Sinusoidal Obstruction Syndrome (SOS), previously known as veno-occlusive disease (VOD), is a rare and potentially fatal liver injury. It typically arises from exposure to certain drugs or toxins. The condition involves damage to the liver’s small blood vessels, leading to impaired blood flow.
Primary Medical Causes
High-dose chemotherapy regimens are a frequent cause of SOS, particularly those used as conditioning treatments before hematopoietic stem cell transplantation (HSCT). SOS can occur in approximately 5% of HSCT recipients.
Among the chemotherapeutic agents commonly implicated, busulfan and cyclophosphamide are notable. Busulfan, when used in conditioning regimens, has been strongly associated with SOS-related death. Cyclophosphamide, another alkylating agent, is also a known contributor to SOS, especially in combination regimens.
Gemtuzumab ozogamicin, a targeted chemotherapy drug, has also been linked to SOS. The incidence of SOS with gemtuzumab ozogamicin can range from 3% when used alone to 15-28% when combined with other agents or if an HSCT is performed within three months of administration.
Radiation therapy, specifically total body irradiation (TBI), is another factor that can contribute to SOS. TBI is often part of the conditioning regimen before HSCT. While improvements in TBI delivery and supportive care have reduced its incidence, SOS can still occur as a subacute toxicity following TBI.
Other Contributing Factors
Beyond high-dose chemotherapy and radiation, other factors increase the risk of SOS. Certain conventional medications have been identified as potential causes, with oxaliplatin being a frequently reported cause of SOS. Methotrexate and carboplatin have also been linked to SOS.
Herbal remedies pose a risk, particularly those containing pyrrolizidine alkaloids (PAs). These compounds are found in various plants, such as Gynura segetum, and have been a major cause of SOS, especially in some Eastern countries, often due to consumption of contaminated food or misuse of herbs for medicinal purposes.
Pre-existing liver conditions or genetic predispositions also increase SOS susceptibility. Underlying liver dysfunction can reduce the liver’s ability to withstand the toxic effects of drugs or radiation. While specific genetic predispositions are still being researched, they are thought to influence an individual’s vulnerability to this condition.
How These Factors Lead to SOS
SOS involves damage to the endothelial cells that line the liver’s sinusoids, which are small, specialized blood vessels. When exposed to damaging agents like certain chemotherapies, radiation, or pyrrolizidine alkaloids, these endothelial cells become injured.
This injury causes the endothelial cells to swell and detach from the sinusoid walls, creating gaps in the sinusoidal barrier. Cellular debris then penetrate into the space surrounding the liver cells, leading to further detachment of the endothelial lining.
The detached endothelial cells, along with other cellular material, can then embolize or lodge downstream in the sinusoids. This creates blockages and narrows the vessels, impeding the normal flow of blood through the liver. The reduced blood flow leads to congestion and oxygen deprivation in liver cells, causing damage and the characteristic symptoms of SOS, such as liver enlargement, fluid retention, and jaundice.