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Pancreatic agenesis is a rare type of congenital genetic disorder, which arises from innate pancreas malformation and the absence of pancreatic tissues. Pancreatic agenesis is classified into four types, and the signs and symptoms vary among the affected individuals. The exact cause of pancreatic agenesis is not well studied due to the limited cases reported worldwide. Nevertheless, the most recognized mechanisms for pancreatic agenesis are genetic mutations in the PDX1 gene and PTF1A gene, which lead to incomplete pancreatic development in the fetus, subsequently inducing agenesis. There is no standard method to diagnose pancreatic agenesis, but medical imaging has been widely used in many medical case reports due to its ability to detect structural abnormalities in patients' pancreas. If symptoms appear, treatments are provided to alleviate the pain and prevent the worsening of tissues and organ dysfunction. Currently, the prevalence of pancreatic agenesis is 1 in 1,000,000 individuals, with less than 100 cases reported globally.

Classification
Pancreatic agenesis is categorized into four types based on the degree and location of congenital malformation in the pancreas, shown in Table 1. Reports suggest that the most common type of pancreatic agenesis is partial dorsal pancreatic agenesis, while the less common types are partial ventral pancreatic agenesis and complete pancreatic agenesis, due to the incompatibility for humans to live. Individuals with dorsal pancreatic agenesis will experience either a partial or complete failure in pancreatic development, referred to as partial dorsal pancreatic agenesis and complete dorsal pancreatic agenesis, respectively. The same theory applies to ventral pancreatic agenesis.

'''Table 1. Types of pancreatic agenesis based on the degree and location of pancreatic malformation. ' ''

Signs and symptoms
Signs and symptoms of pancreatic agenesis vary among individuals due to different levels of disease severity. The disease severity is determined by the degree of congenital malformation and the damaged pancreatic tissues. A greater pancreatic malformation indicates a more severe impairment of the pancreatic tissues, and thus, causing a deterioration in the normal digestive functions which further induces digestive diseases.

Common symptoms associated with pancreatic agenesis are pancreatitis, diabetes mellitus, hyperglycemia, bile duct obstruction, abdominal pain. Recent research has suggested that 50% of the affected individuals developed hyperglycemia or the aforementioned symptoms, while the remaining half did not develop any symptom throughout their lifetime. Other rare diseases associated with pancreatic agenesis are polysplenia and heterotaxy syndrome.

Complications
Medical complications refer to the accompanying symptoms found in affected individuals, which leads to further worsening of health conditions. The common complications of pancreatic agenesis are pancreatitis, diabetes, tumors, and organ malformations. Upon the published studies, most of these complications are not confirmed to have a causal relationship with pancreatic agenesis.


 * Pancreatitis refers to pancreatic inflammation caused by enzyme's autodigestion of tissues, resulting in pancreatic structural damage and dysfunction. Pancreatitis is featured with abdominal symptoms like abdominal pain and bloating. Acute pancreatitis appears suddenly and lasts for several days, while chronic pancreatitis usually onsets after several times of acute pancreatitis. Factions of pancreatic agenesis patients have abdominal pain, which is later confirmed to be pancreatitis-derived rather than agenesis-caused. Several cases demonstrate chronic pancreatitis occurs after treatment to pancreatic agenesis by identifying recurrent alcohol-use associated abdominal pain.
 * Hyperglycemia (high blood sugar level) and hypoglycemia  (low blood sugar level) are associated with pancreas agenesis. Despite proper medications, the unexplained fluctuation of blood sugar levels is frequently observable in pancreas agenesis patients. Pancreatic agenesis patients with hyperglycemia are subjected to type 1 diabetes due to limited insulin production by the damaged pancreas, and these patients usually do not have abdominal symptoms. Commonly, pancreatic agenesis patients develop hypoglycemia as the result of insulin therapy and other diabetes medications.
 * Both pancreatic tumors  and some non-pancreatic tumors are found along with pancreatic agenesis. According to published studies, these non-pancreatic tumors include ampullary tumors, solid pseudopapillary tumors, cystic adenocarcinoma, neuroendocrine tumors, etc.
 * The non-pancreas organ malformation is identified as the accompanying disease with pancreatic agenesis. The organ malformations are multiple splenic deformities, polycystic kidney disease, biliary atresia, and congenital choledochal cysts.

