The Visual Pathway and Related Disorders

Visual Pathway

• Light enters the eye and projects onto the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy, which contains photoreceptors:
  • Cones:
    • Operate in bright-light conditions
    • Responsible for visual acuity Visual Acuity Clarity or sharpness of ocular vision or the ability of the eye to see fine details. Visual acuity depends on the functions of retina, neuronal transmission, and the interpretative ability of the brain. Normal visual acuity is expressed as 20/20 indicating that one can see at 20 feet what should normally be seen at that distance. Visual acuity can also be influenced by brightness, color, and contrast. Ophthalmic Exam and color perception Perception The process by which the nature and meaning of sensory stimuli are recognized and interpreted. Psychiatric Assessment
  • Rods:
    • Operate in dim-light conditions
    • Responsible for low-light vision Vision Ophthalmic Exam
• Signals from the retinal photoreceptors  → go to bipolar Bipolar Nervous System: Histology cells  → then to the ganglion cells
• Axons Axons Nerve fibers that are capable of rapidly conducting impulses away from the neuron cell body. Nervous System: Histology of the ganglion cells converge, forming the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions (CN II).
• Optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions to the optic tract:
  • Optic chiasm (OC): where hemidecussation of optic nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology occurs
  • Optic tract:
    • Nasal fibers from the left eye Left Eye Refractive Errors cross over to join the temporal fibers of the right eye Right Eye Refractive Errors → right optic tract
    • Nasal fibers from the right eye Right Eye Refractive Errors cross over to join the temporal fibers of the left eye Left Eye Refractive Errors → left optic tract
• From the optic tract, most fibers synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission in the lateral geniculate nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles (LGN) of the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus: Anatomy, with some going to the superior colliculus Superior Colliculus The anterior pair of the quadrigeminal bodies which coordinate the general behavioral orienting responses to visual stimuli, such as whole-body turning, and reaching. Cranial Nerve Palsies. 
• From each LGN, neurons Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology travel in the optic radiations (geniculocalcarine fibers):
  • Inferior fibers:
    • Inferior radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma is on the lateral side (lateral bundle) of the optic radiations.
    • Nerves extend anteriorly around the temporal horn of the lateral ventricle before proceeding posteriorly (Meyer’s loop).
    • Carry the information from the superior visual fields
  • Superior fibers:
    • The fibers that go superiorly, passing the parietal lobe Parietal lobe Upper central part of the cerebral hemisphere. It is located posterior to central sulcus, anterior to the occipital lobe, and superior to the temporal lobes. Cerebral Cortex: Anatomy, are on the medial side of the optic radiations.
    • Carry the information from the inferior visual fields
• Primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy (striate or calcarine cortex, or V1 or area 17):
  • Optic radiations head posteriorly to the primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy.
  • The primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy area is mostly in the medial brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification surface of the occipital lobe Occipital lobe Posterior portion of the cerebral hemispheres responsible for processing visual sensory information. It is located posterior to the parieto-occipital sulcus and extends to the preoccipital notch. Cerebral Cortex: Anatomy.

Visual Fields

Anterior to the OC

• Retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy and optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions: carry visual information on the ipsilateral or same eye where the nerve is located
• OC:
  • Visual fields of each eye overlap considerably, to produce binocular vision Vision Ophthalmic Exam and depth perception Depth perception Perception of three-dimensionality. Albinism. 
  • The image from each half of the visual field is processed by the contralateral hemisphere. 
  • Left visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy (blue line) processes the information from the right half of the visual field. 
  • Right visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy (red line) processes the information from the left half of the visual field.
  • In order for each hemisphere to receive visual information from the contralateral visual field, the nasal fibers cross at the OC. 

Posterior to the OC

• Visual field information is processed in the contralateral hemisphere.
• Optic tract and LGN:
  • Left visual field: represented by the right optic tract and LGN
  • Right visual field: represented by the left optic tract and LGN
• Optic radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma and visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy:
  • From the LGN, neurons Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology on both sides course to the visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy.
  • Inferior visual field:
    • Carried by the superior part of the optic radiations 
    • Goes to the superior bank of the primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy
  • Superior visual field:
    • Carried by the inferior part of the optic radiations (through Meyer’s loop)
    • Goes to the inferior bank of the primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy

Visual Field Defects

Unilateral visual field defect

• Ocular pathology (e.g., lens Lens A transparent, biconvex structure of the eye, enclosed in a capsule and situated behind the iris and in front of the vitreous humor (vitreous body). It is slightly overlapped at its margin by the ciliary processes. Adaptation by the ciliary body is crucial for ocular accommodation. Eye: Anatomy, retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy) or complete optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions pathology:
  • Visual deficit: limited to 1 eye, same side of the lesion (monocular visual loss)
  • Contralateral eye sees normally.
• Partial lesion of the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions:
  • If temporal fibers are affected: ipsilateral blindness Blindness The inability to see or the loss or absence of perception of visual stimuli. This condition may be the result of eye diseases; optic nerve diseases; optic chiasm diseases; or brain diseases affecting the visual pathways or occipital lobe. Retinopathy of Prematurity of the nasal visual field (nasal hemianopsia)
  • If nasal fibers are affected: ipsilateral blindness Blindness The inability to see or the loss or absence of perception of visual stimuli. This condition may be the result of eye diseases; optic nerve diseases; optic chiasm diseases; or brain diseases affecting the visual pathways or occipital lobe. Retinopathy of Prematurity of the temporal visual field (temporal hemianopsia)

