Leg weakness is a common and concerning symptom for individuals with cancer, significantly impacting daily life. It can stem from various mechanisms, reflecting the complex ways cancer affects the body. Understanding these causes helps patients and caregivers navigate the condition.
Direct Tumor-Related Causes
A tumor’s physical presence can cause leg weakness. Spinal cord compression occurs when a tumor, primary or metastatic, presses on the spinal cord or nerve roots. Cancers such as lung, breast, prostate, kidney, or multiple myeloma frequently spread to the spine. Symptoms can include back pain, numbness, or paralysis in the legs. Prompt medical attention is important to prevent permanent damage.
Tumors in the pelvis or abdomen can also press on major nerves supplying the legs, such as the sciatic or femoral nerves. This nerve compression can result in localized weakness, pain, or altered sensation in the affected leg. Less commonly, tumors can originate within the muscles or peripheral nerves of the legs, impairing their function. Soft tissue sarcomas, for example, can arise in muscles, fibrous tissue, or nerves in the legs, leading to a lump, pain, or weakness as they grow.
Cancer that has spread to the bones, known as bone metastases, can also cause leg weakness. These metastatic lesions weaken the bone structure, increasing the risk of pain and fractures. When bones like the femur or hip bones are affected, bearing weight becomes difficult, leading to weakness and reduced mobility.
Systemic Effects and Paraneoplastic Syndromes
Beyond direct physical pressure, cancer can cause leg weakness through its systemic effects. Advanced cancer can lead to cancer cachexia, a severe wasting syndrome characterized by significant muscle loss and weight reduction. This systemic process contributes to generalized weakness, often particularly noticeable in the legs, hindering mobility.
Anemia, a low red blood cell count, is another common systemic effect of cancer or its treatments. Red blood cells deliver oxygen to muscles and tissues. When their numbers are insufficient, oxygen delivery is reduced, leading to fatigue and generalized weakness, including diminished leg strength.
Cancer-related fatigue is a pervasive and debilitating tiredness not relieved by rest. This profound fatigue can significantly impact overall energy levels, making the legs feel heavy and difficult to move, affecting strength and mobility. It is a common symptom, affecting 70% to 100% of people with cancer.
Electrolyte imbalances, such as abnormal potassium or calcium levels, can also arise from cancer or its complications. These imbalances can disrupt normal nerve and muscle function, contributing to muscle weakness.
Paraneoplastic syndromes are rare conditions where the immune system, triggered by cancer, mistakenly attacks healthy nerve or muscle cells. This immune response can lead to various neurological symptoms, including leg weakness. Examples include Lambert-Eaton myasthenic syndrome, which affects communication between nerves and muscles, and paraneoplastic neuropathies, both of which can cause muscle weakness.
Treatment-Related Weakness
Cancer treatments can induce leg weakness as a side effect. Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of certain chemotherapy drugs, which can damage peripheral nerves. This damage often manifests as numbness, tingling, pain, and weakness, particularly in the feet and legs, and can persist for weeks to years after treatment.
Radiation therapy, when directed at areas like the spine or pelvis, can also damage nerves or muscles. This damage may lead to weakness or motor deficits that can develop over time, sometimes months or even years after treatment completion.
Surgical interventions for tumors in the spine, pelvis, or legs can temporarily or, in some cases, permanently affect nerves or muscles. This can result in post-surgical weakness as the body recovers or if nerve damage occurs.
Long-term use of corticosteroids, often used in cancer treatment, can cause steroid myopathy. This condition involves muscle weakness, typically more pronounced in the proximal muscles like those in the thighs. The severity of this weakness can be related to the cumulative dose of the steroid.
When to Consult a Doctor
Any new, sudden, or rapidly worsening leg weakness should prompt immediate medical evaluation for individuals with a cancer diagnosis or history. Prompt assessment is important because certain causes, like spinal cord compression, are medical emergencies requiring intervention to prevent permanent neurological damage.
Other concerning symptoms that warrant immediate medical attention include new or worsening numbness, tingling, or loss of sensation in the legs. Difficulty walking, changes in gait, or balance issues are also significant. Loss of bladder or bowel control, or severe back pain, especially if it worsens when lying flat or at night, are red flags.
Early diagnosis is important to identify the cause of leg weakness and initiate appropriate management strategies. While not all leg weakness indicates cancer progression, a thorough investigation can help improve outcomes and prevent further complications.