There are four muscles of facial expression which are inserted into the skin of the eyebrows and
control the eyebrow position:

• Frontalis  elevate the brows
• Orbicularis oculi: closes the eyelids and draw the brows towards the eyes
• Corrugator superiocili: pulls the nasal eyebrow inferiorly and medially, producing vertical glabellar wrinkles
• Procerus: pulls the eyebrow inferiorly.

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The eyebrow is securely fastened along the supraorbital ridge through its attachment to the undersurface of the eyebrow fat pad. As supraorbital ridge is absent in the lateral third of the orbital rim, brow ptosis usually begins laterally.

The main cause of brow ptosis is the result of gravitational pull with age. As explained above, the lateral brow is the most commonly affected. However, medial eyebrow can occur with atrophy of the eyebrow fat.

In brow ptosis, the frontalis muscle is used to elevate the brows and this results in deep forehead furrows. Nonetheless, lateral brow ptosis usually persists due to the lack of frontalis fibres laterally.
 

Lateral brow ptosis

Deep forehead furrows from  frontalis overaction