It is related to arthritis, chronic degeneration and may be congenital(inheritable).
Due to the narrowing of the spinal column, nerve root canal, and/or intervertebral foramina (spaces between vertebrae), this can compress nerve roots and spinal nerves leading to a variety of symptoms that vary on the location of the narrowing. Most commonly is the narrowing of the spinal column in the upper neck (cervical) though it is known to occur in the thoracic (chest) and lumbar (waist) areas. Symptoms include, lower back pain, pains in the lower extremities, neurogenic claudication(inflammation of spinal nerves), numbness, tingling, and weakness in limbs.
Longer bouts of exercise are likely to increase the symptoms, which can initially be somewhat alleviated for some by lying down and/or flexing forward. Eventually endurance exercise becomes almost impossible and pain relievers reduce in efficacy.
If compression worsens then spinal cord injury can occur, resulting in exacerbated symptoms such as loss of balance, chronic pain, loss of bladder function, and problems with fine motor skills.
As in the picture, the vertebrae on the LEFT, shows a healthy cervical(neck), with clean edges around the yellow colored spinal cord. Below it depicts a side view of a health spinal column.
On the RIGHT the picture shows a bony growth along the inner body (large round piece of the vertebrae that forms the actual ‘column’), while the actual spinal canal (large hole in the vertebrae) has decreased in size due to the abnormal bony growth. This compresses the spinal cord, and any nerve branching off from it, resulting in tingling, numbness, and pain. Below it depicts a side view of a spinal cord being compressed.
Note: In Dad’s particular case, there is Spinal Stenosis throughout his entire spinal column, though they are most concerned with the C4-C5 region (Cervical/neck), as it has the most growth and is affecting his quality of life the greatest.
Hope that helps explain what’s going on ‘scientifically’!
P.S. If the link at the top doesn’t work for the picture, it can be found here [http://spinalstenosis.org/blog/wp-content/uploads/2010/08/CERVICAL-SPINAL-STENOSIS1.jpg]