Journal of Neuro-Ophthalmology, September 1996, Volume 16, Issue 3

The Anterior Visual Pathways

Journal of Neuw- Ophllmlmohg}/ 26( 3): 272- 222, 2996. © 1996 Lippincott- Raven Publishers, Philadelphia Annual ( Almost) Review The Anterior Visual Pathways- Part II Alfredo A. Sadun, M. D., Ph. D., and Richard M. Rubin, M. D. Part I of this second annual review of the ante­rior visual pathways covered testing of the anterior visual pathways, basic science relevant to the an­terior visual pathways, and clinical articles on the inner retina and the optic disc. We pick up Part II of the annual review with diseases of the optic nerve starting behind the lamina cribosa, then through the optic chiasm and into the parallel processing of the primary visual nuclei. THE OPTIC NERVE Probably the most important clinical trial in neuro- ophthalmology has been the Optic Neuritis Treatment Trial ( ONTT), which began in 1988 and involved - 460 patients at 15 clinical centers. The initial results were described in our first annual review published in 1992 and 1993. However, since the first descriptions of these exciting results have come several further important follow- up ob­servations. Beck and Trobe summarized these updates at the end of 1995 ( 200). There were many " take-home" messages from the ONTT. Intravenous high- dose corticosteroids led to a more rapid re­covery of visual acuity than lower dose oral corti­costeroids or observation. However, after 1 month, the differences between each treatment group were insignificant. After 1 year, 93% of pa­tients had 20/ 40 vision or better with no difference between groups. Even 67% of patients with initial Manuscript received April 9, 1996; accepted April 14, 1996. From the Department of Ophthalmology, Doheny Eye Insti­tute, University of Southern California School of Medicine, Los Angeles, California, U. S. A. Address correspondence and reprint requests to Dr. Alfredo A. Sadun, Estelle Doheny Eye Institute, 1450 San Pablo Street, Los Angeles, CA 90033, U. S. A. vision of no light perception or light perception achieved 20/ 40 or better ( although, in general, poor vision was the only predictor of a poor visual out­come) ( 201). Even among the patients with the poorest initial visual acuities, neither treatment had any bearing on final visual outcome ( 200,201). Recoveries based on visual field changes basically paralleled those of visual acuities ( 202). As stated in the earlier review, the great surprise of this study was that patients treated with oral corticosteroids demonstrated a greater incidence of recurrent optic neuritis. Furthermore, treatment by high- dose i. v. corticosteroids had a slight pro­tective effect against further attacks ( 203). In an earlier update, Beck recommended MRI of most patients to look for multiple sclerosis ( MS)- related plaques. Coyle, in a letter, suggests treating all op­tic neuritis patients with i. v. corticosteroid, obvi­ating the expense of MRI ( 204). Beck replies there is a need to predict those at high risk for MS, but not to treat the others ( 204). The greatest dramatic development of the ONTT was the discovery that high- dose i. v. corticoste­roids retarded the development of optic neuritis to clinically definite MS ( 205). However, these effects were almost entirely manifested in the first 2 years after study entry ( 206). Thereafter, there was no longer any appreciable change in the likelihood of development to MS ( 200). The risk factors of optic neuritis progressing to definite MS =£ 2 years were ( in order) having an abnormal MRI defined as 5= 3 plaques, having had previous attacks of optic neuritis in the other eye, having an abnormal MRI with 1 or 2 plaques, being of white race, and having had previous nondiag­nostic neurological symptoms ( 200). Curiously, age and sex did not predict development of MS after an attack of optic neuritis despite the many previous good studies that suggested so. 212 THE ANTERIOR VISUAL PATHWAYS 213 Beck and Trobe conclude that with classical pre­sentations of optic neuritis, the full work- up con­sisting of blood test, chest radiographs, and lum­bar puncture is not necessary; that a brain MRI should be offered to help determine the risk of going on to develop definite MS; that treatment with oral prednisone should be avoided; and that treatment with high- dose i. v. methylprednisolone ( 250 mg every 6 h for 3 days) should be considered, especially if the MRI shows a few plaques ( 200). We certainly concur and have interpreted this data to form the following policy. Patients with a typical attack of optic neuritis are told of the ONTT study and of the possible development of MS. We explain the difference between a laboratory or probable diagnosis of MS ( more useful for studies than for a given patient) and a clinical diagnosis of MS, and then emphasize that the latter is the only kind of diagnosis with prognostic value. After ex­plaining their particular risk factors, a brain MRI is offered to further evaluate the patient's risk of go­ing