Unilateral arm weakness, or asymmetric strength, is a noticeable disparity in power between the left and right arms. While a minor difference is common and benign, a new or worsening loss of strength in one arm can signal an underlying medical issue. This symptom makes tasks like lifting, gripping, or opening jars difficult, indicating communication has been disrupted along the pathway from the brain to the muscle. Determining the cause requires identifying where the signal transmission is being blocked or damaged. The location of this disruption—in the brain, spinal cord, or the arm itself—guides the necessary medical investigation.
Understanding Normal Strength Asymmetry
A slight difference in arm strength is a typical feature of human physiology related to handedness. Favoring one arm for daily tasks, such as writing or lifting, leads to greater development and neural efficiency in that limb. This preferential use results in the dominant arm being measurably stronger than the non-dominant arm. Studies often report the dominant hand is approximately 8% to 10% stronger than the non-dominant hand in the general population.
For individuals engaged in sports requiring asymmetrical movement, like tennis or baseball, this difference can be even more pronounced. This asymmetry is functional, meaning it is an expected adaptation to usage rather than a sign of disease. The asymmetry becomes a medical concern only when the strength difference increases suddenly, or when the weaker arm struggles with routine activities.
Localized Musculoskeletal and Nerve Impingement Causes
When weakness is confined to one arm and lacks widespread symptoms, the cause often lies in the local structures of the arm, shoulder, or neck. Injuries like a rotator cuff tear can cause profound weakness because pain inhibits muscle contraction, or the tear compromises the muscle’s ability to pull on the bone. Repetitive strain or acute trauma to muscles or tendons can also lead to localized weakness, sometimes referred to as pain-related inhibition.
Persistent localized weakness frequently results from peripheral nerve compression, where a nerve is physically pinched outside of the spinal cord or brain. Cervical radiculopathy, or a “pinched nerve” in the neck, occurs when a herniated disc or bone spur compresses a nerve root exiting the spine, causing weakness in the muscles that nerve supplies. Peripheral nerve entrapment syndromes also affect specific nerves in the arm. Examples include Carpal Tunnel Syndrome (median nerve compression at the wrist) and Cubital Tunnel Syndrome (ulnar nerve compression at the elbow).
Central Neurological and Systemic Conditions
Weakness originating from the central nervous system (CNS)—the brain or spinal cord—is often more concerning and presents with distinctive patterns. A sudden onset of unilateral arm weakness, known as hemiparesis, is a hallmark symptom of a stroke, which occurs when blood flow to a part of the brain is blocked. Because motor control centers operate contralaterally, a stroke on the right side of the brain typically causes weakness on the left side of the body, often including the face and leg.
Other central conditions cause a more gradual progression of one-sided weakness. Multiple Sclerosis (MS) is an autoimmune disorder that attacks the myelin sheath, disrupting motor signal transmission in the brain and spinal cord. Motor Neuron Diseases, such as Amyotrophic Lateral Sclerosis (ALS), destroy nerve cells controlling voluntary muscles, often beginning with focal weakness in one arm before spreading. Systemic conditions, like peripheral neuropathy caused by diabetes, can also start with asymmetric symptoms before progressing to both sides.
When to Seek Medical Evaluation and Diagnostic Steps
Any sudden or rapidly progressive weakness in one arm requires immediate medical attention, especially when accompanied by other symptoms. Red flags that can indicate an acute stroke include:
- Facial drooping.
- Sudden difficulty speaking.
- Severe headache.
- Loss of balance.
Persistent weakness that worsens over time, or weakness associated with muscle wasting, should also prompt a prompt evaluation.
The diagnostic process begins with a detailed physical and neurological exam to assess muscle strength, reflexes, and sensation. This helps the clinician localize the problem to the muscle, nerve, spinal cord, or brain. If a peripheral nerve issue is suspected, Nerve Conduction Studies (NCS) and Electromyography (EMG) are often performed. NCS measure the speed of electrical signals through the nerves, while EMG assesses electrical activity within the muscles, helping distinguish between a nerve problem (neuropathy) and a muscle problem (myopathy).
Imaging studies are also used, such as Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans. These visualize the brain and spinal cord to check for central causes like stroke, MS lesions, or spinal cord compression.