lesions, as amino acids are essential for epidermal cell proliferation and differentiation. The dermatopathy, known as superficial necrolytic dermatitis (SND) or hepatocutaneous syndrome, is characterized by epidermal hyperplasia, parakeratosis, and superficial necrosis. The exact mechanism is not fully understood, but it is believed that low plasma amino acid levels impair keratinocyte metabolism, leading to the characteristic lesions. Additionally, hyperglucagonemia may cause direct toxic effects on the skin. The tumor may also secrete other peptides, such as pancreatic polypeptide or somatostatin, which can complicate the clinical picture. Metastasis to the liver and other organs can occur, further contributing to morbidity.

The primary predisposing factor for glucagonoma is advanced age, as the tumor is most commonly diagnosed in older dogs and cats. There is no clear breed or sex predisposition, but some breeds, such as Golden Retrievers, may be overrepresented in case reports. Chronic pancreatitis or other pancreatic pathology may increase the risk of neoplastic transformation, but this is speculative. Immunosuppression or concurrent endocrine disorders, such as diabetes mellitus, may complicate the clinical presentation but are not considered direct predisposing factors. Genetic factors, such as mutations in tumor suppressor genes, are likely involved but have not been characterized in veterinary species. Environmental factors have not been identified.

The clinical signs of glucagonoma are often insidious and may be present for months before diagnosis. The most common presenting complaints are dermatologic and metabolic. Dermatologic signs include erythema, crusting, ulceration, and alopecia, typically affecting the footpads, mucocutaneous junctions, and pressure points. The footpads may become hyperkeratotic, fissured, and painful, leading to lameness. The skin lesions are often refractory to conventional therapy. Metabolic signs include polyuria, polydipsia, polyphagia, and weight loss, which are consistent with diabetes mellitus. Some animals may present with signs of hepatic dysfunction, such as icterus or elevated liver enzymes, due to metastasis or concurrent hepatopathy. Neurologic signs, such as seizures or weakness, may occur due to hypoglycemia or other metabolic derangements, although hyperglycemia is more common. In advanced stages, animals may become lethargic, anorexic, and debilitated. The clinical signs can be categorized by stage: early (mild dermatopathy and mild hyperglycemia), intermediate (progressive skin lesions, diabetes mellitus), and advanced (metastasis, severe metabolic derangements, and systemic illness).

Differential diagnoses for glucagonoma include: 1) Diabetes mellitus (uncomplicated) - presents with polyuria/polydipsia and hyperglycemia but lacks the characteristic dermatopathy; skin lesions are not typical. 2) Superficial necrolytic dermatitis (SND) due to other causes, such as chronic liver disease (hepatocutaneous syndrome) or inflammatory bowel disease - these conditions may cause identical skin lesions but are not associated with a pancreatic tumor; diagnosis requires abdominal imaging and biopsy. 3) Other pancreatic tumors, such as insulinoma (causes hypoglycemia, not hyperglycemia) or pancreatic adenocarcinoma (may cause similar nonspecific signs but lacks glucagon excess). 4) Autoimmune skin diseases, such as pemphigus foliaceus or systemic lupus erythematosus - these typically have characteristic histopathology and respond to immunosuppressive therapy, whereas SND does not. 5) Zinc-responsive dermatosis - causes similar footpad lesions but is associated with zinc deficiency and responds to zinc supplementation. 6) Drug eruptions or contact dermatitis - history of exposure and resolution with withdrawal. 7) Fungal or bacterial skin infections - may be secondary but are not the primary cause; cytology and culture can differentiate. 8) Hyperadrenocorticism (Cushing's syndrome) - can cause skin thinning and calcinosis cutis, but the distribution and histopathology differ; also, hyperadrenocorticism is associated with elevated cortisol, not glucagon. 9) Hepatic disease, such as chronic hepatitis or cirrhosis, can cause SND-like lesions due to hypoaminoacidemia; liver function tests and biopsy are needed. 10) Inflammatory bowel disease with protein-losing enteropathy can lead to hypoaminoacidemia and SND; gastrointestinal signs and intestinal biopsy are diagnostic.