on to develop definite MS. Our recommenda­tion is for i. v. methylprednisolone treatment if their risk is " high," but we leave the definition of " high" up to each patient. Many of our patients elect to have the MRI but, not uncommonly, pa­tients defer and cite that they have already con­cluded that the result will not affect their decision for i. v. corticosteroid therapy. Despite the similarity of title, a very dissimilar optic neuritis treatment trial was done in Japan ( 207). By retrospective questionnaire returned by only 50% of the departments of ophthalmology, it was determined that the annual incidence of this disorder was 1.6 per 100,000 and that the male to female ratio was 1: 1.2 ( 207). By comparison, a ret­rospective review of all patients with optic neuritis seen in Olmstead County, Minnesota, from 1935 to 1991 revealed an annual incidence rate - 5.1 per 100,000 ( 208). More interesting, in Minnesota, after 10 years' follow- up, - 40% had progressed to clin­ically definite MS; these numbers went to 50% by 20 years and to 60% by 40 years. This study also did not find any gender difference in the risk of developing MS ( 208). A number of psychophysical parameters used in the detection or characterization of optic neuritis have been described. Losses of luminance and chromatic sensitivity were measured just outside central and paracentral scotomas ( 209). The Pul-frich stereo phenomenon was revisited as a mod­ified technique that involved a moving hand- held black pen and neutral density filters to neutralize the effect as a measure of the severity of optic neu­ritis- produced axonal conduction delays ( 210). Basic as well as clinical science investigations continue on optic neuritis. Does bilaterality influ­ence the diagnosis of optic neuritis? Of 23 adult patients with bilateral simultaneous optic neurop­athy, a complete work- up revealed that 22% had clinically definite MS, 17% Leber's, and 61% pre­sumed idiopathic optic neuritis even 4 years later ( 211). Of these, 21% had MRI abnormalities and 21% had oligoclonal banding of the CSF consistent with MS. But 64% had no risk factors for MS ( 211). In another study, 11 consecutive patients with optic neuritis were compared with 14 MS patients with optic neuritis. In the analysis of the CSF, an-timyelin basic protein measures are not as specific for determining the risk for MS as are measures of antiproteolipid protein ( 212). Others looking at myelin basic protein proteolipid protein, and my-elin- associated glycoprotein found no relations in CSF between optic neuritis or MS patients ( 213). Similarly, patients with mononuclear cells express­ing mRNA for IL- 10 were as likely to have optic neuritis with or without clinically definite MS. However, because the levels kept rising months after the initial episode of optic neuritis, the au­thors concluded that this parameter reflected the degree and stage of CNS inflammation and hence might predict the outcome in optic neuritis ( 214). Both pattern visual evoked potentials ( VEPs) ( es­pecially N70 and P100 latencies and N70- P100 in-terocular amplitude differences) and pattern elec-troretinography ( PERG) were found to be sensitive measures of ( resolved or acute) optic neuritis, and PERG was helpful in distinguishing optic neuritis from non- arteritic anterior ischemic optic neurop­athy ( N- AION) ( 215,216). In a larger study of 126 patients with MS, it was found that delayed P100 and discrepancies between N70 and P100 were common, not only for patients with history of optic neuritis, but also for those without its confirming earlier similar studies ( 217). Simultaneous bilateral optic neuritis has been re­lated to unilateral herpes zoster ophthalmicus, bee sting, and to hepatitis A ( 218- 220). Optic neuritis has been described in association with onchocerci­asis ( but not ivermectin therapy in these patients) as well as with central retinal artery and vein oc­clusions ( 221,222). A case of optic neuritis thought to be secondary to paranasal sinusitis ( rhinoge-nous optic neuritis) reminds us that this used to be a commonly supposed connection ( 223). A second case of rhinogenous optic neuritis was remarkable; the patient's visual acuities ( as well as pupils and visual fields) had a diurnal variation from 20/ 15 in the morning to counting fingers at night ( 224). A number of other neurological disorders have / Ncuw- Ofihthalmol, Vol. 76, No. 3, 7.996 224 A. A. SADUN AND R. M. RUBIN been associated with optic neuropathies in the past. For example, clinical and histopathological investigations have suggested optic atrophy in Alzheimer's disease. However, optic nerves taken from nine Alzheimer's disease patients at necropsy were compared to seven nerves from patients without neuropathology, and image analysis mor­phometry failed to confirm the previously de­scribed differences in axon populations ( 225). Neovascularization of the optic disc and retina were described in cases of chronic myeloid and acute lymphocytic leukemias ( 226,227). Leukemic