Cervical Spondylosis or Cervical Disc Herniation: This condition can cause neck pain radiating to the arm due to nerve compression. The sinus bradycardia could be coincidental or related to pain-induced vagal response. This diagnosis is common in young adults and can explain both the neck pain and the radiation to the arm.

Thoracic Outlet Syndrome (TOS): TOS can cause neck and arm pain due to compression of the nerves and/or blood vessels that pass through the thoracic outlet. While it might not directly cause bradycardia, the pain and stress from TOS could potentially contribute to it.
Musculoskeletal Strain: Muscle strain in the neck or shoulder region can cause pain radiating to the arm. The bradycardia might be unrelated or could be a response to pain or stress.
Referred Pain from Thoracic or Abdominal Pathology: Sometimes, pain from internal organs can be referred to the neck and arm. Conditions like pneumonia, myocardial infarction (less common in this age group), or gastrointestinal issues could potentially cause such pain, though they might not directly explain the bradycardia.

Myocardial Infarction (MI): Although less common in young adults, MI can present with atypical symptoms, including neck and arm pain. The bradycardia could be a sign of an inferior wall MI affecting the heart's conduction system.
Aortic Dissection: This is a life-threatening condition where the inner layer of the aorta tears. It can cause severe, tearing chest pain that radiates to the back or arm and can lead to bradycardia due to involvement of the aortic root or compression of adjacent structures.
Pulmonary Embolism (PE): PE can cause sudden onset of chest pain and shortness of breath. While less typical, arm pain and bradycardia can occur, especially if there's associated hypoxia or stress response.

Pancoast Tumor: A type of lung cancer located at the apex of the lung, which can cause shoulder and arm pain due to invasion of the brachial plexus. Bradycardia could be due to local effects on the stellate ganglion or vagus nerve.
Cervical Spine Infection or Inflammation: Conditions like discitis, osteomyelitis, or spinal epidural abscess can cause neck pain and, if severe, could lead to systemic symptoms including bradycardia due to pain or inflammation affecting the spinal cord.
Neurological Conditions: Such as multiple sclerosis, which could cause a wide range of neurological symptoms including pain and bradycardia, though this would be less common and typically part of a broader clinical picture.