How is ptosis measured?

Chiefly by the margin reflex distance (MRD-1) — how far the upper lid sits from the corneal light reflex — and by levator function, the lid's up-and-down excursion. Together these grade the ptosis and select the operation.

What is the phenylephrine (Neo-Synephrine) test for?

Drops of 2.5% phenylephrine stimulate Müller's muscle; if the lid lifts well, an internal (Müller's muscle–conjunctival resection) repair is likely to work — so the test helps choose the approach.

Why does one eyelid look droopier after the other is lifted?

Hering's law of equal innervation — both lids receive equal lift signals. When a ptotic lid is doing extra work, the fellow lid can look high; correcting one side may unmask droop on the other, which the evaluation anticipates.

Eyelid

Ptosis Evaluation & Diagnosis

How a droopy eyelid is evaluated — margin reflex distance, levator function, the phenylephrine (Neo-Synephrine) test, Hering's law, and visual-field impact.

Medically reviewed by Tamara R. Fountain, MDOculoplastic SurgeonLast updated June 2026

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Part of our complete guide to Ptosis (Droopy Eyelid) — this page covers ptosis evaluation and diagnosis in depth.

Evaluation of Ptosis

Careful measurement is the foundation of ptosis surgery planning. Dr. Fountain records the following at every ptosis evaluation:

Video: What a Ptosis Evaluation Looks Like

Margin-reflex distance (MRD-1): the distance from the corneal light reflex to the upper lid margin in primary gaze. Normal ≈ 4–5 mm; ptosis is present when MRD-1 is < 2 mm
Levator function: total lid excursion from full downgaze to full upgaze with the brow held still. Excellent ≥ 10 mm; fair 5–9 mm; poor ≤ 4 mm — this single measurement largely determines the surgical approach
Lid crease height: the distance from the lash margin to the skin crease, which guides the incision level
Phenylephrine test: a drop of 2.5% phenylephrine behind the upper lid stimulates Müller’s muscle; elevation of ≥ 1 mm predicts a favorable response to internal ptosis repair (MMCR)

Hering’s Law of Equal Innervation

When ptosis affects only one eye — or is much worse on one side — Hering’s law becomes critical to surgical planning. Both levators receive equal central drive from the brain. In unilateral ptosis, the brain increases drive to both sides to keep the ptotic lid open. If surgery elevates the ptotic lid, that extra drive drops — and the fellow lid may fall.

This “see-saw” effect means apparent unilateral ptosis can unmask bilateral ptosis after surgery on one side. The interactive animation below demonstrates this phenomenon.