Is perception of light useful to the blind patient?

Structural and functional consequences of PAX6 mutations in the brain: Implications for aniridia

The paired-box 6 (PAX6) gene encodes a highly conserved transcription factor essential for the proper development of the eye and brain. Heterozygous loss-of-function mutations in PAX6 are causal for a condition known as aniridia in humans and the Small eye phenotype in mice. Aniridia is characterized by iris hypoplasia and other ocular abnormalities, but recent evidence of neuroanatomical, sensory, and cognitive impairments in this population has emerged, indicating brain-related phenotypes as a prevalent feature of the disorder. Determining the neurophysiological origins of brain-related phenotypes in this disorder presents a substantial challenge, as the majority of extra-ocular traits in aniridia demonstrate a high degree of heterogeneity. Here, we summarize and integrate findings from human and rodent model studies, which have focused on neuroanatomical and functional consequences of PAX6 mutations. We highlight novel findings from PAX6 central nervous system studies in adult mammals, and integrate these findings into what we know about PAX6's role in development of the central nervous system. This review presents the current literature in the field in order to inform clinical application, discusses what is needed in future studies, and highlights PAX6 as a lens through which to understand genetic disorders affecting the human nervous system.

Incidence and risk factors for traumatic intraocular pressure elevation and traumatic glaucoma after open-globe injury

PURPOSE

To examine traumatic intraocular pressure (IOP) elevation and glaucoma after open-globe injury.

DESIGN

Retrospective, observational case series.

METHODS

Review of patients with open-globe repair at the University of Washington from May 1997 through July 2010. Traumatic IOP elevation and glaucoma were defined respectively as intraocular pressure (IOP) ≥22 mm Hg at >1 visit or need for glaucoma medication, and long-term (at least 3 months) glaucoma medication use or glaucoma surgery.

RESULTS

We included 515 eyes (515 patients). The mean follow-up was 12.6±20.1 months. One hundred twenty eyes (23.3%) developed traumatic IOP elevation, of which 32 (6.2%) developed glaucoma; six eyes (1.2%) required glaucoma surgery. The mean time to development of traumatic IOP elevation was 1.5±3.4 months (range 1 day to 2 years). Kaplan-Meier 6- and 12-month estimates for development of traumatic IOP elevation were 27.2 and 32.4%, respectively, and for development of traumatic glaucoma were 7.1 and 11.0%, respectively. Multivariate regression revealed associations between traumatic IOP elevation and older age, and traumatic glaucoma and prior penetrating keratoplasty, initial vitreous hemorrhage, Zone II injury, and penetrating keratoplasty after open-globe repair. Traumatic glaucoma was controlled (IOP <22 mm Hg) in 78.1% of eyes at final follow-up, with mean IOP of 18.2 mm Hg on 1.7 medications.

CONCLUSIONS

Traumatic IOP elevation and glaucoma were common after visually salvageable open-globe injury. Most cases developed within 6 months, although longer follow-up remains important for case detection. Penetrating keratoplasty before or after repair, and vitreous hemorrhage were notable risk factors.

Retinal Prostheses for the Blind

Introduction: Using artificial means to treat extreme vision impairment has come closer to reality during the past few decades. The goal of this research has been to create an implantable medical device that provides useful vision for those patients who are left with no alternatives. Analogous to the cochlear implants for some forms of hearing loss, these devices could restore useful vision by converting visual information into patterns of electrical stimulation that excite the remaining viable inner retinal neurons in patients with retinitis pigmentosa or age-related macular degeneration. Methods: Data for this review were selected through a comprehensive literature search. Results: Advances in microtechnology have facilitated the development of a variety of prostheses that can be implanted in the visual cortex, around the optic nerve, or in the eye. Some of these approaches have shown the promise of providing useful visual input to patients with visual impairments. Conclusion: While the development of various retinal prostheses have shown promise in limited clinical trials, there are distinct advantages and disadvantages for each type of prosthesis. This review will focus primarily on the Epiretinal Intraocular Retinal Prosthesis, studied by our group, but will also briefly review other modalities: the subretinal prosthesis, cortical prosthesis, and optic nerve prosthesis.

Stimulation of the retina with a multielectrode extraocular visual prosthesis

BACKGROUND

An extraocular approach to developing a retinal prosthesis for blind patients using electrodes placed on the outer surface of the eye is suggested. Experiments were carried out to determine the feasibility of this approach, and evaluate electrode configurations and parameters for stimulation.