infiltration of the optic nerve was observed in an adult with T- cell leukemia, and a combination of corticosteroid and local radiation proved effective ( 228). Hypertrophy of the optic nerves was noted by serial MRIs and later at autopsy in an infantile form of Krabbe's disease ( 229). Cerebrotendinous xanthomatosis is another storage disease characterized by severe neurologic deterioration in early or middle adulthood. In 10 patients studied with this disease, 4 were found to have optic atrophy ( 230). Even after the " bloom was off the rose" for optic nerve sheath fenestration as a successful treatment of N- AION ( after the Ischemic Optic Neuropathy Decompression Trial), there has been no serious drop- off in investigations on this surgical tech­nique for other applications. In a review of all pa­tients undergoing optic nerve sheath decompres­sion ( ONSD) for chronic raised intracranial pres­sure, investigators found that visual acuity and visual fields had either improved or stabilized in 17 of 20 affected eyes ( 231). In another study of five pseudotumor patients who had undergone ONSD, intraoperative bleeding was found to pre­dict failure of the operation and, in general, the operation did not protect them well against visual loss long term ( 232). We wonder why ONSD sometimes fails to protect pseudotumor patients for visual loss after the first month. Of course, its not entirely understood why it works in the first place, although conventional wisdom suggests that the fenestrations act as shunts, at least for a short while. Tsai and colleagues presented the results of his­topathological and ultrastructural examinations of optic nerves from a 28 year old with severe pseu­dotumor cerebri in whom ONSD was performed because chronic corticosteroid use had endangered the patient ( 232). The patient still died from com­plications related to the corticosteroids 2 weeks af­ter the ONSD, and the authors found that the ar­eas of previously incised optic nerve sheaths had partly fibrosed to create more of a filtering bleb than an open fistula. Spoor and co- workers showed that mitomycin c could be safely used in conjunction with optic nerve sheath decompres­sion for pseudotumor cerebri and led to less orbital scarring and, hence, perhaps better long- term prognosis ( 233). In nine patients undergoing ONSD for pseudo­tumor cerebri, it was noted that the retinal venous caliber decreased significantly and continuously for ~ 3 months after surgery not only in the ipsi-lateral but the contralateral eye as well, suggesting a slow filtration mechanism for the successful ONSD ( 234). ONSD was performed in patients with pseudotumor cerebri who also had macular changes. The exudates decreased, but, not surpris­ing, the subretinal scars, pigment mottling, and striae persisted. Yet, it was the exudates that most correlated with visual losses ( 235). Even for patients in whom the ONSD may have been uneventful, transient and permanent vascu­lar or infectious complications may ensue ( 236- 8). Those patients who have visual loss after ONSD without an identifiable cause may benefit from lumboperitoneal shunting ( 238). Some have advo­cated making the optic nerve sheath fenestration more anterior, to section the posterior scleral ring. This is the Vasco- Posada procedure, which may be appropriate for the treatments of central and branch retinal vein occlusions. However, in a re­view of 4 eyes from three patients for whom this procedure had been done, a large triangular patch of choroidal ischemia was always noted ( 239). There is more to pseudotumor cerebri than treat­ment of the papilledema by ONSD. Unfortunately, the preferred nomenclature of idiopathic intracra­nial hypertension ( IIH) has never caught on. The etiology in most cases of pseudotumor cerebri ( or IIH) can be elusive. Wolin and Brannon describe a case that was diagnosed as Pickwickian syndrome; treatment of the pulmonary problem led to a rapid improvement of the visual loss ( therefore, ONSD was unnecessary) ( 240). In a review of their 30- year experience with childhood pseudotumor cerebri, members of the original Iowa group noted that these cases were seen only about once a year by them, despite the group's reputation as international experts in the field ( 241). In contradistinction to the disease in adults, there is no sexual predilection for pseudo­tumor in children, and only about one third are overweight ( 241). Generally, corticosteroids should be eschewed in the treatment of pseudotumor cerebri. Although they may provide a temporary amelioration, they aggravate management because they encourage / Naiw- Ophlluilmol, Vol. 16, No. 3, 1996 THE ANTERIOR VISUAL PATHWAYS 215 weight gain, and it is difficult to wean the patient off the steroids without provoking much worse papilledema and visual loss. However, Liu and co­workers in Miami successfully treated four pa­tients with high- grade disc edema with high- dose i. v. methylprednisolone for 5 days followed by oral taper ( 242). A case of pseudotumor