Enophthalmos is an uncommon feature of Horner's syndrome. Enophthalmos is due to palsy of:
Enophthalmos (sinking of the eye) is the inward displacement of the eyeball. It results from paralysis of the orbitalis muscle (a smooth muscle that spans the inferior orbital fissure and may contribute to maintaining the normal position of the eyeball). Ref: Gray's Basic Anatomy By Richard Drake, A. Wayne Vogl, Adam W. M. Mitchell, 2012, Page 467.
Levator palpebrae superioris
Superior tarsal muscle
Orbitalis muscle
Inferior tarsal muscle