Nedorim 81a top
The ran learns that the gemoro asks according to reb yosi washing the body must be inuy nefesh because, we see that even washing clothes is chiyusa so obviously washing the body is inuy nefesh. And the gemoro answers that dirty clothes are worse than a dirty body. Because not washing clothes leads to madness.
But by that very same token, it would seem that unwashed hair which leads to blindness is the worst thing of all. So why isn't it inuy nefesh not to wash on the grounds that at least the hair is inuy nefesh and at the very least we should say neder shehutar miktzoso hutar kulo?
Avrumi Hersh, London england
(a) Rav Akiva Eiger (Gilyon ha'Shas on the spot) sends us to Berachos, where we find that Raban Gamliel washed during Aveilus because he suffered from being dirty. Rabbeinu Yona there says that the "Irbuvisah" that brings to blindness is caused by not washing the hair. If so, your question remains valid.
(b) However, the Ran says that the "Irbuvisah" is caused by "not combing". Apparently, according to the Ran (and perhaps due to Rabbi Hersh's question) the lice control is primarily due to a lack of combing, presumably even if the hair is relatively clean. Therefore, the severity of the potential blindness is germane only to combing and irrelevant to the Neder not to wash. If so, the principle of "Neder Shehutar Miktzoso Hutar Kulo" would not apply.
Shimon Brodie
(c) To answer your question according to Rabeinu Yonah - Rashi DH Ledai Shimomisa writes that Shimomisa, madness, is the worst of all. According to this blindness is not the worse thing of all; Shigaon is the worst. This can be understood according to the Gemara above (top of 41a) where Rav Nachman said that "lacking everything" means that one does not possess "De'ah"; he is without intelligence. An intelligent blind person is better than an unintelligent sighted person.
KOL TUV
Dovid Bloom