managed for over 2 years by chronic low- dose corticoste­roids unfortunately gave occasion, through the pa­tient's death by steroid- induced complications, for a morphometric analysis of his optic nerves ( 243). It was found that despite his recent good central vision ( about 20/ 25 in each eye), there were 80 and 90% losses, respectively, of axons in his two optic nerves with a far greater pattern of loss in the areas of the nerves representing nonmacular retina ( 243). In terms of histology of pseudotumor cere­bri, Wall and co- workers, examining two cases af­ter autopsy, found no histological evidence of frank cerebral edema, thus abating the likelihood of this particular theory ( 244). A review of 51 patients with pseudotumor cere­bri asked which cases with papilledema did better or worse ( 245). It was not surprising that the higher the intracranial hypertension, the worse the disc edema and the more severe the visual acuity and field losses. However, counterintuitively, male patients had more severe optic disc edema than the women ( 245). The relationship between subarachnoid pres­sure and optic disc edema was revisited in 16 pa­tients undergoing enucleation of an eye ( 246). Can-nulation of the optic nerve sheath permitted direct measures of subarachnoid pressure while the pa­tients were in and out of Trendelenburg position. The subarachnoid optic nerve pressures were about 6 to 12 mm Hg, and only 1 to 2 mm of Hg change occurred when the head was hanging down ( 246). Also revisited was the spontaneous retinal ve­nous pulse in relation to papilledema. Noting the complexity of interactions between many variables such as the choroidal pulse and anatomic variabil­ity of the optic disc cup, the authors concluded that the spontaneous retinal venous pulse or its absence cannot be used to determine papilledema ( 247). Congenital optic nerve hypoplasia may be an underrecognized problem sometimes associated with functional amblyopia ( 248). Borchert and col­leagues prospectively analyzed 19 children with optic nerve hypoplasia and related clinical findings to measures of the optic disc ( 249). They found that when the horizontal disc diameter was > 30% of the disc- to- macula distance, the patients had good visual acuity ( 249). Optic nerve hypoplasia is often associated with the absence of the septum pelluci-dum; however the neurological abnormalities caused by the latter condition remain unknown ( 250). Other abnormalities associated with optic nerve hypoplasia include lens dislocation as described in chromosome 21 mosaicism ( 251) and angle dys­genesis with blepharophimosis syndrome ( 252). Brodsky has written an excellent review of congen­ital optic disc anomalies ( 253). Fetal alcohol syndrome is characterized by changes in facial appearance and also by optic nerve hypoplasia and poor visual acuity ( 254). A rat model of optic nerve hypoplasia was estab­lished by giving pregnant rats methanol; rats ex­posed to alcohol in utero developed optic nerve hypoplasia with reductions of optic disc size and axonal number ( 255). A case of optic nerve aplasia ( complete absence of the optic disc is rare) is de­scribed in association with a macular coloboma and hypothesized to be due to congenital toxoplas­mosis ( 256). Also on the spectrum of congenital optic disc anomalies is morning glory syndrome, characterized by a funnel- shaped optic disc. High-resolution CT scanning of two such patients re­vealed that the enlarged optic nerve had a radio-density similar to that of the sclera, thus suggest­ing its role in this dysgenesis ( 257). Other optic nerve developmental abnormalities include optic nerve colobomas associated with re­nal anomalies ( 258) and morning glory syndrome associated with CNS and endocrine anomalies ( 259). Patients with dysplastic optic discs may have retinochoroidal pigmentary anomalies; this should prompt neuroimaging for an associated transsphe­noidal encephalocele ( 260). Congenital excavated defects ( usually pits) of the optic disc may lead to serous retinal detachment of the macula that can be managed with vitrectomy, laser, and intravitre-al gas ( 261). Colobomatous defects of the optic disc, although congenital, may continue to evolve as documented through serial ophthalmoscopy ( 262). Optic disc medullation, seen in - 1% of the gen­eral population, represents retinal nerve fiber my-elination that anomalously extends anterior to the lamina cribosa. This was observed to continue not only after birth, but as long as 4 years later ( 263)! Optic disc drusen probably represent a degenera­tive process within the optic nerve head that may, in later years, lead to visual field changes. A com­parison between four modes of imaging optic disc drusen was made on four patients; ultrasound B- scanning was superior and cost effective com- / Neuro- Ophthulmol, Vol. 16, No. 3, 79.96 226 A. A. SADUN AND R. M. RUBIN pared to CT, MRI, and fluorescein angiography of the fundus ( 264). Of 116 eyes from 58 patients noted to have angioid streaks revealed