Botulinum toxin type A or Botox is a nerve blocker used to paralyze muscles in spasm. It's been used to treat a wide variety of problems. These include muscle spasms from brain injury, multiple sclerosis, or stroke. There has been some success using it for spasticity in children with cerebral palsy. You may be more familiar with its recent use for deep facial lines and wrinkles.
Spasms of the hand, including writer's cramp and musician's cramp have been treated with Botox. It has been used in small studies for tennis elbow. The main (intended) side effect in the treatment of muscle spasm is muscle paralysis. Finger drop also known as extensor lag can occur.
The affected individual is unable to extend the finger(s). In other words, the finger(s) stays in a flexed or bent position. The effect is often temporary lasting only three to four months at the most. An occupational therapist can help you with a splint. The splint holds your finger in the right position to use it. A special splint may be needed that will allow you to use your guitar. Take your guitar with you to the appointment.