Homonymous hemianopsia (or homonymous hemianopia, HH) is a field loss deficit in the same halves of the visual field of each eye. This condition most commonly results from stroke for adults, or tumors/lesions for patients under the age of 18. Often, the cause of HH is located at the occipital lobe, followed by an injury to the optic radiations or optic tract. HH can also be characterized as contralateral hemianopsia (unilateral involvement at the optic tract, lateral geniculate nucleus, optic radiations, or occipital cortex opposite to the side of field loss) in contrast to bitemporal hemianopsia (involvement at the optic chiasm).
Homonymous hemianopsia frequently results from vascular injury. In adults, cerebral infarcts and intracranial hemorrhages being the most common (42% to 89%). They are followed by tumors, trauma, iatrogenic events, and neurologic disease. Pediatric cases often originate from neoplasms (39%), stroke (25%), and trauma (19%).
Structure and Function
The optic nerves run near the midline inferior to the frontal lobe, and superior to the cavernous sinus on each side. Their purpose is to transfer retinal information to the appropriate region of the occipital primary visual cortex. The right half of each eye's visual field is recorded by the left half of each retina and vice versa. Ganglion cells within the retina have sensory afferent fibers that course through the optic chiasm. At the chiasm, the nasal portion of each optic nerve crosses to the opposite side, such that each eye transmits information through its ipsilateral temporal fibers and the contralateral nasal fibers to the brain. By crossing at the optic chiasm, all fibers recording the left visual field of both eyes project to the right hemisphere, and the same applies to the right visual field. The optic chiasm is located at the midline inferior to the hypothalamus, and superior to the pituitary gland. Once distal to the chiasm, the fibers collectively are named the optic tract. The tract then communicates with the lateral geniculate nucleus (LGN) within the thalamus. From the thalamus, the fibers then enter the optic radiations before arriving at the primary visual cortex within the occipital lobe. At the same time, some fibers from the optic tract also communicate with the superior colliculus, pretectal nuclei, and suprachiasmatic nuclei, which are involved in the light reflex.
Development and differentiation of the eyes begin around week three of gestation. The retina and optic nerves are derived from neural ectodermal tissue. As a result, the optic nerve is technically part of the central nervous system. Further supporting this claim is the fact that the myelin sheaths surrounding the optic nerve are arranged by oligodendrocytes as opposed to Schwann cells. At about the seventh week of gestation, axons from the retinal ganglion cells begin to extend into the optic stalk, eventually forming the optic nerve.
The major vascular supply to the anterior intracranial structures is the circle of Willis and its associated branches. The optic nerve is supplied by the internal carotid artery (ICA), specifically the superior hypophyseal and ophthalmic arteries. The primary supply to the optic chiasm is delivered by the anterior cerebral (ACA), anterior communicating, posterior communicating, and superior hypophyseal arteries. The optic tract is fed by the ICA and posterior communicating arteries. The LGN is mostly supplied by the ICA and posterior cerebral artery (PCA). The optic radiations are divided into anterior and posterior portions, and so are their vascular supplies. Branches from the circle of Willis and the middle cerebral artery (MCA) supply the anterior proximal sections, whereas branches of the PCA supply the posterior distal radiations. A majority of the visual cortex is supplied from branches of the PCA. However, there are also regions of the occipital pole that are additionally supplied by branches of the MCA.
Each optic nerve transports information from the retinal ganglion cells to the optic chiasm and through the retrochiasmal pathway to varying areas of the brain for use in a number of ways. There exist nearly 2.4 million fibers from the optic nerve that then pass through to the optic chiasm. As mentioned earlier, the temporal fibers communicate visual information to the lateral portions of the optic chiasm, occupying about 35-45% of the chiasm before continuing to the ipsilateral LGN.
As a result, each optic tract leaving the chiasm contains a mix of contralateral nasal (crossed) and ipsilateral temporal (uncrossed) fibers. These fibers receive information from their opposite visual field - the left temporal fibers view the left nasal field. This arrangement coordinates the left retrochiasmal visual pathway with the right half of each eye's visual field and vice versa. Both optic tracts travel near the basal ganglia, internal capsule, and cerebral peduncle on each side.