by ultra­sound evaluation, 22% also have optic disc drusen ( 265). Optic nerve compression can occur, of course, as a result of a number of pathological entities. Per­haps the most curious involved a 35- year- old woman presenting with transient obscurations of vision and decreased visual acuity who turned out to have a frontal basal encephalocele congenital malformation ( 266). Another ususual case in­volved optic canal narrowing caused by bony thickening from familial hypophosphatemic rick­ets ( 267). Decompression for any cause of canal narrowing can be risky. A case of bilateral optic canal decom­pression for craniometaphyseal dysplasia is de­scribed in which there was devastating bilateral visual loss ( 268). Comparisons between pre- and postsurgical electrophysiological examinations may have prognositic value for these patients, and it was found that VEP latency is more sensitive than PERG in detecting compressive optic neurop­athies, but that PERG is more predictive of poor postsurgical prognosis ( 269). Compressive optic neuropathies, as in all causes of optic atrophy, can lead to optic disc excavation that may be confused as glaucomatous. Quantita­tive measurement of optic disc cupping in 29 pa­tients revealed a relative unilateral change in optic disc cupping in the absence of ocular hypertension ( 270). A relatively rare compressive optic neuropathy is the giant cerebral aneurysm that may be unre­sectable. Nineteen of 26 consecutive patients with inoperable giant aneurysms presenting with visual loss underwent embolization with detachable bal­loons and electrocoils; vision improved in 7, was unchanged in 11, and worsened in 11 ( 271). Smaller aneurysms deriving from the origin of the ophthalmic artery with the carotid may be harder to diagnose; at presentation they seem much like optic neuritis. In such a case, the initial MRI was normal and the diagnosis was made by repeat MRI; the authors point out that such aneurysms can grow rapidly. They emphasize the necessity of continued follow- up ( 272). Three cases of unex­plained visual loss after successful clipping of para-clinoid aneurysms were thought to be due to com­promise of large draining veins or to direct damage to the small dural arterioles that feed the perichi-asmal optic nerve ( 273). Traumatic optic neuropathy can be classified as direct and penetrating or indirect and consequent to bone displacement ( 274). Posttraumatic blind­ness after maxillofacial blunt trauma is discussed in 10 patients as being due to fractures of the greater wing of the sphenoid ( 275). Optic canal decompression was recommended for 2 of these patients. Warner and Lessell discuss various pathophysiological mechanisms that may come into play and call for a clinical study ( 276). The management of such cases by optic canal decom­pression is discussed by Levin and colleagues, who analyzed 31 cases of transethmoidal canal de­compression and found that vision improved the most in younger patients. The authors call for a randomized prospective trial that stratifies patients based on age ( 277). In a review of traumatic optic neuropathy, Steinsapir and Goldberg advocate high- dose i. v. corticosteroids and optic canal de­compression if vision does not improve with the steroids ( 278). Meningiomas may present with the classical triad of progressive, painless visual loss associated with optic atrophy and optic disc shunt vessels. A case report in which an altitudinal field defect led to the erroneous diagnosis of N- AION demon­strated that the diagnosis is not always easy ( 279). Chronic progressive visual loss in a 71- year- old woman was first ascribed, after CT imaging, to an optic nerve sheath meningioma. However, many years later, on autopsy, it was revealed that a large focus of metastatic adenocarcinoma existed within the meningioma, which had by now spread intra-cranially to involve the optic chiasm and contigu­ous structures ( 280). Improvements in imaging techniques may have led to more frequent diagno­sis of both kinds of meningiomas that cause optic nerve injury ( 281). A review of 57 patients affected by either optic nerve sheath or sphenoid wing me­ningiomas in whom surgical resection was at­tempted describes the postoperative visual out­comes as satisfactory ( to them but perhaps not to others) ( 282). A case is described in which external-beam irradiation led to significant visual improve­ment ( 281). However, despite newer modalities of radiation treatment, delayed injury to the optic nerve may occur. In a review of 215 optic nerves from 131 patients who had received fractionated external-beam irradiation, investigators concluded that op­tic neuropathy occurred in 17 nerves, 5 due to N- AION and 12 from optic neuritis, usually 2 or 3 years after the radiation therapy. They noted a strong dose- dependent relationship ( 283). Similar findings were found in a review of 219 patients in whom 19 developed optic nerve or optic chiasmal injury but none were found in whom the radiation / Neuro- Ophthalmol, Vol. 