The diagnostic approach to glucagonoma should be systematic: 1) Initial clinical evaluation: Thorough history and physical examination, with special attention to skin lesions, particularly on the footpads and mucocutaneous junctions. 2) Baseline laboratory tests: Complete blood count (CBC), serum biochemistry profile, and urinalysis. Key findings include hyperglycemia, elevated liver enzymes (alkaline phosphatase, alanine aminotransferase), hypoalbuminemia, and hypoaminoacidemia (if measured). 3) If SND is suspected, measure plasma amino acid concentrations; low levels of amino acids, especially glutamine, are supportive. 4) Measure serum glucagon concentration using a validated assay (e.g., radioimmunoassay). Elevated glucagon levels (typically >500 pg/mL in dogs) are highly suggestive of glucagonoma. 5) Abdominal imaging: Ultrasonography is the initial imaging modality of choice to identify a pancreatic mass and assess for metastasis. A well-defined hypoechoic nodule in the pancreas may be seen. Computed tomography (CT) or magnetic resonance imaging (MRI) may provide better characterization and surgical planning. 6) Fine-needle aspiration (FNA) of the pancreatic mass or metastatic lesions for cytology; however, cytology may be nondiagnostic, and histopathology is often required. 7) Surgical biopsy or excisional biopsy of the pancreatic mass for histopathology and immunohistochemistry (positive for glucagon, chromogranin A, synaptophysin). 8) If surgery is not feasible, a liver biopsy may be performed to confirm metastatic disease. 9) Additional tests: Serum insulin levels may be low or normal, and C-peptide may be measured to assess insulin secretion. 10) Rule out other causes of SND, such as primary hepatic disease, via liver function tests (bile acids, ammonia) and liver biopsy if indicated.

Hematology: CBC is often unremarkable, but mild anemia may be present due to chronic disease. Serum biochemistry: Hyperglycemia is a consistent finding, often in the range of 200-400 mg/dL. Liver enzymes, particularly alkaline phosphatase (ALP) and alanine aminotransferase (ALT), are frequently elevated due to hepatic lipidosis or metastasis. Hypoalbuminemia is common due to malnutrition and hepatic dysfunction. Hypoaminoacidemia, especially low levels of glutamine, alanine, and branched-chain amino acids, is a hallmark finding. Electrolyte abnormalities may include hypokalemia and hypomagnesemia. Urinalysis: Glucosuria may be present if blood glucose exceeds the renal threshold. Urine specific gravity may be low if diabetes insipidus-like signs occur, but this is rare. Blood gas analysis: May reveal metabolic acidosis if diabetes is poorly controlled. Specific biomarkers: Serum glucagon concentration is markedly elevated (often >500 pg/mL; normal <200 pg/mL). Insulin levels may be low or normal. C-peptide may be low. Other biomarkers: Chromogranin A may be elevated as a general neuroendocrine tumor marker. Serology/PCR: Not applicable. Endocrinological assays: Thyroid and adrenal function tests may be performed to rule out concurrent endocrinopathies.

Radiography: Abdominal radiographs may reveal a soft tissue mass in the cranial abdomen, but this is not sensitive. Thoracic radiographs are important to detect pulmonary metastasis. Ultrasonography: Abdominal ultrasound is the most useful imaging modality. It may reveal a well-defined, hypoechoic mass in the pancreas, often in the left lobe. The mass may be heterogeneous with cystic areas. Ultrasonography can also detect hepatic metastasis (hypoechoic nodules) and lymphadenopathy. Doppler ultrasound may show increased vascularity. Computed Tomography (CT): CT provides superior contrast resolution and can better delineate the pancreatic mass, its relationship to surrounding vessels, and the presence of metastasis. Triple-phase CT can help identify hypervascular tumors. Magnetic Resonance Imaging (MRI): MRI is less commonly used but can provide detailed soft tissue contrast. Endoscopy: Endoscopic ultrasound (EUS) may be used to visualize the pancreas and obtain FNA, but it is not widely available in veterinary practice. Fluoroscopy: Not typically used. Echocardiography: Not directly relevant, but may be performed to assess cardiac function if metastatic disease is suspected.

