The capillaries of the optic nerve

Biochemical mechanism underlying the pathogenesis of diabetic retinopathy and other diabetic complications in humans: the methanol-formaldehyde-formic acid hypothesis

Hyperglycemia in diabetic patients is associated with abnormally-elevated cellular glucose levels. It is hypothesized that increased cellular glucose will lead to increased formation of endogenous methanol and/or formaldehyde, both of which are then metabolically converted to formic acid. These one-carbon metabolites are known to be present naturally in humans, and their levels are increased under diabetic conditions. Mechanistically, while formaldehyde is a cross-linking agent capable of causing extensive cytotoxicity, formic acid is an inhibitor of mitochondrial cytochrome oxidase, capable of inducing histotoxic hypoxia, ATP deficiency and cytotoxicity. Chronic increase in the production and accumulation of these toxic one-carbon metabolites in diabetic patients can drive the pathogenesis of ocular as well as other diabetic complications. This hypothesis is supported by a large body of experimental and clinical observations scattered in the literature. For instance, methanol is known to have organ- and species-selective toxicities, including the characteristic ocular lesions commonly seen in humans and non-human primates, but not in rodents. Similarly, some of the diabetic complications (such as ocular lesions) also have a characteristic species-selective pattern, closely resembling methanol intoxication. Moreover, while alcohol consumption or combined use of folic acid plus vitamin B is beneficial for mitigating acute methanol toxicity in humans, their use also improves the outcomes of diabetic complications. In addition, there is also a large body of evidence from biochemical and cellular studies. Together, there is considerable experimental support for the proposed hypothesis that increased metabolic formation of toxic one-carbon metabolites in diabetic patients contributes importantly to the development of various clinical complications.

Changes in optic nerve head blood flow and retrobular hemodynamics following calcium-channel blocker treatment of normal-tension glaucoma

BACKGROUND

Because calcium channel blockers reduce vascular resistance, they may have a clinical application in the treatment of normal-tension glaucoma (NTG). This study investigates changes in both the optic disc blood flow and the hemodynamics of retrobulbar vessels in NTG patients after the systemic administration of a calcium channel blocker.

METHODS

Twelve eyes of 12 NTG patients (mean age 57.6 +/- 15.3 years) were examined before and after a 4-week treatment with 2 mg b.i.d. oral nilvadipine, an L-typc calcium channel blocker. By scanning laser-Doppler flowmetry (SLDF), we obtained the velocity, flow, and volume from within a 10 x 10 pixel window placed on the temporal rim region of the optic disc perfusion map. By ultrasound color Doppler imaging (CDI), we measured the peak systolic velocity (PSV) and the end diastolic velocity (EDV) of the ophthalmic artery (OA), central retinal artery (CRA), nasal posterior ciliary artery (NPCA), and temporal posterior ciliary artery (TPCA). We then calculated a resistance index (RI) for each vessel.

RESULTS

After treatment, the flow and velocity of the optic disc blood flow significantly increased (P < 0.05). Nilvadipine also significantly reduced RIs of the CRA, NPCA, and TPCA (P < 0.05), and increased both the PSV of the NPCA and the EDVs of the CRA, NPCA, and TPCA. The percent change in velocity correlated significantly with the percent changes of the CRA RI and NPCA RI.

CONCLUSIONS

Oral nilvadipine appears to reduce orbital vascular resistance, which consequently increases the optic disc blood flow.

Fundus manifestations of orbital disease and treatment of orbital disease

Disorders of the orbit can secondarily involve the eye. Although nonspecific, changes which can be noted on funduscopic examination including abnormalities of the retina, choroid, and optic nerve, can be secondary to an underlying orbital process. Awareness of these findings and their association with orbital disease is of great importance to the practicing ophthalmologist, since many orbital disorders are treatable and indeed, some are life-threatening. In addition, treatment of these disorders can potentially result in a variety of ocular complications. An understanding of the potential risks is of the utmost importance in planning treatment of an orbital disease.

Optic nerve involvement in a case of methanol poisoning

The eyes and optic nerves were studied histologically in a fatal case of methanol poisoning in a 37-year-old man. The most striking findings were bilateral central necrosis of the optic nerves from behind the lamina cribrosa to the orbital apex. Proximal parts of the nerve and the tract showed no necrosis. It is suggested that the complex blood supply of the nerve may be of importance.

Histologic studies of the vasculature of the anterior optic nerve

Thirty-one normal human anterior optic nerves were studied in thin serial sections and their vessels stained by standard histologic stains and a new modification of the silver reticulin stain. The retrolaminar cribrosa was profusely supplied by centripetal and longitudinal vessels of pial origin; small branches of the central retinal artery were also seen. Longitudinal continuity of small vessels extended from the retrolaminar nerve to the retinal surface. This system freely anastomosed with three transverse systems; a significant anteriolar input from short posterior ciliary arteries in the sclera to the lamina cribrosa; smaller short posterior ciliary branches and occasional choroidal capillaries to the prelaminar portion; and branches derived from the central retinal artery in and around the disk. Vessels entering the prelaminar region at the level of the choroid were mainly derived from scleral branches of short posterior ciliary vessels entering through the border tissue of Elschnig, rather than from choroidal branches of ciliary arteries. Venous drainage for the anterior optic nerve was primarily through the central retinal vein, although alternate smaller paths to surrounding tissues were also identified.