Sinusoidal obstruction syndrome overview

# Overview
Sinusoidal obstruction syndrome is characterized by tender hepatomegaly, ascites, and weight gain. It is most commonly a complication of hematopoietic cell transplantation (HCT). The incidence and severity of sinusoidal obstruction syndrome depends on the conditioning regimen used and presence of pre-existing liver disease. There is no established system for the classification of sinusoidal obstruction syndrome. However, it can be classified on the basis of severity as mild, moderate and severe. It is thought that pre-existing liver disease increases the risk of developing sinusoidal obstruction syndrome (SOS) due to impairment of drug metabolism which predisposes to the endothelial injury. The deposition of fibrinogen and factor VIII within the sinusoids leads to their dilation and congestion by erythrocytes. The progressive occlusion of venules leads to widespread zonal liver disruption and centrilobular hemorrhagic necrosis. The two established criteria for the clinical diagnosis of sinusoidal obstruction syndrome are Baltimore criteria and modified Seattle criteria. Patients with a mild or moderate form of sinusoidal obstruction syndrome require no specific therapy and can be managed with supportive care alone. If left untreated, the severe form of sinusoidal obstruction syndrome is characterized by high mortality and progression to multiorgan failure. The most accurate method to confirm the diagnosis and evaluate the severity of sinusoidal obstruction syndrome is the measurement of the hepatic venous gradient pressure (HVGP). The patient with suspected sinusoidal obstruction syndrome (SOS) requires the following lab studies: complete blood count, liver function tests, complete metabolic profile and viral hepatitis serologies. A transjugular liver biopsy can be helpful in the diagnosis of sinusoidal obstruction syndrome. Common findings on liver biopsy include sinusoidal dilation and congestion by erythrocytes and thrombi within the terminal hepatic venules.
The management of sinusoidal obstruction syndrome depends on the severity of the disease. Supportive care is the mainstay of therapy for mild and moderate sinusoidal obstruction syndrome. The severe form of sinusoidal obstruction syndrome needs defibrotide (thrombolytic agent) along with supportive care. 
Surgical treatment of sinusoidal obstruction syndrome is reserved for patients who do not respond to supportive treatment or defibrotide. The surgical options include transjugular intrahepatic portosystemic shunt (TIPS) and liver transplantation.
Primary preventive measures of sinusoidal obstruction syndrome include minimizing risks related to the transplant process such as the source of the graft (allogeneic greater than autologous), choice of chemotherapy and use of antimicrobials. The exposure to the hepatotoxic agents should be minimized and preexisting liver diseases should be managed. The prophylaxis for graft vs host disease should be considered.

# Historical Perspective
Hepatic veno-occlusive disease or sinusoidal obstruction syndrome was first described in 1905 as endophelibitis of the terminal hepatic veins. Ionizing radiations as a cause of sinusoidal obstruction syndrome was identified in the 1960's and bone marrow transplant in the 1970's. However, sinusoidal obstruction syndrome or hepatic veno-occlusive disease was a well-established concept by the mid-1960's.

# Classification
There is no established system for the classification of sinusoidal obstruction syndrome. However, it can be classified on the basis of severity as mild, moderate and severe.

# Pathophysiology
The development of sinusoidal obstruction syndrome begins with the injury to hepatic venous endothelium. It is thought that preexisting liver disease increases the risk of developing sinusoidal obstruction syndrome (SOS) due to impairment of drug metabolism which predisposes to the endothelial injury. The endothelial cells in patients with hepatitis may have abnormal expression of adhesion molecules and pro-coagulant factors. The deposition of fibrinogen and factor VIII within the sinusoids leads to their dilation and congestion by erythrocytes. The progressive occlusion of venules leads to widespread zonal liver disruption and centrilobular hemorrhagic necrosis. Hepatic sinusoidal obstruction syndrome (SOS) is mainly seen in patients of hematopoietic cell transplantation.

# Causes
The most common cause of sinusoidal obstruction syndrome is hematopoietic cell transplantation. Other less common causes include chemotherapeutic agents and pyrrolizidine alkaloids.

