Arm weakness, or paresis, is a reduction in muscle strength that makes it difficult to move the arm or perform daily tasks. This symptom indicates a disruption in the complex communication pathway between the brain, spinal cord, nerves, and muscles. Causes can range from temporary muscle fatigue and localized injuries to progressive neurological diseases or acute, life-threatening events. Understanding the source of the weakness is the first step, as the location of the problem—from the arm itself to the central nervous system—determines the necessary course of action.
Localized Musculoskeletal Causes
Weakness originating in the arm’s muscles, tendons, or joints is often the result of mechanical stress or trauma. Temporary weakness is commonly caused by muscle fatigue or acute strain following strenuous activity. This type of weakness is localized and resolves quickly as the muscle tissue recovers from energy depletion.
More lasting weakness can be a consequence of inflammation in joint structures, such as tendinitis or bursitis. Tendinitis, like that affecting the biceps tendon, involves irritation and swelling that inhibits strength. Similarly, shoulder bursitis, the inflammation of fluid-filled sacs, creates pain that limits movement, leading to functional weakness.
Structural damage within the shoulder joint is another frequent cause, particularly with overhead movements. Rotator cuff injuries can range from inflammation to partial or complete tears. A tear structurally impairs the muscle’s ability to lift or rotate the arm, resulting in a loss of power and restricted range of motion. Acute trauma, such as a sprain or dislocation, can also cause immediate weakness due to structural instability or severe pain.
Weakness Stemming from Peripheral Nerve Compression
When weakness is accompanied by tingling, numbness, or radiating pain, the cause often lies in the compression or damage of peripheral nerves. A common origin point is the neck, known as cervical radiculopathy, where bone spurs or a herniated disc press on a nerve root as it exits the spinal column. The specific nerve root affected dictates the pattern of weakness, which can manifest anywhere from the shoulder down to the fingers.
Further down the arm, nerves are vulnerable to entrapment syndromes, where they are compressed by surrounding tissues. Carpal Tunnel Syndrome (CTS) involves the median nerve being squeezed in the wrist. This causes numbness in the thumb, index, and middle fingers, eventually leading to weakness in the small muscles at the base of the thumb, making fine motor tasks difficult.
Another frequent entrapment is Cubital Tunnel Syndrome, where the ulnar nerve is compressed, often at the elbow. This results in weakness or a claw-like deformity in the hand’s intrinsic muscles, along with numbness in the ring and little fingers. Radial nerve palsy, sometimes called “Saturday night palsy,” involves pressure on the radial nerve, leading to a characteristic “wrist drop” because the muscles responsible for extending the wrist and fingers are paralyzed.
Systemic Conditions and Central Nervous System Involvement
Arm weakness involving the central nervous system (brain and spinal cord) or systemic illness includes acute events like stroke. A Cerebrovascular Accident, or stroke, is a sudden, life-threatening event caused by a blockage or rupture of a blood vessel in the brain. If the stroke affects the motor cortex, it can instantly cause weakness or paralysis, known as hemiparesis, which typically affects one side of the body, including the arm.
Chronic, progressive conditions also target the nervous system. Multiple Sclerosis (MS) is an autoimmune disorder where the immune system attacks the myelin sheath protecting nerve fibers in the brain and spinal cord. Depending on the location of the resulting lesions, MS can disrupt nerve signals that control arm movement, leading to intermittent episodes or a gradual increase in muscle weakness. Guillain-Barré Syndrome is another autoimmune condition, involving the immune system attacking the peripheral nerves. This often begins with rapid, ascending weakness that starts in the legs and progresses to the arms over hours or days.
Metabolic and nutritional factors can also contribute to generalized muscle weakness. A severe deficiency in Vitamin B12, essential for maintaining the myelin sheath, can lead to nerve damage, such as peripheral neuropathy or subacute combined degeneration of the spinal cord. Lack of B12 causes nerve signaling to slow down, manifesting as numbness, tingling, and motor weakness. Endocrine disorders and chronic infections can also lead to fatigue or muscle wasting that results in arm weakness.
When to Seek Urgent Medical Attention
The sudden onset of arm weakness, particularly if it affects one side of the body, requires immediate emergency medical attention. This is a potential sign of a stroke, especially when accompanied by other neurological symptoms like facial drooping, slurred speech, confusion, or sudden vision changes. Rapid assessment is essential for stroke treatment and can significantly improve outcomes.
Any weakness that rapidly progresses over hours or days, especially if it starts in the legs and spreads to the arms, suggests serious conditions like Guillain-Barré Syndrome. Immediate evaluation is also necessary if arm weakness follows significant trauma, such as a car accident or fall, or if it is accompanied by a high fever, severe headache, or loss of consciousness. For chronic, unexplained, or progressive arm weakness that interferes with daily life, consult a primary care physician promptly for diagnostic testing.