If the vocal cord muscles are not exercised regularly by talking (imagine a lighthouse keeper sitting alone on an island year after year), the vocal cord muscles atrophy and can no longer tense to a straight line. They remain concave, even when the vocal processes are completely closed, sometimes the vocal processes override each other in an attempt at overclosure. Consequently, a central gap is created.
Aging contributes to this as well. Much as the skin on the face gradually sags with aging, the vocal cords sag with aging as they lose elasticity. Typically this sagging, or as physicians call it, bowing or presbyphonia, is relatively symmetric. There is a nearly oval shaped gap with pointed ends between the vocal cords. Air leaks out the middle.
An asymmetric central gap is created when tension within one vocal cord is reduced, allowing it to oscillate further lateral than the other side. It will also oscillate out of phase. This suggests a thryoarytenoid muscle paresis. (For further discussion about the nerves supplying the vocal cords, see “Neurolaryngology”). The vocal cord muscle on the side of the injured vocal cord may also atrophy, not be as tight nor thick as the other side, so at any given attempted pitch there is air leak in the central portion of the affected vocal cord, with more leak on the paralyzed side and more leak at lower pitch.
Symmetric central gap: At the beginning of sound production, the vocal processes have touched. The arrows point to a large central gap. He is phonating at pitch E4 which is about an octave higher than the typical male - he is tensioning the cricothyroid muscle to stretch the vocal cords as a means of compensation to reduce the width of the central gap.
Asymmetric central gap: The left vocal cord is not as tense as the right and oscillates further laterally (here at pitch F3#) allowing air leak from the left. She also cannot quite close the vocal processes at this pitch allowing some posterior air leak as well. This likely represents a left anterior branch of the recurrent laryngeal nerve paresis.