Cytology: Fine-needle aspiration of the pancreatic mass may yield cells with neuroendocrine features, such as round cells with moderate cytoplasm and granular chromatin. However, cytology is often nondiagnostic due to the fibrous nature of the tumor. Histopathology: On histopathology, glucagonoma appears as a well-demarcated but unencapsulated mass composed of nests, cords, or trabeculae of polygonal cells with eosinophilic cytoplasm and round nuclei. The cells may show mild to moderate atypia. Mitotic figures are variable. Immunohistochemistry is essential for diagnosis: tumor cells are positive for glucagon, chromogranin A, and synaptophysin. Negative staining for insulin, somatostatin, and pancreatic polypeptide helps differentiate from other pancreatic neuroendocrine tumors. Metastatic lesions in the liver or lymph nodes show similar histologic features. Special stains, such as Grimelius silver stain, may highlight neuroendocrine granules.

The treatment of glucagonoma involves a multimodal approach: 1) Surgical resection: Surgical excision of the pancreatic mass is the treatment of choice if there is no evidence of metastasis. Partial pancreatectomy or pancreaticoduodenectomy may be performed. Surgery can be curative if the tumor is completely excised. However, many tumors are metastatic at diagnosis, making surgery palliative. 2) Medical management of hyperglycemia: Insulin therapy is often required to control hyperglycemia. Regular insulin (0.2-0.5 U/kg SC q8-12h) or NPH insulin (0.5-1 U/kg SC q12h) may be used, with dose adjustments based on serial blood glucose curves. 3) Management of dermatopathy: The skin lesions may improve with amino acid supplementation. Oral amino acid solutions (e.g., 10% amino acid solution at 10-20 mL/kg/day) or intravenous amino acid infusions (e.g., 8.5% amino acid solution at 500 mL over 8-12 hours) have been used. Zinc supplementation (1-2 mg/kg/day PO) may also be beneficial. 4) Somatostatin analogs: Octreotide (5-20 µg/kg SC q8-12h) or lanreotide may reduce glucagon secretion and improve clinical signs, but they are not widely available in veterinary medicine. 5) Chemotherapy: For metastatic disease, chemotherapy with streptozocin (500 mg/m² IV q3 weeks) or doxorubicin (30 mg/m² IV q3 weeks) may be attempted, but response rates are poor. 6) Supportive care: Nutritional support with a high-protein diet, antiemetics, and analgesics for skin pain. 7) Radiation therapy: Not well described for this tumor. 8) Monitoring and management of complications: Treat secondary infections, manage diabetes, and monitor liver function.

The prognosis for glucagonoma is guarded to poor. The median survival time in dogs is reported to be around 6-12 months, even with treatment. Factors associated with a worse prognosis include the presence of metastasis at diagnosis, large tumor size, and poor response to medical management. Surgical resection with complete excision may result in longer survival, but recurrence is possible. The dermatopathy may improve with treatment, but it often persists. The prognosis is better if the tumor is detected early and surgically removed. However, due to the nonspecific early signs, most cases are diagnosed late. In cats, the prognosis is similarly poor, with only a few case reports.

