Thank you for the photos and Question. From your photos I would suggest a posterior approach. You really have no excess skin and only a mild amount of ptosis. In my experience, young patients like you do exceptionally well with the posterior approach.
You have minimal ptosis and therefore you are an excellent candidate for the procedure "Tarsal Plate Resection for Ptosis". This procedure enables a precise correction, which is difficult to achieve with a levator muscle advancement. This was published in Annals of Plastic Surgery, 1990, vol. 25.
Mild ptosis repair surgery
Posterior approach ptosis has several advantages: 1) more predictable result; 2) better eyelid contour; 3) scar-less. It is usually the best technique for mild ptosis repair. That's all I can say without examining you.
Great question! I can understand the difference in opinions. The "posterior approach" is appropriate and will improve some aspects of the differences between the eyes. The "anterior approach" will leave an external scar but allow for greater symmetry and a more accurate result. You can have the "posterior approach" and if you want a greater change afterwards have an "anterior approach" . Both approaches will improve symmetry, but perfection is hard to achieve.
I would place a drop of 2.5% phenylephrine in the eye, if it raises to a level we are both happy with then I prefer the posterior approach in your case. You have no redundant skin/fat and since you only need the left eyelid done there would be no external scar. You are young so we would need to make sure it isn't congenital(meaning you were born with it). In these cases I would not recommend the posterior approach even if the drop works.
You won't find the answer in the text book.
The Mullerectomy (posterior ptosis surgery) will burn a bridge. Every surgery uses up something. The is no substitute for an in person consultation with a detailed examination. However, it is highly likely that you have levator dehiscence ptosis. The problems caused by this will not be addressed with the Mullerectomy and it is highly probably that this surgery will fail. The reason is that when the levator aponeurosis tendon, which is how the levator palpebral superiors muscle inserts into the eyelid, is disinherited, it is not in place needed to make the Mullerectomy effective. The result is that in this situation posterior ptosis surgery fails. Also this approach does not address your other issues. It does not support your upper eyelid lashes which are ptotic and you have loose eyelid platform skin. Ideally, then you have this work done, instead of having a hollow upper eyelid, you will have a crease and an upper eyelid fold. You have a fold now but they are asymmetric and very high because your eyebrows are elevated to compensate for the heavy upper eyelid. What you need in my opinion is anterior surgery to fix the levator dehiscence which will correct the ptosis, you also need an anchor blepharoplasty to define a hard crease in the upper eyelid, snug the eyelid platform skin, and support the upper eyelid lashes. Surgeons like the posterior approach because the procedure is far less complicated than anterior levator ptosis surgery. Many eyelid surgeons do not fully understand the anatomy.
Anterior vs posterior approach ptosis repair
For someone like you with no excess skin, young, and mild ptosis, I would go with a posterior repair. The 2 approaches work on very different muscles so that you have not burned any bridges by doing a posterior repair first. I usually put a drop or 2 of phenylephrine in the ptotic eye and let the patient see the difference. If you are happy with the result of a phenylephrine test, you should be pretty happy with the results of a conjunctivo-Muellerectomy (I have abandoned the Fasanella Servat procedure many years ago).