16, No. 3, 1996 THE ANTERIOR VISUAL PATHWAYS 217 dose was < 50 Gy units ( 5,000 rads) ( 284). Surpris­ingly, a patient with bilateral radiation optic neu­ropathy is described whose visual loss occurred only 3 months after completion of the radiotherapy ( 285). Radiation from brachytherapy plaquing for choroidal melanomas can lead to radiation optic neuropathy in > 50% of cases after 10 years' follow-up ( 286). Gliomas of the optic nerve are also common. An excellent review of optic nerve gliomas by Dutton finds them accounting for 65% of all intrinsic tu­mors of the optic nerve ( 287). Optic nerve gliomas are usually noted in young children, and their growth slows down after puberty. Fortunately, the mortality rate is only 5% when the tumor is con­fined to the optic nerve ( 287). In contradistinction, malignant optic nerve gliomas occur in middle-aged adults and are universally fatal ( 287). A case typical for optic nerve glioma is described in a 14- year- old girl; resection was carried out and the his-topathology turned out to be oligodendroglioma ( instead of the expected pilocytic astrocytoma) ( 288). Optic nerve gliomas are frequently found in as­sociation with neurofibromatosis ( von Reckling­hausen's disease). Is there a different course in op­tic nerve gliomas with and without neurofibroma­tosis type 1 ( NF- 1)? To ascertain this, 17 children with optic nerve gliomas associated with NF- 1 were compared with 19 with optic nerve gliomas without NF- 1. Differences included precocious pu­berty only in the NF- 1 group, and symptoms of increased intracranial pressure only in the non- NF- 1 group ( 289). Chiasmal and optic tract in­volvement were more common in the patients without NF- 1 ( 289). A review of 31 cases of optic nerve gliomas in NF- 1 noted that MR imaging was much better than high- resolution CT scans, al­though the latter did disclose all the tumors ( 290). Three patients with neurofibromatosis and optic nerve gliomas were followed for 10 to 20 years, and their prognosis remains good ( 291). A more comprehensive prospective study of 227 children with NF- 1 revealed that - 20% had tumors of the anterior visual pathways. The usual age of diagno­sis was ~ 4 years ( 292). Fairly long- term follow- up ( 0.2- 7.2 years) led the authors to conclude that op­tic nerve gliomas rarely progress significantly and that neuroimaging is only necessary in NF- 1 pa­tients with visual symptoms ( 292). However, other investigators, attempting early diagnosis of optic nerve glioma in cases of NF- 1, found tumors in 6 of 25 asymptomatic patients by MRI, whereas pattern shift VEPs were not helpful ( 293). In turn, a third study showed that pattern shift VEPs performed on 30 NF- 1 patients ( 10 children with and 20 chil­dren without MRI- demonstrated tumors) had 90% sensitivity ( but also 60% specificity that the au­thors argued reflected gliomas missed on MRI) ( 294). Finally, cerebrovascular abnormalities ( an­eurysms and vascular occlusions) are also ( rarely) associated with NF- 1 ( 295). Two papers described a hemangioblastoma aris­ing from the optic nerve in a patient with von Hip-pel- Lindau disease that led to progressive visual loss until surgically excised ( 296,297). Cerebellar, renal, pancreatic, and adnexal lesions may also be found ( 297). Two cases of cavernous hemangioma involving the retina and optic nerve were used to document continued growth over a 5- to 10- year period ( 298). Graves' dysthyroid ophthalmopathy is a fairly common disorder covered in a separate annual re­view. The disease can lead to optic neuropathy with several presentations. It can take on the fea­tures of N- AION ( 299) or may occur insidiously late ( 7 to 12 years) in the course of the ophthal­mopathy ( 300). Various tests such as VEPs ( 301) or chromatic contrast and discrimination sensitivity testing ( 302) may help determine the need for de­compressive surgery. Color Doppler imaging of venous flow in orbits of patients with Graves' dis­ease discloses severe venous stasis even to the point of reversed blood flow in the superior oph­thalmic vein ( 303). Sinus disease has also been associated with optic neuropathy. A case of presumed compressive op­tic neuropathy turned out to be caused by allergic aspergillus sinusitis ( 304). A case of mucocele of the sphenoid sinus ( a relatively unusual location) led to an initially unexplained compressive optic neuropathy that resolved on sphenoidotomy ( 305). Because intranasal sinus surgery may have the most unfortunate complication of trauma to the optic nerve, a prospective study of 150 CT scans was performed; 8% of all cases disclosed protru­sion of the optic nerve and canal into the sphenoid sinus ( 306). OPTIC CHIASM AND TRACTS Gliomas of the perichiasmal region are not un­common; have a much more unfavorable progno­sis than optic nerve gliomas; and, especially in younger patients, run a more aggressive course ( 307). In a review of 46 children with gliomas around the optic chiasm followed for a median of 6 years, the