Pathologies of the optic nerve and of the visual pathways
Neurophysiology of vision and neurophthalmology
Neurophysiology of vision studies the physiological mechanisms that allow visual perception. It assesses the complex transduction mechanisms of the luminous stimulus in electrical stimulation and its transmission from retinal photoreceptors to the visual cortex.
Currently it is possible to make an objective assessment of the functionality of the different elements that form visual pathways through specific electrofunctional methods:
a) The Electroretinogram (ERG)
ERG is the retinal bioelectric response to a visual stimulus, whether it is a strobe light flash or a television monitor with black and white bars or chess alternating in time (pattern). The ERG flash is an expression of the functionality of the outer retinal layers (pigmented epithelium and photoreceptors, bipolar cells, amacrine cells) and is characterized by a series of alternating polarity waves among which we recognize wave a, wave b and Potential Oscillators (PO). The pattern electroretinogram (PERG) is generated by the most internal retinal layers (cells and ganglionary fibers) and in its trace it is possible to identify peaks that are marked with the letter indicating the polarity and the digit indicating the latency time, namely the time in milliseconds after the presentation of the visual stimulus in which the peak appears on the electroretinographic trace (N35, P50 and N95).
The ERG flash response is a mass response of the whole retina and the contribution made to the genesis of this electrofunctional response by the macular region can be considered negligible. The functionality of localized retinal areas (as the macula) can be evaluated by a particular recording of the electrorestrainographic signal: the focal ERG (F-ERG)or the multifocal ERG (mfERG). Changes in the individual bioelectric responses (ERG, PERG, F-ERG or mf-ERG) are indicative of a dysfunction of the various retinal elements that generated the specific electroretinographic signal recorded.
b) Visual Evoked Potentials (VEPs)
Visual Evoked Potentials (VEPs) are defined as variations of the bioelectrical potentials of the occipital cortex evoked by visual stimuli. They are, therefore, the expression of refined and complex neurosensitive events related to the transduction and transmission phenomena of the nervous impulse along the visual pathways, that is to say from the retinal photoreceptors to the occipital cerebral cortex. The visual stimulus can be provided both by a flash and a pattern. Pattern transient VEP is characterized by a series of waves of alternating polarity in which it is possible to distinguish peaks that are defined by a letter, indicating polarity, and a number to indicate latency time: N75, P100 and N145. The increase in latency time and the decrease in amplitude of the various VEP waves represent the electrofunctional equivalent of a slowing down of the nervous transmission along the visual pathways. This pathological aspect can be ascribed to a primary involvement of retinal photoreceptors, ganglion cells, functional alterations of the macular region and even a delay in the conduction at the level of the central nervous system, that is between the retina and visual cortex. In order to correctly diagnose functional damage, it is essential to associate VEPs with various types of ERGs and, in particular, in order to obtain specific information on post-retinal nervous conduction, it is particularly useful to simultaneously record VEP and PERG, in which the difference between the P100 latency time of the VEP (expression of the occipital response) and the latency time of PERG P50 (expression of the maximum activity of the ganglionary cells) is referred to as "retino-cortical time".
Neuro-ophthalmology. The loss of visual function can be linked not only to eye diseases but also to pathologies that may involve those portions of the brain (optical nerve, optical chiasm, optical tract, lateral geniculate nucleus, optical radiation, occipital cerebral cortex) used for the transmission and decoding of visual information. In this context, the pathologies of the optical nerve are particularly important, which may be due to diabetes, glaucoma, demyelinating pathologies (multiple sclerosis), ischaemic phenomena, toxic or iatrogenic processes, infectious processes, autoimmune diseases (SLE, rheumatoid arthritis), hypo-hydrotoxic diseases, neurodegenerative pathologies (degenerative ataxias, Alzheimer's disease, Parkinson's disease) neoplastic processes, trauma. In addition, many pathologies of the optic nerve are due to genetic mutations such as Leber hereditary optic neuropathy or the dominant or recessive hereditary optic neuritis. The pathologies of the optic nerve manifest themselves with a progressive or acute loss of visual acuity, with a deficit of the visual field, with changes in chromatic perception or contrasts. Depending on the localization of the neuritis process, the pathologies of the optic nerve may be anterior (and in this case typical modifications of the head of the optic nerve or ophthalmoscopic visible optic papilla) or retrobulbarinis (non-negativity of the ophthalmoscopic examination are observed).
In order to provide a correct diagnostic framework, an accurate anamnestic connection, a visual acuity assessment even after cycloplegia, eye motility tests, perimeter or campimetric examinations, electrofunctional tests (ERG, PERG, F-ERG, mf-ERG, VEP), morphological retinal and optic nerve evaluations by OCT examinations, a targeted neuroradiological diagnosis are essential. The therapy, which must be established as early as possible, is mainly based on cortisone or drugs such as cyticoline.
Other pathologies of the central nervous system such as migraine headaches (with or without visual aura), TIA, ictus, cerebral infarctions, neoplastic pathologies, head injuries, can induce visual perception deficits or specific losses of part of the visual field. In this case, the diagnostic framework is very similar to that of neuroticopathies, while as far as therapy is concerned, it is linked to the basic pathology.
In addition, various pathologies that recognize inflammatory, vascular, neoplastic or toxic, metabolic etiology may induce other unnecessary visual system dysfunctions associated with changes in visual perception such as diplopia (paralytic strabisms), adaptive deficits, ptosis or abnormalities of the pupil diameter. Also in this case the diagnostic framework is very similar to that of neuroticopathies and the therapy is related to the basic pathology.