Arm weakness, defined as a loss of strength or power, is a common symptom ranging from minor, temporary issues to signs of serious underlying medical conditions. The medical term for partial or mild weakness is paresis, which is distinct from complete paralysis. Weakness can affect one arm or both, often accompanied by sensations like numbness or tingling (paresthesia). Identifying the origin of this symptom is necessary to determine the appropriate medical response.
Temporary Weakness from Overuse or Injury
The most frequent reasons for arm weakness are temporary and related to the muscles or the body’s overall state. Muscle fatigue follows strenuous exercise, causing a transient inability to generate normal force due to the depletion of energy stores and the build-up of metabolic byproducts. This weakness typically affects both arms and resolves completely with rest.
Minor musculoskeletal issues, such as a muscle strain or tendonitis, also lead to perceived weakness. The pain and inflammation from these injuries trigger a protective mechanism that limits movement and strength. Systemic factors like dehydration or low blood sugar (hypoglycemia) can induce generalized weakness and shakiness, affecting the arms along with the rest of the body. Hypoglycemia starves the muscles and nerves of necessary fuel, leading to weakness and poor coordination.
Weakness Caused by Nerve Compression
When arm weakness is localized and accompanied by sensory changes, the cause is often mechanical compression of a nerve pathway. Since the nerves controlling arm movement originate in the neck, the cervical spine is a common source of trouble. Cervical radiculopathy, or a “pinched nerve,” occurs when a nerve root is compressed as it exits the spinal cord, often due to a herniated disc or bone spurs. This compression causes pain that radiates down the arm, followed by numbness, tingling, and weakness in the specific muscle groups supplied by that nerve root.
Compression can also occur further down the arm, a condition known as peripheral nerve entrapment. Two common examples are Carpal Tunnel Syndrome (CTS) and Cubital Tunnel Syndrome (CuTS). CTS involves the median nerve compressed at the wrist, causing numbness and weakness primarily in the thumb, index, and middle fingers. CuTS is the compression of the ulnar nerve at the elbow, resulting in numbness and weakness in the fourth and fifth fingers and the small muscles within the hand.
Systemic Diseases Affecting Arm Strength
Arm weakness can also manifest as a systemic disease, affecting the body generally rather than being confined to a single nerve pathway. A stroke is an urgent cause, resulting from a blockage or rupture of a blood vessel in the brain. This event causes sudden, usually unilateral weakness (hemiparesis) affecting the arm, leg, and face on one side of the body.
Certain autoimmune disorders attack the nervous system or the connection between nerves and muscles, leading to fluctuating weakness. Myasthenia Gravis (MG) involves antibodies blocking receptors at the neuromuscular junction, preventing effective muscle contraction. This results in weakness that worsens with sustained activity and improves after rest. Multiple Sclerosis (MS) is an autoimmune condition where the body attacks the myelin sheath of nerve fibers, disrupting signal transmission and causing episodic or progressive arm weakness.
Generalized peripheral neuropathy, often linked to long-term conditions like diabetes, involves damage to peripheral nerves throughout the body. This condition causes a gradual, symmetric loss of strength and sensation, typically starting in the hands and feet.
Recognizing Red Flags and Seeking Diagnosis
While many causes of arm weakness are benign, certain associated symptoms indicate a medical emergency requiring immediate attention. Sudden onset of weakness in one arm, especially when accompanied by facial drooping, slurred speech, or a severe headache, signals a possible stroke. Other red flags include rapidly progressing weakness affecting both arms, difficulty breathing, or a change in consciousness.
A thorough medical evaluation is necessary to identify the cause of persistent arm weakness. The diagnostic process begins with a detailed medical history and a physical examination to assess muscle strength, reflexes, and sensation. To pinpoint the issue, a doctor may order diagnostic tests. These tests often include magnetic resonance imaging (MRI) or X-rays to visualize the spine and soft tissues, or nerve conduction studies and electromyography (EMG) to measure the electrical activity of the nerves and muscles.