Can a shoulder muscle spasm cause hand numbness?

Yes: There are many causes for this...Most common is of a nerve being pinched in the neck that can lead to spasming of the shoulder region with numbness of the hand. This is known as a cervical radiculopathy. There are other possibilities including an injury to the brachial plexus or carpal tunnel syndrome. You should see either a physiatrist or neurologist for a full exam.

Thoracic outlet: The muscle spasm can cause compression or inflammation of the brachial plexus resulting in hand numbness. Occurs most commonly in ulnar nerve distribution.

Myofascial Pain: Muscle knots and tight bands from overuse or trauma can cause numbness in classic areas 'downstream' from the problem. A massage, physical medicine, or physical therapy evaluation can help confirm if it is a muscle problem. The test for nerve damage causing numbness is called emg, this is performed by a specialist physician. Seek help if the problem is constant.