My little brother, on a recent Shabbos, had a sharp metal ruler penetrate his eye. He couldn’t open his eye and was in serious need for medical attention. Assuming, that he would have lost function in that eye if he didn’t see a doctor soon, would my father have been allowed to violate an issur deoraisa by driving to the hospital. I ask, because if it’s a matter of pikuach nefesh, it would definitely be permissible. However, i learnt that if he’s a choleh (which i would assume he is), even if he risks losing a limb, he would only be allowed to brake derabanin decrees. So would my father be able to drive my brother to the hospital in this situation?

The gemara states that when the eye is in danger it also involves a danger to one's life, and therefore it is considered as pikuach nefesh, and one breaks Shabbat for it. This is ruled in the Shulchan Aruch (Orach Haim, 328,9). So, in your case when it is not clear exactly what damage the ruler has done, one should certainly break Shabbat to get all the medical treatment needed. A sharp metal object penetrating the eye could involve many different life-threatening situations, and one cannot assume that the damage is limited exclusively to the vision of the eye. An interesting question would be if you knew for certain that there was no threat to his life (a point I stress should only be ascertained by a qualified doctor), but there exists a real possibility of losing the eye – may Shabbat be violated in such a situation? (This can occur when the retina separates, for example). This is a point of argument between the latter-day rabbis (see Nishmat Avraham, 328 (27)) – many of them making a distinction between one eye and two. If we are to understand that there is absolutely no threat to one's life (which depends on how to interperate the gemara we quoted above, and also how to rule halacha in light of modern medical knowledge), then perhaps this would be treated as one treats the laws of a threat to a limb where there is no threat to one's life. [I stress again, that in nearly all situations where there exists a real threat of losing the use of a limb, there is almost always at least a doubt that there could be a threat to one's life, and one should break Shabbat if needed to get professional medical care]. Even in such a case, where there is no danger to life, one should enlist the help of a non-Jew to drive the patient to a hospital. All desecrations of Shabbat that are rabbinic may also be performed in order to help the patient – this includes using a telephone to call a doctor etc. Even Torah prohibitions may be broken if necessary by using a "shinui" (a variation) such as the back of one's hand etc. So, we see that in your case, where there was a chance (even if slight) that there might be danger to life, the correct thing to do would be to take all measures to ensure that medical care was attained as soon as possible – even if this meant a Jew driving on Shabbat. In the theoretical situation where no threat to life exists, but one may lose one's vision, some rabbis say this is equivalent to a life-threatening situation, while others say it is not. Even those stricter opinions though agree that one must seek medical care through the use of a non-Jew, or breaking Torah law with a variation, or breaking Rabbinic law directly. Again – blessings to you and your brother.