For most of us, a feeling of weakness in our left arm, making it heavy and sluggish, is cause for alarm -- public service announcements about left arm weakness being associated with stroke have been effective enough that at the first sign of numbness, panic sets in. But while stroke is a sudden medical emergency that needs immediate treatment, there are other causes of left arm weakness that develop gradually and can be mitigated with medication.
The more information you have on symptoms and potential causes of this sensation, the better equipped you'll be to handle it, should it arise. Start here, with expert-backed information on the most common reasons for left arm weakness and what to do about it.
Strokes, an interruption of blood and oxygen to the brain, typically cause symptoms on only one side of the body, depending on where in your brain it occurs. For instance, if a stroke happens on the right side of your brain, it will affect the left side of your body, including your arm, and vice versa. The symptoms of a stroke show up instantaneously: your arm feels weak and heavy as soon as it happens.
"You would all of a sudden feel the inability to do something with your right or left arm," says Salman Azhar, M.D., the director of the stroke program at Northwell Lenox Hill Hospital in New York City. "I may not be able to lift a glass of water, for example. The glass would fall through my hands."
Impairments like this happen when a stroke affects a certain area of your brain -- namely, your motor cortex. "If the motor cortex, which is responsible for voluntary movement, is impacted, weakness in the arm can manifest as a symptom of the stroke," says Jan Bittar, M.D., a neurologist at The Ohio State University Wexner Medical Center in Columbus, OH.
That said, arm weakness isn't the only indicator of a stroke -- they often have multiple symptoms. According to Dr. Bittar, those can include:
Not every stroke will present with these symptoms, and sometimes, "arm weakness can be the only sign of stroke," says neurologist Benedict Pereira, M.D., a stroke specialist at Cedars Sinai in Los Angeles.
So if you're only experiencing arm weakness, how can you know if it's a stroke or something else? Experts say it has to do with how quickly the sensation occurs. "What makes a case more likely to be a stroke, as opposed to other causes, is its sudden onset," says Dr. Pereira. "A stroke doesn't gradually start happening -- it happens instantly." And when it does, Dr. Bittar adds, you need to call 911.
"If you experience a sudden or acute onset of arm weakness or find it challenging to lift your arms, you need to seek prompt medical assistance," Dr. Bittar says.
To recognize the signs of a stroke, experts like Dr. Bittar use the F.A.S.T. acronym:
Time is critical, says Dr. Pereira, because one of the most effective stroke treatments, called TNKase (tenecteplase), must be administered within 4.5 hours of symptoms starting to limit the stroke's damage.
Not every case of arm weakness is caused by a stroke. Other causes range from muscular and nerve issues to autoimmune conditions. Unlike a stroke, these conditions usually develop over time, and the symptoms appear gradually versus all at once. Here's what else could be causing your arm weakness.
Pinched Nerve in the Neck (Cervical Radiculopathy)
Cervical radiculopathy, a painful condition caused by compression or inflammation of nerves in your neck, is one of the most common causes of left arm (or right arm) weakness, says Dr. Azhar. Usually due to aging or injury, cervical radiculopathy can trigger sudden loss of strength and numbness in part of your arm. It usually gets better on its own, according to the Cleveland Clinic. But if your doctor feels you do need treatment, it may include the following:
Peripheral Neuropathy
As the name suggests, this condition causes damage to your peripheral nerves, which includes any nerves not found in your brain or spinal cord. As those nerves deteriorate, muscle weakness can develop in your left arm and elsewhere in your body, and you also may feel numbness or tingling. The most common cause is undermanaged type 2 diabetes, which can lead to diabetic neuropathy. Circulation problems, infections, alcohol use disorder, and some autoimmune diseases also can trigger peripheral neuropathy, according to the Cleveland Clinic. In addition to addressing its cause, treatment for it may include:
Brachial Plexus Injury
"Brachial plexus injury typically results from trauma that disrupts the network of nerves responsible for transmitting signals from the upper spinal cord to the shoulder, arm, forearm, and hand," says Dr. Bittar. Car crashes, falls, and sports accidents are among the most common causes -- but this kind of arm weakness can also result from tumors, cancer treatment, and inflammation from an infection or autoimmune disease, says Dr. Azhar.
