Concurrent impaired vision and pupillary light reflex deficits are suggestive of a lesion affecting the proximal portion of the visual pathway, from the retina to just prior to the lateral geniculate nucleus, which is common to both the visual pathways and the pupillary light reflex pathway.
Retinal, optic disc and optic nerve lesions
Unilateral lesions will usually result in impaired vision in the affected eye and loss of the direct and consensual pupillary light reflex on stimulating the affected eye. Both the direct and consensual pupillary light reflex should still be present on stimulating the normal eye. Bilateral lesions will usually result in impaired vision, mydriasis and loss of the pupillary light reflex (both the direct and consensual reflexes) in both eyes.
Sudden acquired retinal degeneration: SARD is characterized by an acute loss of vision (although in some cases this may develop over a few days), and occurs occasionally in dogs in the UK (). Affected dogs are typically adult (middle-aged), can be of pedigree or mixed breed descent, and present bilaterally blind with dilated unresponsive pupils. In the acute stages no abnormalities are evident on ophthalmoscopic examination, but overtime (weeks later) a bilaterally symmetrical retinal degeneration becomes evident, with hyper-reflectivity of the tapetal fundus and attenuation of retinal blood vessels. An electroretinogram is required to demonstrate photo-receptor death in the acute stage in order to distinguish SARD from other lesions responsible for acute-onset blindness (). There is no treatment for SARD; although the blindness is permanent, no further clinical signs develop.
Optic disc hypoplasia: Optic disc hypoplasia may be unilateral or bilateral; it occurs sporadically in many dog breeds and has been suggested to be inherited in the Miniature Poodle (). Depending on the severity, optic disc hypoplasia may result in decreased to absent vision with decreased pupillary light reflex proportional to the degree of optic disc hypoplasia. In unilateral cases the owner may not notice the problem. Extreme cases present bilaterally blind with bilaterally dilated and unresponsive pupils from the time that the puppies first open their eyes. On ophthalmoscopic evaluation, affected optic discs appear small and grey in colour (). There is no treatment, but in those cases with a possible hereditary cause preventive measures should be taken against breeding.
Optic disc atrophy: Damage to the retinal ganglion cells or proximal axonal processes may occur due to a variety of causes, including generalized retinal degeneration, glaucoma, trauma and inflammatory lesions. The consequence of this is axonal loss or the development of Wallerian-like degeneration of axons, with loss of the surrounding myelin sheath. The amount of axonal loss determines the degree of optic disc atrophy, which appears grey and shrunken-looking on ophthalmoscopic examination. This process is gradual and not immediately apparent at the time of insult.
Papilloedema: Papilloedema is defined as oedema of the optic disc and usually results from raised intra-cranial pressure (due to cerebral tumours and inflammation) but may occur secondary to optic nerve tumours and inflammation and potentially in conditions causing widespread myelin oedema (seen in certain metabolic and toxic disorders, such as hexachlorophene poisoning). Papilloedema is evident as an irregular and swollen optic disc margin (), frequently with evidence of retinal congestion and haemorrhage (). Papilloedema needs to be differentiated from hypermyelination of the optic disc (pseudo-papilloedema) where myelination extends beyond the periphery of the optic disc as a normal feature, giving the disc margin an irregular and fluffy appearance. Hypermyelination is more evident in certain large breed dogs, including Boxers, German Shepherd Dogs and Golden Retrievers. The presence of papilloedema (although it is inconsistently present), with evidence of central nervous system (CNS) signs, is a reliable indicator of raised intracranial pressure and suggestive of an increased risk of brain herniation (). Historically it has been reported that papilloedema spares vision and that this can be used to distinguish it from optic neuritis; however, in the majority of cases in clinical practice, any forebrain or optic nerve lesion severe enough to cause papilloedema will usually interrupt the visual pathways resulting in concurrent visual deficits.
Optic neuritis: Optic neuritis or papillitis is defined as inflammation of the optic nerve and is characterized by visual loss in the presence of optic disc changes similar in appearance to papilloedema and, as such, can be difficult to distinguish from papilloedema on ophthalmoscopic examination (). Typically the optic disc appears swollen, with frequent haemorrhages. Potential causes () include granulomatous meningoencephalitis (GME), canine distemper, cryptococcosis () and histoplasmosis (), though often no underlying cause is identified and the disorder is presumed to be immune-mediated. Many cases will respond to immunosuppressive treatment with steroids; however, relapses are likely and the prognosis for recovery of vision remains guarded ().
- • Neoplasia (primary optic nerve neoplasia versus secondary compressive orbital neoplasms) may interrupt optic nerve function
- • Trauma may cause optic nerve compression, traction or haemorrhage
- • Retinal degeneration (also known as progressive retinal atrophy) and retinal dysplasia are hereditary in many different breeds of dog and can result from taurine deficiency in cats. The reader is referred to standard ophthalmology textbooks for full details of retinal disease.
Optic chiasm lesions
Lesions affecting the optic chiasm will usually result in decreased to absent vision and mydriasis with loss of both the direct and consensual pupillary light reflex in both eyes.
