Diseases of the Vitreous

AMYLOIDOSIS

Amyloidosis is a condition of abnormal protein deposition in one or more organ systems of the body. The ocular form of amyloidosis occurs only with familial amyloidotic polyneuropathy (FAP), a rare genetic disorder, and follows an autosomal dominant transmission pattern.

Symptoms

Patients with ocular amyloidosis in the vitreous report blurred vision in one or both eyes. The condition is bilateral but may be asymmetric. Patients may occasionally report dry eye symptoms, which are due to lacrimal gland involvement. Although FAP is a genetic disorder, patients do not become symptomatic until the fourth or fifth decade of life.

Clinical Features

Vitreous opacities are the ocular hallmark of amyloidosis. These vitreous opacities typically appear whitishgray, thick, and fibrillar and may completely obscure the view to the retina. Other posterior segment features of amyloidosis include sheathing of the retinal vessels, perivascular infiltrates, and superficial gray retinal patches. The infiltrates and gray patches are thought to be intraretinal amyloid.

The anterior segment features of amyloidosis involve the conjunctiva and pupil. Patients with amyloidosis may have perilimbal microaneuryms on the conjunctiva. An unusual feature of this condition is a scalloped or irregular pupillary margin that may be due to denervation of the pupillary sphincter muscle. Patients may also have anisocoria or decreased pupillary reaction to light. On the posterior lens surface, a clinical feature termed pseudopodia lentis can be seen. Pseudopodia lentis appears as a cluster of small white dots that are thought to be the footplates of the amyloid fibers.

The differential diagnosis of amyloidosis includes asteroid hyalosis, large cell lymphoma, posterior uveitis, old vitreous hemorrhage, and chronic endophthalmitis.

Ancillary Testing

Electroretinography and electro-oculography may show subnormal findings; however, these tests are not usually required to make the diagnosis. A biopsy can be performed of either the conjunctiva or the vitreous. Analysis of the vitreous samples should include bacterial and fungal cultures for endophthalmitis, cytology for lymphoma, and histopathologic analysis for the presence of amyloid.

Pathology/Pathogenesis

Amyloid is a homogeneous, acellular protein. A vitreous biopsy specimen stains positively with Congo red stain (a marked green color) and exhibits birefringence when viewed under a polarizing light microscope. Amyloid in the vitreous is also eosinophilic, with hematoxylin and eosin staining. The pathogenesis of amyloidosis is related to the aberrant formation of a single protein. Instead of the normal folding of a mature protein, amyloid forms abnormal sheets or folds of protein called “beta pleats.”

Treatment/Prognosis

Pars plana vitrectomy is now the standard method for obtaining a biopsy specimen as well as for clearing the vitreous opacities. The prognosis for improved vision is good following surgery, although amyloid can recur in 20% to 25% of patients.

Systemic Evaluation

Patients with clinical signs consistent with ocular amyloidosis should be asked about their family history and ancestry, as FAP is much more common in people of Portuguese descent. Biopsies may be done of the vitreous, conjunctiva, skin, rectum, and sural nerve. Patients who have FAP should have cardiac evaluation for possible cardiomyopathy or thickening of the atrial or ventricular wall.

This 61-year-old man complained of reduced vision and floaters. His examination revealed prominent whitish gray vitreous opacities obscuring the view of the retina.

Cytologic examination of the vitreous fluid removed during pars plana vitrectomy of his left eye demonstrated positive Congo red/Sirius red staining for amyloid.

The fellow eye had similar vitreous findings. Note the prominent vitreous strands. A diagnostic and therapeutic pars plana vitrectomy confirmed the presence of amyloid.

The same patient had peripheral neuropathy. Sural nerve biopsy revealed marked loss of the myelinated axons and focal areas of amyloid deposition in the endoneurium, mainly involving the blood vessel walls.

ASTEROID HYALOSIS

Asteroid hyalosis is a common vitreous abnormality characterized by innumerable deposits throughout the vitreous. The term asteroid refers to the refractile appearance of the deposits as seen with slit lamp biomicroscopy. Asteroid hyalosis is observed most commonly in the older population and affects men and women equally. Asteroid hyalosis is usually unilateral but may be bilateral or asymmetric.

Symptoms

Most patients with asteroid hyalosis are asymptomatic or complain of mild floaters. The report of floaters is rare, even in patients with extensive vitreous involvement.

Clinical Features

Patients with asteroid hyalosis have normal visual acuity. Examination reveals a variable number of yellowish white, oval or round deposits within the collagen framework of the vitreous that may form clumps or strands. The deposits are highly refractile in some patients.

Ancillary Testing

Fluorescein angiography is helpful in evaluating retinal abnormalities when the asteroid deposits prevent direct visualization of the fundus. Ultrasonography also may be useful in evaluating the fundus when asteroid deposits are prominent.

Pathology/Pathogenesis

The cause of asteroid hyalosis is unknown. Historically, asteroid bodies were believed to be calcium soaps. More recent reports employing electron spectroscopic imaging reveal a homogeneous distribution of calcium, phosphorus, and oxygen similar to hydroxyapatite.

Treatment/Prognosis

Most patients with asteroid hyalosis are asymptomatic and do not require treatment. In rare cases where patients have visual alterations related to the asteroid deposits or where visualization of the fundus is essential, pars plana vitrectomy may be useful.

Systemic Evaluation

Although the majority of patients with asteroid hyalosis are otherwise healthy, asteroid hyalosis has been associated with diabetes mellitus. The relationship between asteroid hyalosis and diabetes is unclear and remains controversial.

Asteroid hyalosis is viewed by slit lamp biomicroscopy. The yellowish white deposits may be quite refractile in some patients. The deposits move in synchrony with the vitreous.

In some instances, the extent of asteroid deposits may interfere with visualization of the fundus. This patient had reduced vision in her left eye attributed to asteroid hyalosis.

Asteroid hyalosis is viewed through the fundus camera. The deposits are variable in shape and size and tend to have a softer, yellowish appearance.

Fluorescein angiography of the patient was extremely helpful in determining the cause of visual loss. It revealed idiopathic juxtafoveal retinal telangiectasis.