As age advances, the brow is under a constant “tug of war” between the combined forces of gravity and the periorbital protractor muscles (the orbicularis oculi, procerus, and corrugator supercilii) on one hand, which tend to pull the brows downward, and the frontalis muscle on the other, which elevates the brow.⁷ The downward forces tend to prevail, and the brow is observed to descend in the majority of older patients.⁷ However, paradoxical brow elevation is observed in a sizable proportion of individuals.⁸ Some authors claim that the odds of brow elevation or descent with age are roughly equal and that in the majority of cases the brow level does not change appreciably with age.⁹ This controversy probably depends on which part of the brow the observer is evaluating. The central and medial portions of the brow may remain constant in position or even elevate with age,⁸ and one study failed to observe any age-related change in the position of the nasal part of the brow.¹⁰ The lateral part of the brow, however, invariably shows age-related descent.¹¹ A recent study demonstrated that the distance between the nasal ala to the lateral brow decreases by 0.15 mm per year, accounting for an average descent of 3 mm over 20 years.¹¹ Therefore, it could be conceptualized that what is happening with aging is not an overall change in the position of the eyebrows, but rather a change in brow shape.⁸ In younger individuals, the lateral brow usually is stationed at a higher position than the medial or the central parts of the brow, but as aging progresses, the brow acquires a flatter contour because of inevitable lateral descent. This may or may not be associated with paradoxical elevation of the rest of the brow.⁸ These changes are more pronounced laterally because the lateral eyebrow is not densely attached to periosteum lateral to the supraorbital ridge,¹¹ and because the frontalis muscle stops short of the tail of the brow.¹¹,¹² The lateral border of the frontalis muscle almost always ends, or becomes extremely attenuated, beyond the temporal fusion line of the skull.¹¹,¹² Therefore beyond this point, the gravitational effects, as well as the downward pull of the protractor muscles, are unchecked.⁸,¹¹,¹²

Another frequently cited cause for involutional brow ptosis is volume loss and tissue deflation rather than actual tissue descent.²,⁴,¹³,¹⁴ This has led to a trend in brow rejuvenation procedures focusing on volume replenishment instead
of brow elevation.¹⁴,¹⁵ The existence of lateral brow drooping has been refuted, with the argument that hollowing of the eyelid-brow complex that occurs with age causes shadowing beneath the brow, causing an illusionary brow descent.¹⁴ This concept has been challenged in a recent study that showed that total eyebrow volume does not decline with age and may slightly increase in males.¹⁵ Furthermore, this same study showed a paradoxical increase in brow fat volume.¹⁵ Although these findings may seem incompatible with clinical observations, the authors argue that the pseudodeflation that is observed in clinical practice is due to thinning and loss of skin elasticity, along with widening and deepening of the supraorbital rim with age.¹⁵

Brow ptosis may also occur secondary to paralysis or weakness of the frontalis muscle. The loss of frontalis tone that accompanies facial nerve paralysis generally causes a severe brow droop.⁴,¹⁶,¹⁷ Other rarer causes include myasthenia gravis, myotonic dystrophy, and oculopharyngeal muscular dystrophy, although in these patients the frontalis muscle is less severely affected than the extraocular muscles.⁴,¹⁸ Brow ptosis may also occur from the involuntary contraction of the orbicularis oculi in conditions such as blepharospasm or other facial dystonias.⁴ Mechanical brow ptosis can also occur due to neoplasms involving the brow, like basal cell carcinoma, keratoacanthoma, melanoma, or squamous cell carcinoma.⁴