Nearly 80% of nerve fibers from the optic tract enter the ipsilateral lateral geniculate nucleus. The remainder project to the Edinger-Westphal nuclei (afferent signal for pupillary light reflex). The LGN contains an array of axons distributed in various laminae depending on whether the signal comes from the ipsilateral or contralateral eye. Such a retinotopic organization is crucial to properly merge binocular images at the occipital cortex.
From the LGN, neurons spread to the primary visual cortex via radiations. These optic radiations, also called geniculocalcarine tracts, are divided into superior and inferior segments (sometimes also termed posterior Baum's loop and anterior Meyer loop). Like the relationship between nasal and temporal fibers and their visual fields, the superior radiation projects inferior field information, and the inferior radiation projects superior field information. The superior pathway courses posteriorly through the parietal lobe before contacting the visual cortex. The inferior radiation first courses anteriorly, along the roof of the lateral ventricle's temporal horn, before turning laterally and projecting through the temporal lobe as Meyer loop.
At the primary visual cortex, Brodmann area 17, even further organization of visual information takes place in order to best merge the images received from each eye. Similar to the rest of the pathway, the visual cortex is also retinotopically arranged. In this way, the central field of vision (macular portion) is received by the posterior pole of the visual cortex.
In a review of 904 cases by Zhang et al., homonymous hemianopsia appeared in a nearly even distribution among genders (52% male, 48% female) with an average age of around 50 years old. An increased likelihood of left-sided HH (55%) to right-sided (45%) was noted. Incomplete HH was more common than complete, 62.4% to 37.6%, with the most common incomplete presentation being homonymous quadrantanopia (29.2%). HH appeared as an isolated deficit in 46.5% of patients, while 53.5% experienced motor symptoms, cognitive changes, or a combination of the two.
Corresponding projections to the retinal ganglion cells of the visual fields can be vertically divided into temporal and nasal halves. With respect to homonymous hemianopsia, the field deficit can be described as complete or partial/incomplete. The involvement of the entire half is complete hemianopsia, whereas any other loss is partial.
Complete homonymous hemianopsia is a visual field deficit with loss of the complete hemifield on the affected side bilaterally, including that half of the macula. Lesions at any point of the retrochiasmal visual pathway can cause this defect.
Homonymous quadrantanopia, often partial but may be complete, is a visual field deficit with a superior or inferior component in addition to the nasal or temporal involvement. This presents itself as a loss of ¼ of the visual field. Most often, damage to the superior or inferior banks of the primary visual cortex (e.g., infarct) will present as a bilateral quadrantanopia. Quadrantanopia may also result from unilateral superior or inferior optic radiation involvement. For instance, right-sided Meyer loop (inferior radiation) damage would cause a loss of vision in the superior-left quadrant bilaterally.
Homonymous hemianopsia with macular sparing presents with nearly identical field loss as homonymous hemianopsia with macular sparing. However, sparing of the macula permits a centrally localized (about 5-25 degrees) region of functional vision on the affected side. Commonly, this deficit results from infarcts/strokes involving the most posterior portion of the occipital cortex in a classic posterior cerebral artery distribution. The occipital pole, which manages macular field information, has an anastomotic vascular supply with the contralateral side. This manages to abate macular field loss while the surrounding tissue, and therefore surrounding visual field, remain involved.
Homonymous scotomatous defects are rare visual field losses limited to the central 30 degrees, respecting the vertical meridian, and surrounded by functional peripheral vision. These deficits are most often caused by occipital tip injury (infarct or stroke) that damages the cortex beyond the dual vascular reserve. As a result, this field loss can be described as a reversal from macular sparing hemianopsia.
Homonymous sectoranopia is a rare defect resulting in wedge-shaped field loss. The loss is often a result of the involvement of the lateral geniculate ganglion, which has a dual vascular supply from both anterior and posterior choroidal arteries. Anterior choroidal artery involvement characteristically causes a superior and inferior wedge-shape loss straddling the horizontal meridian. Posterior choroidal artery involvement typically spares the superior and inferior sectors and has a field loss of the wedge in between.