"With inflammation, you're going to get multiple nerves involved, which can lead to multiple parts of your arm being weak," he says. Milder injuries often heal slowly on their own, but your doctor may recommend physical therapy and avoiding strenuous activity with your affected arm. You may need surgery if your injury does not improve on its own, but that's by no means the only outcome.
Radial Nerve Palsy
Radial nerve palsy, also known as "Saturday night palsy," is an issue you can bring on yourself, experts say. "This can happen if you've had too much to drink and you fall asleep in a chair with your arm hanging over the back," says Dr. Azhar. "If you sleep like that for many hours, and you wake up with weakness of your arm, it's because you've caused a compression of the nerves feeding the hand and the arm."
While it seems like a casual mishap, this can cause permanent injury to the radial nerve. The arm weakness may be accompanied by wrist drop, in which your wrist becomes limp, making it difficult to lift objects. Treatment, if needed, often involves:
In short: "Avoid falling asleep on your arm," says Dr. Pereira.
Autoimmune Diseases
Inflammation caused by certain autoimmune diseases, such as Guillain-Barré syndrome (GBS), myasthenia gravis (MG), and multiple sclerosis (MS), can damage nerves and lead to left arm weakness, as well as weakness in other parts of your body.
In GBS, for example, the immune system attacks the peripheral nerves (reminder: these are the nerves not located in the brain or spinal cord). It usually affects both sides of your body, starting with your feet and arms, says Dr. Pereira. "Chances are you would know you have it before it reached your arms," he says.
Myasthenia gravis disrupts the signals from your nerves to your muscles, leading to arm weakness and weakness in your legs and neck. It also affects the muscles in your eyes, face, and throat, per the Mayo Clinic.
As for MS, the resulting inflammation damages myelin, a layer of cells that protects the nerves in your brain and spinal cord, comprising your central nervous system. That inflammation impacts different parts of the central nervous system, and if it damages a part of the brain that controls your arm muscles, it can cause arm weakness. According to the Mayo Clinic, that weakness usually develops on one side of the body, so it could just affect your left arm.
None of these autoimmune diseases have a cure, but medications can help manage them. For example, corticosteroids can tamp down your immune system and reduce inflammation in patients with MS and MG. GBS, which is rare, often requires hospitalization, according to the Cleveland Clinic. Most people make a full recovery, but according to Dr. Pereira, the disease can become lifelong, a condition called chronic inflammatory demyelinating polyneuropathy.
Rotator Cuff Injury
Another common cause of arm weakness is a rotator cuff injury, says Dr. Azhar. To lift your arm, you need a working rotator cuff, composed of muscles and tendons that hold your shoulder in place. Ergo, if you damage it through a fall or a sports injury, your arm will become weak as a result. Sometimes, though, such injuries come with age. "As we get older, we start to have more wear and tear in our shoulders," says Dr. Azhar. "And that can definitely lead to an issue with lifting up your arm and arm weakness." Treatment for rotator cuff injuries generally includes:
Your doctor may also give you an injection of corticosteroids in your shoulder if other approaches don't ease your pain.
Although not as likely as other causes, Dr. Bittar adds the following conditions to the list of possible culprits causing left arm weakness. Each of these conditions comes with other symptoms, too, so as you're researching, make sure to do a crosscheck.
Certain types of medication also can cause muscle weakness, including in the arms. Dr. Bittar lists the following:
Talk with your doctor about managing the side effects of the medication you're taking.
As you can probably tell by the plethora of possible conditions, arm weakness is a common symptom in many patients. "Arm weakness and heaviness is a common chief complaint," says Dr. Bittar. "Left arm weakness has different causes ranging from minor injuries to serious medical conditions. We ask patients to seek immediate medical attention if they experience a sudden or severe onset of left arm weakness or paralysis."
If you experience confusion, difficulty finding words or slurred speech, dizziness, facial droop, imbalance or loss of coordination, or visual disturbances, you should act with particular urgency, experts say. The same goes for you have severe pain, numbness, or symptoms of stroke. Otherwise, make the earliest available primary care doctor appointment, in-person or virtually -- whichever gets you seen the soonest.