Causes
Pancreatic agenesis is the failure of pancreatic development due to the absence of primordial germ cells in the embryonic stage. Nonetheless, the exact causes of pancreatic agenesis are not fully known. Many research studies proposed that the pathophysiological mechanisms are associated with genetic mutations and genetic inheritance. In research laboratories, the more well-known genetic mutations related to pancreatic agenesis are the PDX1 gene and PTF1A gene, while the less studied genetic mutations are the GATA4 gene, GATA6 gene, and Mnx1 gene.

Genetic mutation in PDX1 gene
A mutation in the Pancreatic Duodenal Homeobox-1 (PDX1) gene is associated with pancreatic agenesis. The PDX1 gene codes for the proliferation of the pancreas in the embryonic stage. The PDX1 gene develops signals during the first three to eight weeks of embryogenesis (embryonic development), which is received by the pancreatic bud. Upon release of signals, the pancreatic bud will differentiate into a complete pancreas composed of a neck, body, and tail. However, a mutation in the PDX1 gene will cause abnormal development of the pancreatic bud into an incomplete pancreas, and thus, leading to structural abnormalities and pancreatic dysfunctions.

Additionally, the PDX1 gene is suspected to be associated with various metabolic diseases in pancreatic agenesis patients. One commonly associated metabolic disease is diabetes mellitus. The PDX1 gene is a major transcription factor for the insulin gene, which controls the absorbing ability of pancreatic beta-cells and serves to maintain the body's blood glucose level. A mutation of the PDX1 gene depletes the functional pancreatic beta-cells that sustain the body's blood glucose level in the normal range. Thus, the mutation induces hyperglycemia (high blood sugar level) and diabetes mellitus.

Genetic mutation in PTF1A gene
A missense mutation in the Pancreas Associated Transcription Factor 1a (PTF1A) gene is associated with the incidence of pancreatic agenesis. A missense mutation is a specific type of genetic mutation in which a single based nucleotide in the DNA is changed, and results in a substitution of an amino acid that codes for a different protein. The PTF1A gene promotes the development of embryonic foregut endoderm to form a proper pancreas. At an early stage, the PTF1A genes are expressed in progenitors of pancreatic ducts, endocrine, and exocrine cells of the pancreas. At a later stage, the PTF1A gene controls the expression of the PDX1 gene for pancreatic development in the embryo[9]. When the PTF1A gene mutates, the embryonic foregut endoderm will fail to develop into a complete pancreas, and therefore, inducing pancreatic agenesis.

Furthermore, a missense mutation in the PTF1A gene is responsible for the inheritance of neonatal diabetes mellitus, which is associated with pancreatic agenesis. Neonatal diabetes mellitus is an autosomal recessive syndrome, and it is a type of diabetes onset in the first six months in infancy. If neonatal diabetes mellitus remains untreated, symptoms such as dehydration, muscle weakness, learning disabilities, and other health complications may develop. Nonetheless, the occurrence of both neonatal diabetes mellitus and pancreatic agenesis is uncommon.

Genetic inheritance
Pancreatic agenesis is inherited by autosomal recessive or X-linked dominant mode at the neonatal or infancy stage. Research suggested that individuals who carry these affected genes may temporarily exhibit no symptoms or a historical record of pancreatic agenesis.

Diagnosis
The diagnosis of pancreatic agenesis is not well-established due to the limited cases reported worldwide. Although there is no standard method to diagnose this disease, many clinicians use imaging examinations as the primary diagnostic tool for detecting pancreatic structural abnormalities. The choice of imaging techniques is based on the specific symptoms and clinical availability. In some cases, clinicians apply multiple imaging techniques to confirm the diagnosis of pancreatic agenesis.