Follow-up should be structured and regular: 1) Recheck examinations every 1-3 months initially, then every 3-6 months if stable. 2) Serial blood glucose measurements to adjust insulin therapy. 3) Monitor serum glucagon levels every 3-6 months to assess tumor progression or recurrence. 4) Repeat abdominal ultrasound every 3-6 months to evaluate for metastasis or recurrence. 5) Monitor liver enzymes and amino acid levels to guide nutritional support. 6) Dermatologic evaluation to assess response to treatment. 7) If the patient is on octreotide, monitor for side effects such as diarrhea or injection site reactions. 8) Adjust amino acid supplementation based on plasma amino acid levels. 9) Provide client education on signs of hypoglycemia or hyperglycemia and when to seek emergency care.

Pearls: 1) Always consider glucagonoma in older dogs with diabetes mellitus and characteristic skin lesions, especially footpad hyperkeratosis. 2) Measure plasma amino acids in any dog with suspected SND; hypoaminoacidemia is a key finding. 3) Serum glucagon measurement is the most specific diagnostic test; request it early. 4) Abdominal ultrasound is essential for identifying a pancreatic mass; consider CT for surgical planning. 5) Surgical resection offers the best chance for long-term survival; refer to a surgical specialist. 6) Amino acid supplementation can dramatically improve skin lesions, even if the tumor is not resectable. Pitfalls: 1) Misdiagnosing the skin lesions as autoimmune or allergic dermatitis, leading to inappropriate immunosuppressive therapy. 2) Assuming that hyperglycemia is due to uncomplicated diabetes mellitus without investigating the skin lesions. 3) Failing to measure glucagon levels because the test is not readily available; consider sending samples to a specialized laboratory. 4) Overlooking the possibility of metastasis; always perform thoracic radiographs and abdominal ultrasound. 5) Using insulin without addressing the underlying tumor, which may lead to poor glycemic control. 6) Not providing amino acid supplementation, which is crucial for skin healing.

Based on Plumb's Veterinary Drug Handbook: 1) Insulin: Regular insulin (Humulin R) - initial dose 0.2-0.5 U/kg SC q8-12h, titrate based on blood glucose curves. NPH insulin (Humulin N) - 0.5-1 U/kg SC q12h. 2) Amino acid supplementation: For IV infusion, use 8.5% amino acid solution (e.g., Aminosyn) at 500 mL over 8-12 hours, repeated every 1-2 weeks as needed. For oral, use a 10% amino acid solution at 10-20 mL/kg/day, divided q8h. 3) Zinc: Zinc sulfate or zinc methionine at 1-2 mg/kg/day PO, monitor for gastrointestinal upset. 4) Octreotide: 5-20 µg/kg SC q8-12h, titrate to effect. 5) Chemotherapy: Streptozocin (Zanosar) - 500 mg/m² IV q3 weeks, with saline diuresis to prevent nephrotoxicity. Doxorubicin - 30 mg/m² IV q3 weeks, with cardiac monitoring. 6) Supportive care: Antiemetics (e.g., maropitant 1 mg/kg SC q24h), analgesics (e.g., tramadol 2-5 mg/kg PO q8-12h), and gastroprotectants (e.g., omeprazole 0.5-1 mg/kg PO q24h) as needed. 7) For diabetes management, monitor for hypoglycemia and adjust doses accordingly. 8) In renal or hepatic impairment, adjust doses of chemotherapeutic agents and octreotide as needed.

The veterinary literature on glucagonoma is limited to case reports and small case series. A landmark case series by Gross et al. (1990) described the clinical and pathologic features of glucagonoma in dogs, establishing the association with superficial necrolytic dermatitis. Another study by Miller et al. (1991) reported successful surgical treatment in a dog. More recent reports have documented the use of octreotide and amino acid supplementation for palliative management. There are no prospective clinical trials or consensus guidelines from ACVIM or ECVIM specifically for glucagonoma. The evidence is primarily level IV (case reports and expert opinion). However, the diagnostic criteria and treatment approaches are extrapolated from human medicine and adapted to veterinary patients. The importance of measuring serum glucagon and plasma amino acids is well supported. Surgical resection remains the only potentially curative treatment, but the prognosis is guarded due to late diagnosis. Further research is needed to establish standardized treatment protocols.