METHODS

In anaesthetized cats, a 21-electrode extraocular retinal prosthesis (ERP) array was sutured to the sclera over the lateral surface of the eye. Electrically evoked potentials (EEP) were recorded at the visual cortex bilaterally in response to retinal stimulation with the electrode array. Bipolar stimulation of the ERP array electrodes in horizontal and vertical configurations and at different interelectrode separations was investigated with biphasic constant-current pulses.

RESULTS

Electrical stimulation of the lateral retina with an ERP elicited EEP that were higher in the ipsilateral visual cortex. The threshold for bipolar retinal stimulation was 500 microA. EEP amplitude increased with increases in stimulus pulse duration and current intensity. Retinal stimulation was slightly more effective with electrodes in a vertical as opposed to horizontal orientation. A larger interelectrode separation resulted in a higher EEP amplitude.

CONCLUSIONS

Retinal stimulation with a prototype ERP array is demonstrated. The thresholds for retinal excitation are below safe charge-density limits for chronic neural stimulation. Ipsilateral localization of the EEP suggests that localized retinal stimulation is occurring. An ERP is a new approach to retinal prosthesis research, and might lead to the development of a low-resolution visual prosthesis for blind patients.

Advances in the development of visual prostheses

Visual prostheses are based on neuronal electrical stimulation at different locations along the visual pathway (ie, cortical, optic nerve, epiretinal, subretinal). In terms of retinal prostheses, advances in microtechnology have allowed for the development of sophisticated, high-density integrated circuit devices that may be implanted either in the subretinal or epiretinal space. Analogous to the cochlear implants for some forms of deafness, these devices could restore useful vision by converting visual information into patterns of electrical stimulation that would excite the remaining spared inner retinal neurons in patients with diseases such as retinitis pigmentosa and age-related macular degeneration. The different types of implants and recent results are discussed, but special emphasis is given to retinal implants.

LINAC based stereotactic radiotherapy of uveal melanoma: 4 years clinical experience

PURPOSE

To study local tumor control and radiogenic side effects after fractionated LINAC based stereotactic radiotherapy for selected uveal melanoma.

PATIENTS AND METHODS

Between June 1997 and March 2001, 90 patients suffering from uveal melanoma were treated at a LINAC with 6 MV. The head was immobilized with a modified stereotactic frame system (BrainLAB). For stabilization of the eye position a light source was integrated into the mask system in front of the healthy or the diseased eye. A mini-video camera was used for on-line eye movement control. Tumors included in the study were either located unfavorably with respect to macula and optical disc (<3 mm distance) or presented with a thickness >7 mm. Median tumor volume was 305+/-234 mm3 (range 70-1430 mm3), and mean tumor height was 5.4+/-2.3 mm (range 2.7-15.9 mm). Total doses of 70 (single dose 14 Gy @ 80% isodose) or 60 Gy (single dose 12 Gy @ 80% isodose) were applied in five fractions within 10 days. The first fractionation results in total dose (TD) (2 Gy) of 175 Gy for tumor and 238 Gy for normal tissue, corresponding values for the second fractionation schedule are 135 and 180 Gy, respectively.

RESULTS

After a median follow-up of 20 months (range 1-48 months) local control was achieved in 98% (n=88). The mean relative tumor reductions were 24, 27, and 37% after 12, 24 and 36 months. Three patients (3.3%) developed metastases. Secondary enucleation was performed in seven patients (7.7%). Long term side effects were retinopathy (25.5%), cataract (18.9%), optic neuropathy (20%), and secondary neovascular glaucoma (8.8%).

CONCLUSION

Fractionated LINAC based stereotactic photon beam therapy in conjunction with a dedicated eye movement control system is a highly effective method to treat unfavorably located uveal melanoma. Total doses of 60 Gy (single dose 12 Gy) are considered to be sufficient to achieve good local tumor control.

Visual and electrical evoked response recorded from subdural electrodes implanted above the visual cortex in normal dogs under two methods of anesthesia

Sensitive methods are required to record electrical evoked potentials over the visual cortex to evaluate the efficacy and safety of a retinal prosthesis before it can be implanted on the retinal surface of patients afflicted by outer retinal diseases. This study was designed to examine subdural electrodes as a mean to evaluate cortical evoked potentials in response to light and electrical stimulation of the retina in three dogs under two methods of anesthesia-halothane and propofol. Results showed that subdural electrodes could be stabilized over the visual cortex for several (3-5) months, and that they were 6.95 times more sensitive than subdermal electrodes in recording cortical visual evoked potentials (VEPs) and 4.31 times more sensitive in recording cortical electrical evoked potentials under both methods of anesthesia. The waveforms' shape changed for each electrode in the subdural array during 6/6 (100%) and 20/38 (52%) multi-channel recording sessions under halothane and propofol, respectively. This change could point to a cortical retinotopic organization versus hierarchical organization of different cortical areas for a given retinal stimulus. In summary, subdural electrodes show promising results for recording visual and electrical evoked responses (EERs) and thus for evaluation of the retinal prosthesis.

