Horner's Syndrome: Pupillary & Iris Function

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Questions and Answers

Why might melanin content in the iris affect drug response?

Melanin directly alters the chemical structure of drugs, neutralizing their effects.
Melanin binds drugs, reducing their availability and affecting their onset and duration of action. (correct)
Melanin enhances the drug's receptor binding affinity, leading to an exaggerated response.
Melanin interferes with the blood-aqueous barrier, preventing drugs from reaching the iris.

Why is the pupillary light reflex useful in veterinary medicine?

It solely assesses the sympathetic nerve function, crucial for diagnosing specific neurological disorders.
It uses both oculomotor and sympathetic nerves, providing insights into neurological and ophthalmic health. (correct)
It helps differentiate between central and peripheral nervous system lesions affecting the eyes.
It solely assesses the oculomotor nerve (CN III) function, crucial for diagnosing cranial nerve damage.

Which of the following scenarios would necessitate surgical intervention as a treatment for Horner's syndrome in horses?

Horner's syndrome secondary to guttural pouch infection. (correct)
Mild trauma to the sympathetic trunk.
Spontaneous resolution of clinical signs within hours.
Idiopathic Horner’s syndrome.

What is a key difference between the constrictor and dilator muscles of the iris in horses?

The constrictor muscle is stronger and causes miosis, while the dilator muscle is weaker and causes mydriasis. (B) Signup and view all the answers

How do intrinsically photosensitive retinal ganglion cells contribute to the pupillary light reflex?

By detecting blue light (480 nm) and modulating pupil size independently of visual signals. (A) Signup and view all the answers

In a horse with Horner's syndrome, what clinical sign is most directly related to the loss of sympathetic innervation to the eye?

Miosis due to unopposed parasympathetic activity. (B) Signup and view all the answers

Why do prostaglandin analogs cause miosis in horses?

Through a complex mechanism involving the activation of prostanoid receptors in the iris. (B) Signup and view all the answers

What is the primary reason for the variation in pupil shapes among different species?

Pupil shape is an adaptation to specific environmental conditions and predatory strategies. (A) Signup and view all the answers

How does neuropeptide Y influence the adrenergic-induced contractions in the iris dilator muscles?

By enhancing the response of the iris dilator muscle to adrenergic stimulation. (A) Signup and view all the answers

What is the most critical factor in determining the long-term prognosis for a horse diagnosed with Horner's syndrome?

The underlying cause of the Horner's syndrome. (B) Signup and view all the answers

What common characteristic links the iris constrictor and dilator muscles?

Both muscles are derived from neural ectoderm. (B) Signup and view all the answers

What is a potential consequence of carotid artery catheterization in horses, related to Horner's syndrome?

Damage to the vagosympathetic trunk, leading to sympathetic denervation of the eye. (C) Signup and view all the answers

Which statement accurately contrasts the α- and β-adrenergic receptor effects on feline and canine pupils?

Feline pupils constrict with timolol due to β-adrenergic nerve fibers in the iris sphincter muscle, while canine pupils do not show this response. (D) Signup and view all the answers

Why are prey species more likely to have horizontally elongated pupils?

To provide a wider field of vision, aiding in the detection of predators. (A) Signup and view all the answers

What is the significance of the observation that young horses tend to have more circular pupils compared to adult horses?

It reflects a developmental change in the balance of sympathetic and parasympathetic tone affecting the iris muscles. (B) Signup and view all the answers

In cases of Horner's syndrome caused by traumatic brain injury, what is the most likely mechanism leading to sympathetic dysfunction?

Disruption of the sympathetic pathways in the brainstem or spinal cord. (A) Signup and view all the answers

How does equine protozoal myeloencephalitis (EPM) potentially lead to Horner's syndrome in horses?

By inducing lesions in the central nervous system that disrupt sympathetic pathways. (D) Signup and view all the answers

What is the rationale behind using topical phenylephrine in the treatment of Horner's syndrome, and why is it rarely used long-term?