Medical imaging
Imaging examination is commonly used by physicians to detect abdomen symptoms. Recent research recommends to confirm pancreatic agenesis diagnosis by integrating one or more imaging examinations. Despite limited clinical reports, endoscopic retrograde cholangiopancreatography (ERCP) is found to give the highest diagnostic values for pancreatic agenesis.


 * Ultrasound scan: also known as sonography. It is a medical test that captures the pancreatic image through the use of high-frequency sound waves. This method is commonly used to detect pancreatic pathology. However, the detection of pancreatic agenesis becomes inaccurate when intestinal gas blocks the view of the partial pancreas near the splenic vessels.
 * Computed tomography (CT) & Magnetic resonance imaging (MRI): CT mainly uses X-ray to capture cross-sectional images while MRI is magnet-based. These two diagnostic tools are usually applied to display the internal structure of the pancreas. However, both CT and MRI remain arguable in diagnosing pancreatic agenesis. Some clinical reports used to state that both CT or MRI can reveal the integrity and partiality of the pancreatic head, body, and tail. Conversely, recent clinical reports have declared the CT's and MRI's inability to fully display the pancreatic body and tail.




 * Endoscopic retrograde cholangiopancreatography (ERCP) &  Magnetic resonance cholangiopancreatography (MRCP) : ERCP is used for detailed examination on the pancreatic duct by inserting a flexible plastic tube (endoscopic) into the stomach and duodenum. MRCP is a non-invasive imaging technique that combines magnetic and radioactive information to evaluate the pancreas and pancreatic duct. Both ERCP and MRCP can expose the detailed shape of the main and accessory pancreatic ducts. The detection of a short pancreatic duct indicates the absence of pancreatic tissues.  Therefore, the two methods complement the information of the pancreatic duct that is missed by other imaging techniques.

Differential diagnosis
Differential diagnostic tools are applied to differentiate pancreatic agenesis from diseases with overlapping symptoms. These diseases include pancreatic fat infiltration, chronic pancreatitis, atrophy in pancreatic body and tail. For example, pancreatic fat infiltration bears a higher fat signal fraction (FSF) in MRI images and unique echo characteristics in ultrasound examination. Chronic pancreatitis, the common mimicker to pancreatic agenesis, is featured by pancreas dividum and autodigestion despite it possesses similar structural abnormality under imaging examinations. Recently, published research shows that chronic pancreatitis owns dilated pancreatic ducts compared with pancreatic agenesis.

Treatments
There is no universal agreement regarding the standard treatment of pancreatic agenesis. Most of the management measures and treatments focus on relieving symptoms and preventing further damage to pancreatic tissues. Recent studies suggest simple pancreatic agenesis requires no special treatment, but severe complications need medical treatments, such as the top two common complications: hyperglycemia (up to 50%) and pancreatitis.


 * Pancreatitis: Most acute pancreatitis patients are administered with an oxygen supply and intravenous transfusion of medication fluids with nutrition. Bowel rest is recommended that no food or liquid intake from the mouth. For better recovery, stomach juices is removed by a nasogastric tube for further resting the intestine. In cases of chronic pancreatitis, medication for pain or nausea is prescribed to alleviate symptoms.
 * Hyperglycemia: Management of hyperglycemia varies across patients with different causes. For pancreatic agenesis patients, insulin deficiency is caused by damaged pancreatic tissues, including insulin-secreting beta-cells. A pancreatic agenesis patient may present both endocrine and exocrine dysfunctions, requiring supplementation of pancreatic enzymes and insulin. Half of the pancreatic agenesis patients with hyperglycemia have received insulin therapy. At the same time, exercise is beneficial to lower high blood sugar level when urine ketone is absent. In the condition of moderate to large amounts of urine ketone, exercise will exacerbate blood sugar level. Diet is modulated by the dietitian to keep the sugar level in a normal range.

Epidemiology
Pancreas agenesis is a rare congenital anomaly inherited by autosomal recessive alleles or X-linked dominant mode. Up to now, only around 50-100 cases of partial pancreas agenesis are reported, and the disease prevalence is estimated to be 1 in 1,000,000.