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Hello from JustAnswer.
Does the left sided weakness also affect the left leg?
Does she have times when the strength in the left arm is completely normal between periods of weakness?
Thank you for the additional information.
If the leg is also affected, that is much stronger evidence that the underlying problem is within the brain or spine, and since her pain is in the neck, the problem is most likely in the neck. The arm weakness is also frequently from cervical spine disorders, but could also occur from disease or pressure on the nerves after the nerves exit the spine. However, any condition that affects the nerves after they exit the spine would not cause leg weakness, but a problem in the cervical spine can also affect the spinal cord fibers that will ultimately affect the leg.
The most common problems in the cervical spine are degenerative changes, but it also could be related to inflammatory or infectious conditions or anatomic growths in the cervical spine.
The MRI of the cervical spine would usually identify the problem that is causing such symptoms, but since the MRI of the cervical spine was negative, then the lumbar puncture would be an appropriate next step. Occasionally, a myelogram may also need to be done.
Is there any other information that would be helpful?
Any time that a needle is placed into the body, there is a risk of bleeding or infection, but the risk of bleeding or infection due to a lumbar puncture is quite low. The most common adverse outcome from a lumbar puncture would be a severe headache, which will resolve with time.
If the lumbar puncture is negative, the next step to consider would be a myelogram, usually done in conjunction with CT imaging. There also are certain tests that assess the electrical function of the spinal cord, such as somatosensory evoked potentials or cortical motor evoked potentials.
That option would be OK if the lumbar puncture is negative, since it will have excluded an infectious cause of the symptoms.
Please let me know of any additional information would be helpful.