Neoplasia and other space-occupying lesions:
Space-occupying masses () may occasionally occur at the level of the optic chiasm and, despite the slow development of the underlying disease process, some animals may present with an acute onset of visual deficits (). Lymphoma is a common cause in cats but consideration should be given to other tumours in dogs and cats, in particular pituitary macroadenomas (), meningiomas and tumours of the nasal cavity extending into the region of the optic chiasm. An unusual brain tumour called a suprasellar germ cell tumour has been reported in young adult dogs, in particular Dobermann Pinschers () This tumour expands in the region of the pituitary fossa and can become extremely large, causing compression of the optic chiasm and the adjacent cavernous sinus and associated cranial nerves (see Cavernous sinus syndrome).
- • Vascular lesions, including haemorrhage and vascular malformations
- • Inflammatory lesions, e.g. fungal granuloma ().
Optic tract lesions
Optic tract lesions usually cause visual deficits in the lateral visual field of the contralateral eye and the medial visual field of the ipsilateral eye. The pupillary light reflex is intact in both eyes but the degree of pupillary constriction may be slightly reduced in the contralateral eye (most evident on the swinging flashlight test).
Space-occupying masses: As for other CNS structures, the optic tracts are vulnerable to disruption or compression by mass lesions, including tumours and haemorrhage.
Inflammatory lesions: The most common causes of CNS inflammation affecting the visual tracts in dogs include GME and infectious agents including Toxoplasma and Neospora, canine distemper virus, tick-borne agents and Cryptococcus (). In cats, feline infectious peritonitis (FIP) is a significant cause, but consideration should be given to feline leukaemia virus (FeLV), feline immunodeficiency virus (FIV), toxoplasmosis, cryptococcosis and immune-mediated encephalitis.
Decreased vision with intact PLRs
Lesions affecting the central projections of the visual pathway (from the lateral geniculate nucleus to the visual cortex) will result in visual deficits but do not affect the PLR. The pupillary light reflex requires fewer intact axons than conscious perception of vision and therefore the situation may occasionally arise that in partial lesions of the proximal visual pathways there is loss of vision but sparing of the PLR, creating the illusion of a more central lesion.
Unilateral lesions affecting the central projections of the visual pathways usually demonstrate a diminished to absent lateral visual field in the contralateral eye and (less obviously) a diminished to absent medial visual field in the ipsilateral eye. Other clinical signs of forebrain disease, including decreased consciousness, seizures, circling, conscious proprioceptive deficits and hemi-neglect/hemi-inattention syndrome () would usually be expected in forebrain lesions severe enough to cause visual deficits ().
Central visual disturbances may also be a feature of some degenerative disorders, particularly the lysosomal storage diseases (reported in gangliosidoses and sphingomyelinosis).
Hydrocephalus: Congenital hydrocephalus, characterized by massive dilation of the ventricular system of the brain and in particular the lateral ventricles, is usually characterized by a dome-shaped forehead (). In congenital hydrocephalus, and to a lesser extent acquired hydrocephalus, the optic radiations (as they pass adjacent to the lateral ventricles) are particularly vulnerable to injury, and visual deficits are therefore one of the potential clinical signs. Congenital hydrocephalus is further characterized by the presence of bilateral ventrolateral strabismus ― the ‘setting sun sign’ ― as discussed under disorders of eyeball position and movement.
Lissencephaly: Lissencephaly is a rare developmental disorder, described in the Lhasa Apso dog, where there is congenital absence of cerebrocortical convolutions. Neurological abnormalities become apparent within the first year of life and include seizures, behavioural abnormalities and visual disturbances (). Treatment is limited to management of seizures, and the prognosis is grave in this progressive disorder.
Diffuse encephalopathies, most commonly secondary to metabolic disorders or hypoxic episodes, may present with visual deficits (see Chapter 8). Any animal with a metabolic encephalopathy severe enough to cause visual deficits will usually demonstrate marked CNS depression. Potential metabolic encephalopathies that should be considered include: hepatic encephalopathy, hypoglycaemia, renal-associated encephalopathy, profound electrolyte derangements, endocrine-associated encephalopathies(hypothyroidism), profound acid-base disturbances and mitochondrial encephalopathies.
Global cerebral ischaemia may occur as a consequence of anaesthetic accidents orfollowing prolonged seizures. Post-ictal depression (functional forebrain suppression following seizure activity) may also present as central blindness.
Space-occupying lesions within the cerebral hemispheres are primarily tumours. The most common primary brain tumours include meningiomas (particularly in cats) and gliomas, but ependymomas, choroid plexus papillomas and metastatic tumours should also be considered ().
Immune-mediated (in dogs, particularly GME) and infectious causes (including viral, bacterial, protozoal, rickettsial and fungal agents) of encephalitis should be considered ().
The forebrain is vulnerable to trauma, though it is relatively well protected (more so in dogs) by the skull and overlying masticatory muscles. Blindness, as one of the potential clinical signs of forebrain trauma, may be evident immediately following the injury or the onset may be delayed in the event of secondary processes, including CNS infection or abscessation.
Toxins are rare causes of cortical blindness, but in the presence of other suggestive clinical signs, particularly gastrointestinal disturbances, lead poisoning should be considered ().
Although an uncommon cause of blindness, vascular lesions (haemorrhagic or ischaemic in nature) may occursecondary to underlying diseases, including bleeding disorders () and trauma ().
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