Temporal crescent-sparing or unilateral loss involves varying field loss of about 30 degrees of the farthest peripheral temporal field, which is not overlapped by the contralateral eye’s nasal field (the temporal crescent). Unilateral loss is the only mentioned defect thus far that involves retrochiasmal lesions and presents as a monocular field defect (the others are bilateral and homonymous). Temporal crescent-sparing homonymous hemianopsia is hemianopsia with the temporal crescent of the contralateral eye field being spared, often caused by injury to the occipital cortex with preservation of the anterior portion of the cortex.
Homonymous hemianopsia involves loss of visual field zones, and patients often present with bilateral field loss, though sometimes they complain of monocular loss or dyslexia. In addition, unilateral lesions in these following anatomical locations do not alter acuity. Further examination of visual acuity should be planned if patients present with visual acuity complaints.
Optic Tract Lesions
Optic tract lesions may result in complete or incomplete hemianopsias. The involvement of the optic tract often includes afferent pupillary nerve fibers. This can present as a pupillary defect in the eye contralateral to the tract lesion. Other times, the patient may present with a Wernicke pupil – light shone to the functioning half of the affected eye will cause pupillary constriction. However, light shone to the half with field loss may cause lessened or loss of pupillary constriction. On retinal exams, often characteristic optic atrophy can be visualized. The optic disc of the eye contralateral to the lesion shows a horizontal “bow-tie” about one to one-and-a-half months after injury. The ipsilateral eye may present with diffuse optic disc pallor. Less common are signs of damage to surrounding anatomical structures, such as the hypothalamus and internal capsule. Visual acuity and color perception are often preserved unless there exists bilateral involvement of the anterior extension of the lesion involving the optic nerve or chiasm.
Lateral Geniculate Nucleus
The lateral geniculate nucleus has a complex array of neurons and axons correlated with the retinotopic arrangement of the eye. Because of this, defects tend to be variable and incomplete. As stated above, characteristic wedge-shaped field losses (sectoranopia) are seen depending on the involvement of the anterior or posterior choroidal arteries. Unless further involvement of the optic tract occurs, patients present with normal pupillary reflexes. Signs of ipsilateral thalamic or pyramidal tract involvement may also be present.
The involvement of the optic radiations often causes complete, contralateral homonymous hemianopsia. Injury to the radiations can occur proximally or distally. Because of the anatomical relationship with the posterior limb of the internal capsule near the proximal radiations, corticospinal and thalamocortical nerve fibers are often involved with lesions in this location. Distal injury can involve the temporal or parietal lobes. Defects from injury of the optic radiations in the temporal lobe are more likely to be incomplete, superiorly situated “pie-in-the-sky” field losses. Temporal lobe association may also result in aphasia, seizures, hallucinations, or memory complications (when on the dominant hemisphere). Parietal lobe optic radiations are more often associated with incomplete, inferiorly located lesions. When affecting the nondominant hemisphere, these lesions may produce contralateral hemineglect, differing from dominant hemisphere Gerstmann syndrome (acalculia, agraphia, finger agnosia, and right-left disorientation). The involvement of the parietal lobe may also cause difficulty with smooth pursuit towards the side of the lesion, presenting as nystagmus in that direction. Either hemisphere may involve the primary and association sensory cortices.
The most common homonymous hemianopsia deficits are from occipital lobe lesions. These deficits typically present without other associated neurologic symptoms. However, the patterns may vary. Often these lesions include macular sparing as a result of the dual blood supply and bilateral macular representation at the occipital cortex. At the occipital pole, most posteriorly, homonymous scotomas are produced. Most anteriorly, temporal crescent loss and other similar peripheral vision patterns occur. If the lesion extends anteriorly enough to involve the left corpus callosum, patients may present with alexia without agraphia (if the lesion spares the angular gyrus). Bilateral occipital lobe lesions will produce bilateral homonymous hemianopsia of various types. Most notably is Anton’s syndrome involving a complete bilateral homonymous hemianopsia (cortical blindness) with which the patient also experiences anosognosia, being unaware of their blindness. With bilateral occipital lesions, it is possible that visual acuity may be impacted.
