Vitreous

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12

Vitreous

Normal Anatomy

I. The transparent vitreous body, or hyaloid (Fig. 12.1), is one of the most delicate connective tissues in the body.

A. It occupies the posterior or largest compartment of the eye (~80% of the eye’s volume), filling the globe between the internal limiting membrane of the neural retina and the posterior lens capsule.

B. The structure is composed of a framework of extremely delicate, embryonic-like collagen filaments closely associated with a large quantity of water-binding hyaluronic acid.

II. Embryology

A. Embryologically, the developing avascular secondary vitreous surrounds and compresses the vascularized primary vitreous into a tube or canal that extends from the optic disc to the back of the lens, forming the hyaloid canal (canal of Cloquet).

B. The hyaloid vessel, which atrophies and disappears before birth, passes through the canal.

C. Persisting remnants of the primary vitreous or hyaloid vessel produce congenital anomalies (see later), the most common being retention of tissue fragments on the back of the lens (Mittendorf’s dot; Fig. 12.2), retention of tissue on the nasal optic disc (Bergmeister’s papilla), and persistent primary vitreous (see Chapter 18).

Congenital Anomalies

Persistent Primary Vitreous

I. Remnants of the primitive hyaloid vascular system, either anterior or posterior, may persist, or the entire hyaloid vessel from the optic disc to the back of the lens may remain.

Hyaloid vessel remnants are observed in more than 90% of infants of less than 36 weeks’ gestation and in more than 95% of infants weighing less than 5 pounds (2.275 kg) at birth.

A. Anterior remnants (see Fig. 12.2; see also Fig. 9.3)

1. The lenticular portion of the hyaloid artery may hang free in the vitreous from its lens attachment site.

2. Mittendorf’s dot is an opacity just below and nasal to the lens posterior pole at the lenticular attachment site of the hyaloid artery.

B. Posterior remnants (Fig. 12.3)

1. Vascular loops from the optic disc may remain within Cloquet’s canal.

2. Bergmeister’s papilla is the glial remnant of the hyaloid system at the optic disc. The papilla, which usually occupies the nasal portion of the optic disc, may appear as a solid mass of whitish tissue; as a delicate, ragged strand; or as a well-defined membrane stretching over the disc.

3. Congenital cysts, which are usually pearly gray, wrinkled, and translucent, are the cystic remains of the hyaloid system.

They usually float freely but may be attached to the optic disc or suspended by a pedicle. Some have been shown histologically to consist of gliotic retinal or vascular remnants, whereas others resemble pigment epithelial cells (i.e., choristoma of the primary hyaloid system).

Inflammation

Acute

See Chapter 3.

Chronic

See Chapters 3 and 4.

Vitreous Adhesions

Post Nonsurgical and Surgical Trauma

Postinflammation

See Chapters 3 and 4.

Idiopathic

Idiopathic vitreous adhesions may follow partial vitreoretinal separation (posterior vitreous detachment).

Vitreous Opacities

Hyaloid Vessel Remnants

Muscae volitantes are minute remnants of the hyaloid vascular system or detachments of small folds of poorly differentiated retinal tissue, usually present in the anterior vitreous.

Muscae volitantes also is a historical, obsolete term for acquired vitreous floaters.

Inflammatory Cells

I. “Snowball” opacities (microabscesses) may occur with mycotic infections (especially with the mold fungi).

II. Whitish masses (“white balls”) may be seen inferiorly with vitreitis (e.g., sarcoidosis).

Macrophages and small T lymphocytes are the major cell types found in vitreous opacity samples diagnosed as inflammation.

III. Numerous vitreous opacities, foamy “Whipple” macrophages, may be found in persons with Whipple’s disease (Fig. 12.4).

A. Whipple’s disease is a disorder of men, usually older than 35 years of age.

1. The detection of the causative agent, Tropheryma whippelii, by polymerase chain reaction allows confirmation of the clinical diagnosis.

2. The gram-positive bacteria, Arthrobacter species, phylogenetically related to T. whippelii, may also be causative agents.

B. Arthritis, fever, serous effusions, cough, lymphadenopathy, and malaise may occur for several years preceding the intestinal malabsorption, steatorrhea, and cachexia.

C. Ocular findings, in addition to vitreous opacities, include inflammations and ophthalmoplegia.

D. Foamy macrophages containing periodic acid–Schiff (PAS)-positive cytoplasm may be found in intestinal and rectal mucosa, mesenteric and extra-abdominal lymphatic tissue, heart, lungs, liver, adrenals, spleen, serous membranes, neural retina, and vitreous.

1. In addition, intracellular and extracellular rod-shaped bacillary bodies and serpiginous membranes are found by electron microscopic examination of macrophages.

2. It is now assumed that the characteristic macrophages derive their PAS-positivity from ingested rod-shaped bacilli (Tropheryma whippelii) and also from their residue in autophagic vacuoles of the macrophages.

Iridescent Particles

I. Asteroid hyalosis (Benson’s disease; Figs. 12.5 and 12.6) consists of complex lipids embedded in an amorphous matrix containing mainly calcium and phosphorus and attached to the vitreous framework.

A. Diabetes mellitus is a major risk factor; other risk factors include systemic hypertension, atherosclerotic vascular disease, and hyperopia.

B. Asteroid hyalosis has the following clinical properties:

1. Asteroid bodies remain attached to collagenous framework and move only when the framework oscillates. They are seen as gold balls when viewed with side illumination (e.g., with the ophthalmoscope) and appear white with direct illumination (e.g., with the slit lamp).

2. The condition is usually unilateral and most common in the seventh and eighth decades of life. It is infrequently associated with neural retinal detachment.

Asteroid hyalosis may be a risk factor for the development of calcification of silicone lens implants.

C. Histologically, asteroid bodies consist of an amorphous matrix that is both PAS-positive and acid mucopolysaccharide-positive and contains birefringent, small crystals when viewed with polarized light. Electron microscopically, the bodies are composed of finely laminated ribbons of complex lipids, especially phospholipids, lying in a homogeneous background and intertwined with filaments of vitreous framework.