Scalenus syndrome is a medical condition caused by the compression of nerves in the neck, leading to pain, numbness, and weakness in the shoulders, arms and hands. It is also commonly known as Thoracic Outlet Syndrome (TOS) or Cervical Rib Syndrome. TOS arises from the compression of the neurovascular bundle of the brachial plexus. Other synonyms for Scalenus Syndrome include Adson's Syndrome, Costoclavicular Syndrome and Halstead's Maneuver Syndrome. These conditions occur due to the compression of nerves and blood vessels that run between the collarbone and first rib. Common causes of Scalenus Syndrome include repetitive strain, poor posture, and trauma. If left untreated, the condition can lead to debilitating pain and permanent nerve damage.