median age of diagnosis was 2 years and 33% had NF- 1. Of the children studied, 5 died ( 11%), 4 became blind, and 20 had serious endo- / Neum- Ophthalmol, Vol. 16, No. 3, 1996 218 A. A. SADUN AND R. M, RUBIN crinopathy ( 52% morbidity). The children with NF- 1 did not best ( 307). A review of 20 patients with gliomas of the optic chiasm and hypothalamic regions disclosed that 6 patients had NF- 1; for a mean 9- year follow- up, severe tumor progression occurred in only 7 pa­tients ( 35%), who tended to be younger and have NF- 1 ( 308). Precocious puberty was a common (- 40%) complication in children with gliomas of the optic chiasmal area with NF- 1 ( 309). A group of 45 patients with giant gliomas of the optic chiasm were retrospectively reviewed for 17 years. Surgi­cal resection led to improvement or stabilization of vision in 77%, and there was only a 6% mortality in the immediate postoperative period ( 310). In a re­view of 20 patients with gliomas of the optic chi­asm and hypothalamus, 6 were noted to have NF- 1, and postoperative morbidity occurred in 5 pa­tients ( 25%) ( 308). Another retrospective review of 33 patients with gliomas of the anterior visual pathways involving the optic chiasm and hypothalamus looked at var­ious treatments including debulking ( subtotal re­section), radiation, and/ or chemotherapy; 28 ( 85%) were still alive a mean of 11 years later ( 311). Oral etoposide was the treatment in 14 patients with unresectable gliomas of the optic chiasm and hy­pothalamic region. After a mean follow- up of 8 years, 8 had shown radiographic response or sta­bilization ( 312). In another case report, an infiltrat­ing glioma of the optic chiasm presented in a boy aged 16 months as gigantism thought to be caused by involvement of the somatostatinergic pathways ( 313). Pituitary adenomas are the most common cause of chiasmal syndrome. Perhaps, given their com­monness, there are few reports of them. Microad­enomas ( pituitary adenomas < 10 mm in diameter) are often picked up as incidental findings during CT or MR I scanning. How should these be man­aged? Because 15% of microadenomas become sig­nificantly enlarged, periodic follow- up is advised ( 314,315). In contradistinction to pilocytic or low- grade as­trocytomas, glioblastomas are a highly malignant cause of chiasmal syndrome. A biopsy- proven case of glioblastoma multiforme of the optic chiasm was presented ( 316). Other rare lesions described in­clude neurosarcoidosis of the optic chiasm that mascaraded as childhood glioma ( 317). Another case was tuberculous meningitis involving the op­tic chiasm and causing blindness ( 318). A 21- year-old woman presented with progressive loss of vi­sion and irregular menses; her MRI scan suggested a chiasm meningioma, but histopathology demon­strated a hemangioblastoma and further work- up re­vealed that she had von Hippel- Lindau disease ( 319). Lesions of the optic chiasm may be detectable on laboratory testing other than neuroimaging. VEP delays and the asymmetric distribution in both eyes ( crossed asymmetry) can be evidence of a compressive lesion of the chiasm ( 320). Trauma may also produce a chiasmal syndrome. In two cases, blunt frontal head trauma led to bitemporal visual field defects ( 321). Another case of traumatic chiasmal syndrome pointed out the difficulty of making this diagnosis on CT; yet, MRI may help ( 322). A prospective study of 29 patients with pituitary gliomas disclosed that although bitemporal visual field defects were the most common, they were usually asymmetrical. Generalized constriction was also fairly common ( 323). How much pressure can the optic chiasm withstand? This question was revisited in a retrospective study of 50 cases of chiasmal syndrome from pituitary adenomas and 17 control cases with measurements made by CT scanning. Visual disturbances only occurred when the optic chiasm was displaced - 10 mm upward ( 324). This conforms fairly closely to the older cal­culation that a pituitary tumor must be 3= 22 mm in diameter ( 12 mm to get to the chiasm + 10- mm displacement) to cause any visual effects. The optic chiasm is interesting for a number of reasons including being a simple model for neuro­nal specificity. How do the axons know whether to decussate or not? Guillery ( one of the great names in modern visual pathway neuroanatomy) and col­leagues studied why axons from the temporal ret­ina take an uncrossed course ( 325). Decussation is probably the default command; most of phylogeny is dominated by decussation. Optic nerve axons actually lose their retinotopic order as they pro­ceed posteriorly. These investigators propose that molecular cues act on neurons in the development of the chiasm ( 325). In the mouse, just before birth, crossed and uncrossed axons diverge in a 100 to 200-| xm zone in the center of the chiasm where there is a thin raphe of glial cells that produce stage- specific embryonic antigen 1 ( SSEA- 1). Tem­poral retinal fibers are turned back by this zone, thus suggesting that they are