vitreous humor

**Anatomy and composition** - Occupies ~80% of eye volume, in the vitreous chamber (posterior segment) between the posterior lens and the retina. - ~98-99% water; also type II collagen fibrils (some type IX/V/XI) and hyaluronic acid (hyaluronan), which gives the gel its viscosity. Contains very few cells (hyalocytes). - Avascular and acellular for the most part. It is not replaced, unlike aqueous humor, which is continuously produced and drained. - Attachments are strongest at the vitreous base (straddling the ora serrata), the optic disc margin, the macula/fovea, and along retinal vessels. The hyaloid canal (Cloquet's canal) is a remnant of the embryonic hyaloid artery. - Embryology: the primary vitreous contains the hyaloid artery, which normally regresses. The secondary (definitive) vitreous forms from neuroectoderm and fills most of the chamber.

**Function** - Maintains globe shape and intraocular volume/turgor. - Keeps the neural retina apposed to the retinal pigment epithelium/choroid. - Transparent medium for light transmission, with a refractive index of ~1.33. - Shock absorption, and a reservoir/diffusion medium for metabolites.

**Vitreous vs. aqueous humor** - Aqueous humor is produced by the ciliary body epithelium (nonpigmented), flows from the posterior chamber through the pupil to the anterior chamber, and drains via the trabecular meshwork to Schlemm's canal. Impaired outflow causes glaucoma. - Vitreous does not circulate and is not a major determinant of IOP.

**Clinical correlates** - **Posterior vitreous detachment (PVD):** age-related syneresis (liquefaction) causes separation of the posterior hyaloid from the retina. Presents with floaters and photopsias (flashes). It is usually benign but can cause retinal tears, which can lead to rhegmatogenous retinal detachment (curtain-like visual field loss). Any new floaters or flashes require a dilated fundus exam. - **Vitreous hemorrhage:** causes include proliferative diabetic retinopathy (neovascularization, the most common cause in adults), retinal tears, and trauma. Presents with sudden painless vision loss or a shower of floaters/"cobwebs," and loss of the red reflex. - **Asteroid hyalosis:** calcium-lipid (calcium soap) deposits, usually asymptomatic. - **Persistent fetal vasculature (persistent hyperplastic primary vitreous):** failure of hyaloid vasculature regression, causing leukocoria, usually unilateral. It is on the leukocoria differential with retinoblastoma and congenital cataract. - **Vitritis:** inflammation of the vitreous from posterior uveitis or infection. Examples include toxoplasmosis ("headlight in the fog"), CMV retinitis, and endophthalmitis (an ophthalmologic emergency, with intravitreal antibiotics). - **Stickler and Marfan syndromes:** abnormal vitreous (Stickler: type II collagen defect, with an optically "empty" vitreous, high myopia, and retinal detachment risk) and association with lens dislocation and retinal detachment in Marfan. - **Treatment:** vitrectomy is used for nonclearing hemorrhage, tractional detachment, and macular hole.

**High-yield pearls** - Vitreous = gel, not replenished. Aqueous = fluid, continuously produced. - Floaters + flashes → think PVD → rule out retinal tear/detachment. - Diabetic neovascularization → vitreous hemorrhage and tractional retinal detachment.