Blindness and behavioral changes Flashcards
What is peripheral blindness
Peripheral blindness is blindness due to a lesion rostral to the thalamus (i.e., within the eye, cranial nerve II, optic chiasma or proximal optic tract) that is a lesion along the visual pathway shared with the pupillary light reflex pathways
What is central blindness
Central blindness is blindness due to a lesion caudal to the thalamus (i.e., the forebrain including the distal optic tract, lateral geniculate nucleus, optic radiation or occipital cortex) that is a lesion in the visual pathway not shared with the PLR pathways
Describe the anatomatic pathway supporting conscious perception of vision
Each cerebral hemisphere receives information from the contralateral visual field
The first neuron in the visual pathway is the bipolar cell of the retina, which receives impulses from the neuroepithelial cells of the retina (rods and cones)
- these synapse with a second neuron (i.e., the retinal ganglion cells) and the axons of this neuron course in the optic nerve
In the cat, nasal (medial) retinal ganglions fibres (representing 65% of the retinal fibres) decussate at the optic chiasm temporal (lateral) retinal fibres (the remaining 35%) do not
Following the chiasm the ganglion fibres then course in the optic tract to the level of the thalamus and at this point the fibres follow one of two routes:
- conscious perception of vision (80% of the optic fibres continue to the occipital cortex)
- reflex activity (the remaining 20% of fibres are involved in two possible pathways:
- parasympathetic pathway (i.e., the afferent arm of the PLR)
- somatic motor responses responsible for reflex movements of the head
What is the basis of binocular vision and depth perception
Overlap of the visual field (estimated at 75% in humans and 50% in carnivores) is the basis of binocular vision and depth of perception
What is required about the viasual pathways to observe a normal menace response
A normal menace response requires intact central and peripheral visual pathways in addition to connections from the cerebrum to the brainstem and cerebellum
The menace response is a learnt response and not a reflex
- it may not be present before 12 weeks of age
What is important to check if the menace response is negative
If the menace response is not intact it is important to ensure that the facial and trigeminal nerves are functioning correctly
this is undertaken by touching the lateral and medial canthi of both eyelids
Explain why electroretinography is limited in cats compared to dogs
Electroretinography may determine whether blindness is caused by retinal or post-retinal disease
In cats, the majority of retinal diseases would cause visible ophtalmoscopic abnormalities and, hence electroretinography use is limited
What is suggested by an abnormal PLR and menace response test
An abnormal PLR and menace response suggest the lesion lies within the pathways shared by these two tests
What is suggested by a normal PLR and abnormal menace response
A normal PLR and abnormal menace response suggest a lesion within the visual pathways not shared by the PLR
What is the particularity of cats regarding behavior changes and neurological diseases
In cats, it is not uncommon to observe abnormal behavior as the sole manifestation of a neurological disease
What are the neuroanatomical structures that control behavior in normal cats
Structures controlling behavior are all contained within the brain and are collectively known as the limbic system
The intricacies and complexities of the limbic system means that a lesion almost anywhere within the brain can affect the normal function of the limbic system, leading to abnormal behavior
The limbic system controls the psychic and motor aspects of behavior
- changes in a normal cat’s habits and attitudes are likely to be seen with lesions in the forebrain, involving the limbic system components
What are the clinical signs associated to a lesion of the limbic system
Cats with limbic system disease may experience any number of the following signs:
- changes in personality
- dementia
- lethargy
- listlessness
- aggression
- failure to recognize the owners or familiar environments
- loss of learnt habits
- inappropriate house soiling
- compulsive behaviors (e.g., aimless pacing)
- restlessness
- polyphagia or anorexia
- pica
Explain why it is important in cats to differentiate “compulsive behavior” from seizures
Seizures in cats, unlike dogs, are often of the “psychomotor” type and the owner can misinterpret them as “compulsive behavior”
Seizures may manifest as:
- episodic aggression
- disorientation
- fear or anxiety
- cats may suddenly hide or run in the absence of any specific trigger
The paroxysmal nature of the event combined with the possible presence of prodromal or post-ictal changs is usually suggestive of a neurological condition
Whatare main differentials for secondary behavioral change
Hyperthyroidism and pain can result in aggressive behavior
Hypertension, either primary or secondary to hyperthyroidism, renal failure or diabetes mellitus, can also cause behavioral changes such as disorientation, dullness and aggression
