Thank you for posting your query.
The commonest cause of head shaking is essential tremors.
If you can upload a small video of the head shaking, we can confirm the diagnosis.
There are medications available, if your head shaking affects your daily life.
Twitching of the left thumb is not related to head shaking. It is possibly due to more strain/fatigue
Please get back if you require any additional information.
Dr Sudhir Kumar MD (Internal Medicine), DM (Neurology)
Senior Consultant Neurologist
Apollo Hospitals, Hyderabad,
My personal URL on this website: http://bit.ly/Dr-Sudhir-kumar
My email: firstname.lastname@example.org