Fluid-filled cavity at the center of the spinal cord is seen in:

In syringomyelia, a cystic cavity forms within the spinal cord. This cyst, called a syrinx, can expand and elongate over time, destroying the spinal cord. Typically seen in cervical areas, there is dissociative anaesthesia (pain and  temperature ie spinothalamic sensations are lost, while posterior coloumn sensation ie proprioception and vibration are preserved) in a cape shaped distribution. Brown sequard syndrome is seen in hemisection of the cord . Tabes dorsalis is syphilitic damage to cord. SACD occurs due to Vitamin B12 deficiency.