Nausea and vomiting are common symptoms experienced by people living with cancer. While many associate these symptoms primarily with treatment like chemotherapy, the cancer itself can directly cause significant distress. The mechanisms are complex, involving biochemical signals released by the tumor, physical obstruction, systemic chemical imbalances, and pressure effects on the nervous system. Understanding these distinct causes is important for effective management.
Tumor-Driven Biochemical Signaling
The presence of a tumor can trigger a systemic inflammatory response, which is a non-mechanical cause of nausea. Cancer cells and the body’s immune cells release chemical messengers known as inflammatory mediators, such as cytokines like Interleukin-1 (IL-1), IL-6, and Tumor Necrosis Factor-alpha (TNF-a). These circulating substances travel through the bloodstream. They eventually reach a specialized area in the brainstem called the Chemoreceptor Trigger Zone (CTZ), or Area Postrema. The CTZ monitors the blood for toxins and chemical irritants. It is located outside the protective blood-brain barrier, allowing chemicals like cytokines to directly stimulate its receptors. When activated by these cancer-related biochemical signals, the CTZ sends signals to the nearby Vomiting Center, which coordinates the physical act of emesis. This mechanism is a component of the paraneoplastic effects of malignancy.
Physical Compression and Obstruction
Tumor growth causes nausea through mechanical forces, depending heavily on the cancer’s location. When a tumor develops near the gastrointestinal tract (e.g., stomach, pancreas, or intestines), its size can physically narrow the passage for food and digestive fluids. This physical blockage prevents the normal movement of contents through the digestive system. The resulting backup causes the stomach and intestines to become distended, leading to painful fullness, bloating, and intense nausea. This pressure buildup mechanically irritates nerve endings in the gut wall, sending distress signals directly to the brain’s vomiting center. Tumors can also cause non-obstructive issues by pressing on important nearby nerves. Compression of the celiac plexus or the vagus nerve, which runs from the brainstem to the abdomen, disrupts normal digestive motility. This nerve interference slows the rate at which the stomach empties, a condition known as gastroparesis. Gastroparesis leads to chronic feelings of nausea and satiety after only a few bites of food.
Metabolic and Electrolyte Imbalances
Cancer disrupts the body’s internal chemical environment, leading to systemic imbalances that trigger the CTZ. The most common paraneoplastic metabolic imbalance is hypercalcemia, an abnormally high level of calcium in the blood. This often results from tumors producing parathyroid hormone-related protein (PTHrP) or from extensive bone destruction caused by metastatic disease. Excess calcium acts as a toxin, directly stimulating the CTZ in the brainstem. High calcium levels also negatively affect the smooth muscle of the digestive tract, slowing gut movement and contributing to constipation and nausea. Systemic organ dysfunction caused by malignancy also introduces toxins that trigger nausea. Advanced liver cancer or widespread liver metastases can cause hepatic encephalopathy, where the liver fails to clear ammonia and other waste products. Similarly, cancer-related kidney failure, known as uremia, causes a buildup of nitrogenous waste. These accumulated toxins circulate in the blood and activate the CTZ.
Indirect Effects of Pain and Neurological Pressure
Nausea can be a secondary reaction to other severe symptoms, particularly uncontrolled pain. Chronic severe pain, regardless of its source—whether from a tumor pressing on a bone or from nerve infiltration—stimulates the central nervous system. This widespread activation lowers the threshold for the vomiting reflex, increasing the likelihood of nausea. A distinct neurological cause occurs when cancer affects the brain, either as a primary tumor or metastatic lesion. Tumors growing within the confined space of the skull increase the pressure inside the head, known as intracranial pressure (ICP). The rigid structure of the skull means that any added volume compresses sensitive brain tissue. This increased pressure directly affects the brainstem, where the vomiting center is located, causing its involuntary activation. Nausea from high ICP is often accompanied by a headache that is typically worse in the morning and can lead to sudden, sometimes projectile, vomiting.