Most of the abdominal organs are covered by a double layer sac known as the peritoneum. The visceral peritoneum adheres to the organs while the parietal peritoneum lines the inner wall of the abdomen. Between these two layers is a potential space known as the peritoneal cavity. A small amount of peritoneal fluid serves as lubrication to reduce the friction when the two layers rub against each other. This amount is very small and healthy men may have little to no peritoneal fluid while in women it may be up to 20mL at most. Sometimes there is an excess of fluid within the peritoneal cavity to the extent that it can cause abdominal swelling and cause weight gain.
What is ascites?
Ascites is the medical term for fluid accumulation within the abdomen, or more specifically in the peritoneal cavity (intraperitoneal). It is mainly linked to liver disorders (hepatic) such as cirrhosis but can be due to other conditions that do not involve the liver (non-hepatic). A slight excess of peritoneal fluid will cause no symptoms but as the condition progresses and more fluid accumulates, ascites can present as abdominal swelling, weight gain and difficulty breathing. Ascites may or may not be related to peritonitis which is the inflammation of the peritoneal membranes caused by various conditions including an infection, malignancy and certain immune-mediated reactions.
The exact mechanism responsible for ascites is not clearly understood. It is associated with portal hypertension meaning that the pressure of the blood flowing to the liver is higher than normal. The pressure difference inside and outside the blood vessel in conjunction with the lower than normal blood proteins (hypoalbuminemia) may cause fluid to be leave the vessel. Retention of sodium and water may be another contributing factor with an overall increase of fluid in the tissue and potential spaces. This occurs as a result of the activation of the renin-angiotensin system which may be related to portal hypertension and any decrease in circulating blood volume. With peritonitis, inflammation makes the blood vessels more porous allowing fluid to leak out into the tissue spaces and subsequently collect in the peritoneal cavity.
Causes of Ascites
The causative factors can be divided between the hepatic and non-hepatic causes. Ascites needs to be differentiated from various other causes of abdominal distention, abdominal bloating and functional bloating. Hepatic causes account for most cases of ascites and of these portal hypertension that arises with several liver diseases is the main factor.
Hepatic causes of ascites includes :
- Liver cirrhosis
- Alcoholic hepatitis
- Non-alcoholic steatohepatitis
- Budd-Chiari syndrome
- Liver cancer
- Biliary diseases (bile ascites)
Non-hepatic causes of ascites may also be responsible for portal hypertension. Other mechanisms may also account for non-hepatic ascites. It may include :
- Cancer of the pancreas or peritoneum
- Congestive heart failure
- Constrictive pericarditis
- Nephrotic syndrome
- Protein-losing enteropathy
- Ovarian disease
- Renal dialysis, particularly peritoneal dialysis.
- Systemic lupus erythematosus (SLE)
Not every person with ascites may be aware of the underlying causative conditions mentioned above. Risk factors therefore includes :
- Chronic viral hepatitis
- Intravenous drug users
- Multiple sexual partners
- Regular blood transfusions
- Diabetes mellitus (type 2)
Signs and Symptoms
Symptoms are rarely seen or very mild if present when the fluid accumulation within the peritoneal cavity is less than 500mL. Despite the presence of symptoms beyond this level, clinical signs such as a shifting dullness upon percussion and a fluid wave may be absent until the total accumulated fluid exceeds 1,500mL.
Ascites can be graded as :
- Grade 1 (mild ascites) where there is usually no symptoms and the fluid accumulation can only be detected upon an ultrasound examination of the abdomen.
- Grade 2 (moderate ascites) presents with symptoms like moderate distention of the abdomen.
- Grade 3 (massive ascites) presents with major abdominal distention, bloating (sensation of fullness) and difficulty breathing.
Picture of massive ascites due to cirrhosis. Sourced from Wikimedia Commons.
There is usually no abdominal pain due to the ascites itself but apprehensive patients may report the discomfort associated with bloating as pain. When pain is present, it is more likely due to other causes although the ascites may exacerbate pain in these instances. Difficulty breathing may be a result of the pressure in the abdominal cavity preventing the diaphragm from contracting fully during inhalation. Sometimes the fluid can migrate across the diaphragm and cause a pleural effusion (fluid around the lungs) which may also present with difficulty breathing. Other features like weight gain and a flattened or everted umbilicus may only be significant in massive ascites. Nausea and a lack of appetite may be seen at any stage but may not always be related to ascites itself.
Depending on the underlying cause, there may be concomitant signs and symptoms such as :
- Muscle wasting
- Spider nevi
- Palmar erythema
Diagnosis of Ascites
Diagnostic techniques are aimed at first confirming the presence of ascites, then identifying the most likely cause of ascites and finally monitoring any changes to determine the progression of the condition as well as the effectiveness of any treatment.
Physical examination and medical history may confirm ascites by the presence of the signs and symptoms and risk factors mentioned above. The patient’s abdomen is observed in the supine position (lying down on the back) and then when standing upright. Fullness in the flanks when lying flat subsequently eases when standing upright and causes the abdomen to hang. The umbilicus may become flattened or everted and any abdominal or inguinal hernias may protrude. Measuring the abdominal girth and recording the body weight at regular intervals is helpful in monitoring the ascites.
These clinical features, however, may not be evident in mild to moderate ascites and imaging studies such as an ultrasound and computed tomography (CT) scan can be useful. These studies may be able to detect the presence of as little as 100mL to 200mL of ascitic fluid. Chest and abdominal X-rays will only be able to detect the presence of 500mL or more of ascitic fluid. Nevertheless this is still of value when compared to the most prominent clinical signs being detectable only at levels of 1,000 to 1,500 mL or more of fluid.
Ascitic fluid can be collected at the time of an abdominal paracentesis and sent for laboratory evaluation. This will assist with diagnosing the cause of ascites. Apart from visual inspection to identify the color and therefore possible pathogenesis, other features that are investigated includes the cell count, SAAG (serum ascites albumin gradient), total protein, culture and cytology.
Additional laboratory studies like a complete blood count (CBC), liver function tests (LFT), clotting profile and renal function tests may also be necessary to reach a diagnosis. In the event that other diagnostic investigations do not reveal the most likely cause of ascites, a laparoscopy may then be considered.
Treatment of Ascites
The treatment is almost entirely dependent on the underlying cause but various measures may assist with reducing the volume of ascitic fluid. This includes :
- Sodium-restricted diet to decrease water retention.
- Diuretics like spironolactone and furosemide to increase the excretion of fluid.
- Therapeutic paracentesis which is the removal of ascitic fluid, usually not surpassing 4L/day.