8- Neurophysiology (Vestibular System)- Pt 2.docx

- **Optic Nerve (CN2) Decussation in Different Species** - Birds and Lizards: (100%) No consensual PLR (pupillary light reflex) - Mice: (97%) Not detectable consensual PLR - Large animals: (80-90%) Very weak consensual PLR - Dogs (75%) and Cats (65%): Direct PLR...

- **Optic Nerve (CN2) Decussation in Different Species** - Birds and Lizards: (100%) No consensual PLR (pupillary light reflex) - Mice: (97%) Not detectable consensual PLR - Large animals: (80-90%) Very weak consensual PLR - Dogs (75%) and Cats (65%): Direct PLR is better than their consensual PLR - Primates: (50%) Direct PLR is equal to their consensual PLR - **Vestibular Dysfunction: Clinal Signs** - Motion sickness (vomiting) is a common clinical sign for vestibular dysfunction. - Vomiting is beneficial for removing any potential toxins in the body, that could be the source of the motion sickness. - Head tilt - Wide stance- meaning that the patient may be unbalanced, causing the wide stance - Vestibular ataxia - Leaning/Falling - Circling - "Alligator" rolling - Nystagmus - A severe presentation of vestibular signs does NOT mean that there is poor prognosis, nor does it indicate the severity of the disease. - **Non-Vestibular Signs** - It is common to mistake an animal experiencing (only) circling as a vestibular dysfunction. - However, if that patient is circling, and lacks the vestibular ataxia and head tilt, they likely have a vision or cerebral cortex issue, causing the compulsive behavior of circling. - **Vestibular System: Posture** - The vestibular system acts as a sensor of gravity and head acceleration (controlling balance), making it crucial in controlling posture. - The vestibular system also contributes directly to motor control. - The descending motor pathways, such as the vestibulospinal nerves, receives vestibular and other types of information to control the orientation of the eyes, head, and trunk, and to coordinate postural movements. - Therefore, if an animal has vestibular disease, they may have difficulty moving. - When all of the component of the vestibular system are working, there is equal extensor tone on each side, and balance and equilibrium are maintained and adjusted. - With a lesion on one side, there will be a lack of facilitation of the extensor muscles on that side, leading to imbalance. - The normal side will "push" the body and head towards the abnormal side. - Essentially, with a vestibular lesion, the side of the brain where the lesion is will be the same side of the body that the patient is tiling their head. - For example, [if the patient has a lesion on the left side of the brain from vestibular dysfunction, they will experience a head tilt to the left, rather than to the right.] - The same applies to other vestibular abnormalities, such as circling, leaning, falling, or rolling in the same direction where the lesion is on the brain. - **Nystagmus** - Nystagmus is the rhythmic, involuntary oscillation of the eyes, and it can be "pendular nystagmus" or "jerk nystagmus". - **Pendular nystagmus** involves short, equal eye movements. - This type of nystagmus is not a sign of vestibular disease, and is often [benign]. - Some breeds are more predisposed to pendular nystagmus. - An example is the Siamese cat which can have a congenital defect in their visual pathway, causing this. - **Jerk nystagmus** involves a fast a slow phase of eye movement, with the fast phase moving away from the side where the lesion is located on the brain. - For example, [if the lesion is on the right side of the brain, the fast phase of jerk nystagmus would be towards the left, rather than the right.] - Jerk nystagmus can be horizontal, vertical, rotatory, or conjugate. - **Conjugate jerk nystagmus** means that both eyes are moving in the same direction. - Unconjugated jerk nystagmus is very rare. - Jerk nystagmus can also be spontaneous or positional. - Jerk nystagmus tends to be more problematic (less benign) than pendular nystagmus. - **Spontaneous nystagmus** is probably caused by abnormal, asymmetric action potential inputs to the brainstem from the vestibular apparatus on the 2 sides of the head. - The imbalance of neural activity is interpreted by the brainstem as a rotation or movement of the body. - Nystagmus is generated even though the body and head are stationary. - **Vestibular System: Eye Position** - The vestibular system is responsible for maintaining the eyes position relative to the position of the head in space by controlling the extraocular muscles of the eye, known as the **vestibulocochlear reflex**. - The vestibular system coordinates eye and head movements so that the head turns, and the eyes remain fixed on the original field of vision for as long as possible. - This allows the animals to interpret a field of vision despite angular acceleration of the head. - **Physiological nystagmus** is when the eyes slowly move opposite to the head rotation, followed by a fast flick back in the direction of the head's rotation. - The reflex eye movement pattern requires normal sensory input from semicircular ducts, an intact pathway in the brainstem, and normal function of motor neurons and extraocular muscles. - During a physical exam, it is normal if the doctor turns the patient's (such as a dog's) head to the left, and their eyes also turn to the left, and vice versa. - It would be abnormal to turn the animals' head in one direction and the eyes move in the opposite direction. - **Vestibular Dysfunction** - **Central vestibular dysfunction** involves the brainstem and cerebellum, so if central vestibular dysfunction is suspected, the doctor should look further into brainstem/cerebellar signs. - **Vestibular nuclei** are located in the brainstem. - Central vestibular dysfunction can have the following symptoms which peripheral will always lack: altered mentation (reduced consciousness), proprioceptive deficits, and cerebellar signs. - **Peripheral vestibular dysfunction** involves the inner ear (vestibular apparatus) or cranial nerve 8. - Deficits in cranial nerve 7 would indicate peripheral vestibular dysfunction only. - If the peripheral vestibular dysfunction is due to otitis, that is the only case where treatment is required. Otherwise, symptoms will likely resolve on their own with time. - Peripheral vestibular dysfunction can also be due to an idiopathic vestibular disease. - If a cat presented for vestibular ataxia and horizontal nystagmus, they likely have a peripheral vestibular dysfunction. - The following clinical signs can indicate either central or peripheral vestibular dysfunction: head tilt, vestibular ataxia, falling/rolling, nystagmus, and strabismus. - However, [vertical] nystagmus, specifically, will ALWAYS indicate [central] vestibular dysfunction. - Animals with vestibular dysfunction will usually present with a primary complaint of head tilt, nystagmus, or ataxia (or a combination of all 3). - "**Vestibular disease**" is not a diagnosis, rather it is a group of clinal signs showing that the vestibular system is not working properly, so the primary disease must be investigated. - Vestibular disease can be caused by any disease affecting the peripheral or central vestibular components. - It is better to refer to "vestibular disease" as "vestibular dysfunction" or "vestibular syndrome" instead, until an actual diagnosis is made. - **Idiopathic Vestibular Disease** - Idiopathic vestibular disease is the most common cause for vestibular dysfunction in dogs and cats. - Idiopathic vestibular disease often has severe clinical signs with rolling and rapid nystagmus being very evident. - Idiopathic vestibular disease will ALWAYS be from peripheral dysfunction and will NEVER be from a central dysfunction cause. - It has no sex or breed predilection. - In **cats**, idiopathic vestibular disease is most common in 4yr olds, but any age can be affected. - In **dogs**, usually senior/geriatric dogs develop idiopathic vestibular disease. - Idiopathic vestibular disease tends to have a good **prognosis**, with signs resolving rapidly without definitive treatment in 1-3weeks. - Some animals will require fluid and antiemetic therapy. - **Diagnosis** of this disease is done through excluding all other causes of peripheral vestibular dysfunction (diagnosis of exclusion), like ruling out otitis.