Before considering the Samprapti of Netraroga, I would like to highlight the following important points.
Importance of equilibrium of Dosha-
The homeostatic condition of Dosha, Dhatu, and Mala is essential for maintaining the normal functioning of various Dnyanendriya, Karmendriya, Mann (mind) and Atma (soul). Their deviation either from physiology or towards pathology triggers genesis of disease (A.H.Su.1-20).
Six stages of Samprapti / Pathogenesis-
According to Sushruta( S. Su. 21-36), 6 stages are necessary for generating disease of a particular structure. They are Chaya (accumulation), Prakopa (excitation), Prasara (spread), Sthanasanshraya (localization), Vyakta and Bheda (complete manifestation of disease).
Importance of Srotovaigunya i.e. Abnormality-
The spreading vitiated Dosha get localized / settled / lodged wherever Kavaigunya / Srotovaigunya i.e. abnormality is present (S. Su. 24-10).
General Samprapti of Netraroga-
Dosha vitiated by various etiological factors; especially the ones specifically acting on eyes (caused by unwholesome diet and injudicious activities); when passes upwards through sira; occupy different structures like Vartma, Sandhi, Shukla, Krushna, Drushti and Sarva / all parts of eye; and then precipitate diseases of that particular structure only if Srotovaigunya is present there (S.U. 1-20, 21 and A.H.U. 8-1, 2; A.H.Ni.1-12).
In very few diseases Sushruta described some specific Samprapti e.g. Srava, Savrana Shukra, Ajakajata, Timira, etc. Timira, Kacha, and Linganasha are 3 main diseases of Drushtigata Roga causing partial or total blindness if not treated properly and promptly. Hence their specific Samprapti is described here.
Specific Samprapti / Pathogenesis of Timira / Kacha / Linganasha
Dosha, vitiated by various etiological factors / specifically some Timira generating dosha (Dalhana), when enters the Eye through sira and get lodged in the First Patala i.e. Tejojalashrita (due to pre-existing Srotovaigunya); generate pathology there i.e. Timira. If neglected / untreated, these Dosha become more and more intensified / aggravated and invade 2nd, 3rd, and 4th Patala and precipitate Kacha and Linganasha. The mild blurring of vision in the affliction of 1st Patala gradually increases leading to the visual loss in the 4th Patala.
Thus derangement of the environment in the vicinity of Tejojala i.e. aqueous humor is responsible for the development of Timira / Kacha / Linganasha. If the environment remains within the physiological limits, vitiated dosha shall not be able to get settled in the 1st Patala.