Visual field confrontation test is the most commonly used screening tool to assess for visual field defects. Typically, the patient will cover one eye and be asked to look at a stationary object (the wall or instructor’s finger) while noting the number of fingers visible in their periphery. These tests, however, are instructor dependent and have low sensitivity. Another option is the automated or manual (Goldmann, tangent screen) perimetry test. This test provides information not only on the field loss but also on the size and form of the deficit.
In addition to identifying the field deficit, all patients experiencing homonymous hemianopsia should undergo further imaging, such as an MRI, to identify the cause of such symptomatology.
Most treatment options offered for patients experiencing homonymous hemianopsia involve forms of rehabilitation. One option utilizes prisms to project the image from the visual field defect side to the intact side. This can allow the patient to see what is in their blind spot, though it tends to be disorienting.
More commonly are therapies anchored on working with the visual field loss. For instance, programs aimed at teaching patients how to read with assistance (following the text with a finger) or move their eyes into the field loss (saccadic training) may improve overall functionality.
Any injury to the retrochiasmal visual pathway may result in homonymous hemianopsia. As a result, in addition to the various derivations of cancer, other diseases exist, which may cause such a visual field deficit.
Spontaneous improvement of homonymous hemianopsia varies dramatically, with great emphasis on the timeline from when the injury occurred to when the visual field test was administered. Even with reported differences in spontaneous improvement, after six months from the incident, it is unlikely that the patient will experience a spontaneous recovery. However, improvement in underlying diseases has shown to provide visual field improvement even after such time (eg., multiple sclerosis).
In addition to the visual field deficit, patients experiencing homonymous hemianopsia may feel disoriented and complain of dizziness, vertigo, or nausea. These symptoms all increase the risk of trauma. Patients are more likely to fall as a result of their field loss.
Risk factors for homonymous hemianopsia are those related to the common causes. Vascular insult being most frequent, patients should be aware of their possible stroke risk. Atherosclerosis, hypertension, diabetes, smoking history, obesity, heavy alcohol intake, and previous vascular injury are all factors.
For patients already experiencing homonymous hemianopsia, driving is a serious concern. It should be made aware to the patient that operating machinery with a visual field loss may put themselves and others at risk. Each state has different visual criteria regarding driving licensure. If determined by the state that the patient does not meet the minimum visual criteria, the physician should counsel the patient on driving and safety.
Homonymous hemianopsia is a field loss deficit in the same halves of the visual field of each eye. These patients may also exhibit non-specific signs and symptoms such as nausea, coordination difficulty, vertigo, and dyslexia. The cause of homonymous hemianopsia is often due to a myriad of diagnoses, including metabolic, neurologic, immunologic, traumatic, infectious, and vascular etiologies. While the physical exam may reveal that the patient has a visual field deficit, the cause is difficult to know without proper imaging studies.
While the primary care or emergency physician is almost always involved in the care of patients with a visual field deficit, it is important to consult with an interprofessional team of specialists that often include an ophthalmologist, neurologist, neurosurgeon, radiologist, or interventional radiologist. The nurses are also vital members of the interprofessional group as they will monitor the patient's vital signs and assist with educating the patient and their family. If the causal lesion is classified as surgical in nature, the patient may undergo an operation as part of the treatment plan. In the postoperative period for pain, wound infection, and ileus, the pharmacist will ensure that the patient is on the right analgesics, antiemetics, and appropriate antibiotics.
The radiologist also plays a vital role in determining the cause. Although homonymous hemianopsia is most commonly secondary to cerebrovascular injury, trauma, or tumors, the specific location of the causal lesion can be difficult to glean from the visual field loss alone. With that said, to best improve outcomes, prompt consultation with an interprofessional group of specialists is recommended.[Level 5] In addition, patient adaptation after experiencing the visual field loss continues to be explored. Therapy options vary and are best followed through with the inclusion of multiple specialists. [Level 4]
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