# Differentiating Sinusoidal obstruction syndrome from Other Diseases
The differential diagnosis of sinusoidal obstruction syndrome includes other causes of hepatic failure that may have abnormal liver function tests such as increased conjugated bilirubin and alkaline phosphatase or a clinical presentation as right upper quadrant abdominal pain, jaundice or ascites.

# Epidemiology and Demographics
The incidence of sinusoidal obstruction syndrome depends on the presence of risk factors, chemotherapy regimen and the clinical criteria used for the diagnosis. The incidence after allogeneic hematopoietic cell transplant ranges from a low of 10,000 persons per 100,000 persons to a high of 15,000 persons per 100,000 persons. The incidence of sinusoidal obstruction syndrome after autologous hematopoietic cell transplant is below 5,000 persons per 100,000 persons.

# Risk Factors
Common risk factors in the development of sinusoidal obstruction syndrome are stem cell transplantation, preexisting liver dysfunction and high-dose conditioning regimens.

# Screening
There is insufficient evidence to recommend routine screening for sinusoidal obstruction syndrome.

# Natural History, Complications, and Prognosis
Patients with a mild or moderate form of sinusoidal obstruction syndrome require no specific therapy and can be managed with supportive care alone.The mild and moderate form of sinusoidal obstruction syndrome has a good prognosis. If left untreated, the severe form of sinusoidal obstruction syndrome is characterized by high mortality and progression to multiorgan failure.

# Diagnosis

## Diagnostic study of choice
The two established criteria for the clinical diagnosis of sinusoidal obstruction syndrome are Baltimore criteria and modified Seattle criteria. The most accurate method to confirm the diagnosis and evaluate the severity of sinusoidal obstruction syndrome is the measurement of the hepatic venous gradient pressure (HVGP).

## History and Symptoms
Patients with sinusoidal obstruction syndrome may have a positive history of hematopoietic cell transplantation (HCT) or pre-existing liver disease. Common symptoms of sinusoidal obstruction syndrome include tender hepatomegaly, weight gain and ascites.

## Physical Examination
Common physical examination findings of sinusoidal obstruction syndrome include abdominal pain or distention, tender hepatomegaly, signs of ascites and jaundice.

## Laboratory Findings
The patient with suspected sinusoidal obstruction syndrome (SOS) requires the following lab studies: complete blood count, liver function tests, complete metabolic profile and viral hepatitis serologies.

## Electrocardiogram
There are no ECG findings associated with sinusoidal obstruction syndrome.

## X-Ray
There are no x-ray findings associated with sinusoidal obstruction syndrome.

## CT
CT scan findings suggestive of sinusoidal obstruction syndrome include hepatomegaly, nutmeg liver, portal vein dilatation, ascites.

## MRI
The MRI contrast studies for sinusoidal obstruction syndrome will show a diffuse hypointense reticular pattern.

## Ultrasound
Ultrasound can be helpful in the diagnosis of sinusoidal obstruction syndrome. Common findings on ultrasound may include: hepatomegaly, heterogeneous echotexture and abnormal portal vein waveform.

## Other Imaging Findings
There are no other imaging findings associated with sinusoidal obstruction syndrome.

## Other Diagnostic Studies
A transjugular liver biopsy can be helpful in the diagnosis of sinusoidal obstruction syndrome. Common findings on liver biopsy include sinusoidal dilation and congestion by erythrocytes and thrombi within the terminal hepatic venules.

# Treatment

## Medical Therapy
The management of sinusoidal obstruction syndrome depends on the severity of the disease. However, supportive care is the mainstay of therapy for mild and moderate sinusoidal obstruction syndrome. The severe form of sinusoidal obstruction syndrome needs defibrotide (thrombolytic agent) along with supportive care. Patients are advised to avoid supplements and medications that are linked to hepatic injury.

## Surgery
Surgical treatment of sinusoidal obstruction syndrome is reserved for patients who do not respond to supportive treatment or defibrotide. The surgical options include transjugular intrahepatic portosystemic shunt (TIPS) and liver transplantation.

## Prevention
Primary preventive measures of sinusoidal obstruction syndrome include minimizing risks related to the transplant process such as the source of the graft (allogeneic greater than autologous), choice of chemotherapy and use of antimicrobials. The exposure to the hepatotoxic agents should be minimized and preexisting liver diseases should be managed. The prophylaxis for graft vs host disease should be considered.