Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine and obtain a history, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
Unilateral arm weakness may be due to a problem in the brain or peripheral nerves. Many possibilities exist, and depend somewhat on your age, your risk factors, and the nature of your symptoms (whether or not the weaknes started suddenly or gradually, whether it is a complete loss of motion, whether it is an objective finding on examination or it is only a feeling that you have etc).
Several concerning possibilities for unilateral weakness are stroke, MS, etc, but many times these may be associated with other clinical findings. Other possibilities include a peripheral nerve injury/neuropathy, plexopathy, or radiculopathy.
It is difficult to provide more objective information without knowing more about the location of the weakness, if other symptoms are present, etc. I would suggest you be evaluated by a neurologist. You may need an MRI of the brain (depending on the history and examination) and/or EMG/NCS (depending on the history and examination).
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.
To Dr. Newey
Thank you very much for a quick response. And I'm sorry for not providing enough information.
The weakest part of my left arm is deltoid, and a little of triceps. I have no problem raising my arm because it has no pain, but it gets tired so quickly.
And these are the other symptoms;
* Slight pain in my left wrist when I bend it. It used to bother me before, but deltoid weakness bothers me more now.
* My neck & shoulder have been stiff lately. I feel very good when I get a massage at Acupuncture clinic.
* Since I've been using my right arm for most of the regular work lately (I usually use left hand even though I'm not lefty), right arm gets a little tired (slightly numb).
But these are not the major problems for me. And when I'm not moving my arm, nothing bothers me. When I'm lying on the bed or sitting on the couch watching TV, I feel like a healthy person.
Anyway, I guess you still suggest I see a neurologist, but I just thought I'd add more information.
Thank you very much.