repulsed by SSEA- 1 ( 326). Despite the presence of molecular cues, there is apparently a local loss of retinopy ( fiber organiza­tion related to the retinal map/ visual field) in the optic chiasm. For example, injection of a label in the monkey lateral geniculate nucleus ( LGN) shows a gross disorder of fibers in the optic chiasm even though the order exists in the anterior optic / Naiw- Ophllmlmol, Vol. 16, No. 3, 1996 THE ANTERIOR V1SUAE PATHWAYS 219 nerve and posterior optic tract ( 327). Two unre­lated children had an inborn absence of an optic chiasm, forcing nasal retinal fibers to anomalously project ipsilaterally; this achiasmal condition had been dubbed the nondecussating retinofugal fiber syn­drome ( 328). The vascular anatomy of the human optic chi­asm was investigated in 85 autopsy specimens: The midline received a scanty watershed distribu­tion, making it the most vulnerable to compressive lesions ( 329). This is the latest of many hypotheses seeking to explain the common feature of bitem­poral hemianopia. It is strange then that a case of homonymous hemianopia is attributed to MRI-confirmed chiasmitis secondary to c/ s- platinum therapy ( 330). The anatomy of the human optic tract also re­ceived some attention. Investigators looking at normal human optic tracts found anatomical evi­dence of large- caliber fibers ( likely to reflect the M- cell contribution) segregated to the ventral sur­face of the tract compared to the small- caliber fi­bers thought to correspond to the P- cell pathway ( 331). Another anatomical study of the human op­tic tract involved three patients with postgenicu-late cerebral arteriovenous malformations in whom band optic atrophy was found in the con­tralateral optic disc ( 332). That would usually have been deemed evidence of retrograde transsynaptic degeneration ( which is more likely to occur in chil­dren or with long- standing lesions). However, the presence of remote vascular abnormalities causing deep venous drainage near the posterior optic tract offered a more plausible explanation ( 332). After the optic chiasm, not all retinofugal fibers proceed down the optic tracts to the LGN. Three hypothalamic nuclei also receive a direct retinal in­put presumably to help entrain the diurnal hypo­thalamic clock. Some evidence exists that this ret­inal projection to the hypothalamus also benefits the neuroendocrine " orchestra"; light deprivation can lead to seasonal affective disorder ( SAD). In­cidentally, the bright light that successfully miti­gates SAD has no deleterious effect on the eye or anterior visual pathways ( 333). In the rat, tracer experiments labeled the supra-chiasmatic nucleus of the hypothalamus, and in­vestigators followed the fibers back to what ap­peared to be type W ( cat X, Y, W system) retinal ganglion cells in the contralateral retina ( 334). What do projections to the pulvinar subserve? A patient with a hematoma of the left pulvinar showed impaired stereoacuity, without the losses of reaction times for targets in the contralateral pe­riphery previously noted in animal studies ( 335). Morphometric measurements carried out on the LGNs of 21 human fetuses at gestational ages 13 to 38 weeks demonstrated a growth spurt at 15 to 20 weeks simultaneous with a dying back reflected by the presence of pyknotic cells ( 336). In a follow- up study, the same investigators demonstrated by ul-trastructural examination that synapses first occur in the human LGN at about gestational week 14 and take on the more complex triad arrangement at - 21 weeks ( 337). Deoxyhemoglobin- sensitive functional MRI permitted the detection of simulta­neous signal in the LGN and primary visual cortex in the human brain during periodic photic stimu­lation, hence establishing a physiological manifes­tation of this basic anatomy ( 338). Parallel processing through the LGN can be measured a number of ways. For example, it has been thought that the P- cell system ( or midget sys­tem) subserves visual afterimages. However, Schiller and Dolan found that ablating the parvo-cellular LGN ( the P- cells) did not eliminate the vi­sual aftereffects in monkeys ( 339). A good review article traces visual neurotrans­mission from retinal G- proteins to the visual cortex and points out that whereas the retina has a circu­lar center- surround physiology, the LGN uses rectangular receptive fields surrounded by antag­onistic areas ( 340). As this review also illustrates, information processing in the human visual sys­tem is a complex and wondrous thing that goes beyond the simple concepts of information relays or even the cascade specification first illustrated by Hubel and Weisel. The visual pathways do much more than send­ing information along, and far more than generat­ing higher and higher levels of abstraction from visual images. The visual pathways are in fact a fully integrated part of the human brain; their study brings insight into the structure, function, and very nature of the human brain. 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