Bilateral visual field defects

• Site of lesion is the OC:
  • Where nasal fibers cross: Temporal visual fields are affected.
  • Visual defect: bitemporal heteronymous (different visual field affected in each eye) hemianopsia (loss of half of the visual field)
• Site of lesion is the optic tract or lateral geniculate body (LGB):
  • Visual deficit is on the side contralateral to the lesion.
  • Contralateral homonymous (similar deficit in each eye) hemianopsia (loss of half of the visual field)
• Site of lesion is the optic radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma or primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy:
  • Visual deficit is on the side contralateral to the lesion.
  • Optic radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma:
    • Inferior radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma or Meyer’s loop: contralateral superior quadrantanopsia
    • Superior radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma: contralateral inferior quadrantanopsia
    • All of optic radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma: contralateral homonymous hemianopsia
  • Primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy lesion: 
    • Inferior bank of the primary visual  cortex: contralateral superior quadrantanopsia
    • Superior bank of the primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy: contralateral inferior quadrantanopsia
    • Primary visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy: contralateral homonymous hemianopsia

Synopsis

Clinical Relevance

• Ischemic optic neuropathy Neuropathy Leprosy (ION): a condition secondary to reduced blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure leading to optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions damage. Risk factors include age > 50, diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus, and hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension. Ischemic optic neuropathy Neuropathy Leprosy can be arteritic (due to giant cell arteritis Giant Cell Arteritis Giant cell arteritis (GCA), also known as temporal arteritis, is a type of large-vessel vasculitis that predominantly affects the aorta and its major branches, with a predilection for the branches of the carotid (including the temporal artery). Giant cell arteritis is defined by inflammatory leukocytes in the vessel walls leading to reactive damage, ischemia, and necrosis. Giant Cell Arteritis) or non-arteritic (other causes), which is more frequent. Non-arteritic ION presents as an acute, painless monocular visual loss associated with an altitudinal visual field defect. 
• Middle cerebral artery Middle cerebral artery The largest of the cerebral arteries. It trifurcates into temporal, frontal, and parietal branches supplying blood to most of the parenchyma of these lobes in the cerebral cortex. These are the areas involved in motor, sensory, and speech activities. Cerebrovascular System: Anatomy (MCA) infarction: the middle cerebral artery Middle cerebral artery The largest of the cerebral arteries. It trifurcates into temporal, frontal, and parietal branches supplying blood to most of the parenchyma of these lobes in the cerebral cortex. These are the areas involved in motor, sensory, and speech activities. Cerebrovascular System: Anatomy supplies the frontal Frontal The bone that forms the frontal aspect of the skull. Its flat part forms the forehead, articulating inferiorly with the nasal bone and the cheek bone on each side of the face. Skull: Anatomy, parietal Parietal One of a pair of irregularly shaped quadrilateral bones situated between the frontal bone and occipital bone, which together form the sides of the cranium. Skull: Anatomy, and temporal lobes. Ischemic stroke Ischemic Stroke An ischemic stroke (also known as cerebrovascular accident) is an acute neurologic injury that occurs as a result of brain ischemia; this condition may be due to cerebral blood vessel occlusion by thrombosis or embolism, or rarely due to systemic hypoperfusion. Ischemic Stroke affecting the MCA area can manifest as contralateral homonymous hemianopsia. Presentation is usually with other findings including contralateral hemiparesis Hemiparesis The term hemiparesis refers to mild to moderate weakness involving one side of the body. Epidural Hemorrhage and sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology loss with aphasia Aphasia A cognitive disorder marked by an impaired ability to comprehend or express language in its written or spoken form. This condition is caused by diseases which affect the language areas of the dominant hemisphere. Clinical features are used to classify the various subtypes of this condition. General categories include receptive, expressive, and mixed forms of aphasia. Ischemic Stroke.
• Pituitary Pituitary A small, unpaired gland situated in the sella turcica. It is connected to the hypothalamus by a short stalk which is called the infundibulum. Hormones: Overview and Types adenoma: benign Benign Fibroadenoma tumor Tumor Inflammation of the pituitary Pituitary A small, unpaired gland situated in the sella turcica. It is connected to the hypothalamus by a short stalk which is called the infundibulum. Hormones: Overview and Types gland, which sits inferior to the OC. Suprasellar extension Extension Examination of the Upper Limbs of the adenoma compresses the OC and produces bitemporal hemianopsia. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship also have headaches and symptoms of pituitary Pituitary A small, unpaired gland situated in the sella turcica. It is connected to the hypothalamus by a short stalk which is called the infundibulum. Hormones: Overview and Types dysfunction: infertility Infertility Infertility is the inability to conceive in the context of regular intercourse. The most common causes of infertility in women are related to ovulatory dysfunction or tubal obstruction, whereas, in men, abnormal sperm is a common cause. Infertility and galactorrhea Galactorrhea Excessive or inappropriate lactation in females or males, and not necessarily related to pregnancy. Galactorrhea can occur either unilaterally or bilaterally, and be profuse or sparse. Its most common cause is hyperprolactinemia. Hyperprolactinemia, among others.
• Foster-Kennedy syndrome: presents with bilateral visual problems and is due to an enlarging intracranial mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast compressing on the ipsilateral optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions, resulting in ipsilateral optic neuropathy Neuropathy Leprosy. Blurry vision Vision Ophthalmic Exam is bilateral because the increased intracranial pressure Intracranial Pressure Idiopathic Intracranial Hypertension from the mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast causes papilledema Papilledema Swelling of the optic disk, usually in association with increased intracranial pressure, characterized by hyperemia, blurring of the disk margins, microhemorrhages, blind spot enlargement, and engorgement of retinal veins. Chronic papilledema may cause optic atrophy and visual loss. Idiopathic Intracranial Hypertension on the contralateral eye.