Retinal prosthesis for the blind

Most of current concepts for a visual prosthesis are based on neuronal electrical stimulation at different locations along the visual pathways within the central nervous system. The different designs of visual prostheses are named according to their locations (i.e., cortical, optic nerve, subretinal, and epiretinal). Visual loss caused by outer retinal degeneration in diseases such as retinitis pigmentosa or age-related macular degeneration can be reversed by electrical stimulation of the retina or the optic nerve (retinal or optic nerve prostheses, respectively). On the other hand, visual loss caused by inner or whole thickness retinal diseases, eye loss, optic nerve diseases (tumors, ischemia, inflammatory processes etc.), or diseases of the central nervous system (not including diseases of the primary and secondary visual cortices) can be reversed by a cortical visual prosthesis. The intent of this article is to provide an overview of current and future concepts of retinal and optic nerve prostheses. This article will begin with general considerations that are related to all or most of visual prostheses and then concentrate on the retinal and optic nerve designs. The authors believe that the field has grown beyond the scope of a single article so cortical prostheses will be described only because of their direct effect on the concept and technical development of the other prostheses, and this will be done in a more general and historic perspective.

Functional MRI of congenital hyposmia: brain activation to odors and imagination of odors and tastes

PURPOSE

Our goal was to use functional MRI (fMRI) to define brain activation in response to odors and imagination ("memory") of odors and tastes in patients who never recognized odors (congenital hyposmia).

METHOD

Functional MR brain scans were obtained in nine patients with congenital hyposmia using multislice echo planar imaging (EPI) in response to odors of amyl acetate, menthone, and pyridine and to imagination ("memory") of banana and peppermint odors and to salt and sweet tastes. Functional MR brain scans were compared with those in normal subjects and patients with acquired hyposmia. Activation images were derived using correlation analysis, and ratios of areas of brain activated to total and hemispheric brain areas were calculated. Total and hemispheric activated pixel counts were used to quantitate regional brain activation.

RESULTS

Brain activation in response to odors was present in patients with congenital hyposmia. Activation was significantly lower than in normal subjects and patients with acquired hyposmia and did not demonstrate differential vapor pressure-dependent detection responsiveness or odor response lateralization. Regional activation localization was in anterior frontal and temporal cortex similar to that in normal subjects and patients with acquired hyposmia. Activation in response to presented odors was diverse, with a larger group exhibiting little or no activation with localization only in anterior frontal and temporal cortex and a smaller group exhibiting greater activation with localization extending to more complex olfactory integration sites. "Memory" of odors and tastes elicited activation in the same central nervous system (CNS) regions in which activation in response to presented odors occurred, but responses were significantly lower than in normal subjects and patients with acquired hyposmia and did not lateralize.

CONCLUSION

Odors induced CNS activation in patients with congenital hyposmia, which distinguishes olfaction from vision and audition since neither light nor acoustic stimuli induce CNS activation. Odor activation localized to anterior frontal and temporal cortex, consistent with the hypothesis that olfactory pathways are hard-wired into the CNS and that further pathways are undeveloped with primary olfactory system CNS connections but lack of secondary connections. However, some patients exhibited greater odor activation with response localization extending to cingulate and opercular cortex, indicating some olfactory signals impinge on and maintain secondary connections consistent with similar functions in vision and audition. Activation localization of taste "memory" to anterior frontal and temporal cortex is consistent with CNS plasticity and cross-modal CNS reorganization as described for vision and audition. Thus, there are differences and similarities between olfaction, vision, and audition, the differences dependent on unique qualities of olfaction, perhaps due to its diffuse, primitive, fundamental role in survival. Response heterogeneity to odors may reflect heterogeneous genetic abnormalities, independent of anatomic or hormonal changes but dependent on molecular abnormalities in growth factor function interfering with growth factor/stem cell interactions. Patients with congenital hyposmia offer an unique model system not previously explored in which congenital smell lack as measured by fMRI is reflective of congenital dysfunction of a major sensory system.