Phenylephrine stimulates α-adrenergic receptors, temporarily improving clinical signs, but does not address the underlying cause. (B) Signup and view all the answers

Which of the following best describes the anatomical pathway affected in Horner's syndrome?

The sympathetic nerve fibers traveling from the hypothalamus to the eye. (C) Signup and view all the answers

Why might an obstructive esophageal disorder contribute to the development of Horner's syndrome in horses?

Esophageal obstruction causes local inflammation and compression of the vagosympathetic trunk in the neck. (C) Signup and view all the answers

What is the mechanism by which cervicothoracic ganglion blocks can unintentionally cause Horner's syndrome?

By directly disrupting the sympathetic pathway as it passes through the ganglion. (C) Signup and view all the answers

How does sympathetic activity affect pupillary function through α- and β-receptors?

α-receptors mediate pupil dilation, while β-receptors mediate pupil constriction. (B) Signup and view all the answers

In domestic cats, how does pupil shape change under different lighting conditions, and what is the functional significance of this change?

From vertically slit when constricted to circular when dilated, optimizing light entry and minimizing aberrations. (C) Signup and view all the answers

What is the significance of avian pupils being highly motile?

It allows birds to rapidly adjust their focus and track moving objects. (B) Signup and view all the answers

What is a potential mechanism by which metastatic neoplasia can cause Horner's syndrome?

By compressing or invading the sympathetic trunk in the neck or thorax. (C) Signup and view all the answers

Why do dogs under anesthesia exhibit rapid pupil constriction and redilation phases with light stimulation?

Anesthesia reduces the inhibitory control of the cerebral cortex on the pupillary light reflex pathway. (D) Signup and view all the answers

Which treatment approach is most effective for managing Horner's syndrome caused by guttural pouch infections in horses?

Treating the underlying guttural pouch infection, such as with antibiotics or surgery. (A) Signup and view all the answers

What is the primary reason idiopathic Horner’s syndrome typically resolves spontaneously in weeks to months?

The underlying cause is often a transient inflammatory process that resolves on its own. (B) Signup and view all the answers

How does the location of a lesion along the sympathetic pathway influence the clinical presentation of Horner's syndrome?

Lesions in the central nervous system may be associated with other neurological deficits, while peripheral lesions are more isolated to the eye and surrounding structures. (B) Signup and view all the answers

What is the effect of anesthesia on the sensitivity of the pupillary light reflex (PLR) and how does it alter the interpretation of PLR results?

Anesthesia decreases the sensitivity of the PLR, potentially masking neurological deficits. (D) Signup and view all the answers

What role do adrenergic nerve fibers play in the iris sphincter muscle of felines, and how does this differ from their role in other species?

Adrenergic nerve fibers cause constriction of the pupil in felines, while they cause dilation in most other species. (B) Signup and view all the answers

Why is it important to differentiate between central and peripheral causes of Horner's syndrome when determining prognosis and treatment?

Central causes often indicate more severe underlying conditions (e.g., neoplasia), leading to a poorer prognosis and different treatment approaches. (D) Signup and view all the answers

In cases of Horner's syndrome secondary to neoplastic disease, what factor most significantly influences the prognosis?

The type, location, and extent of the neoplasm. (A) Signup and view all the answers

What mechanism explains why Horner's syndrome can sometimes result from jugular venipuncture?

The vagosympathetic trunk, which contains sympathetic fibers, runs close to the jugular vein and can be damaged during venipuncture. (C) Signup and view all the answers

A horse owner reports that their horse's pupil constricts in bright light and dilates in the dark, but that the pupil is smaller than normal size even in the dark. What is the most likely problem?

Sympathetic nerve damage (A) Signup and view all the answers

List the potential causes of Horner's syndrome in the Horse?

• Idiopathic: No identifiable cause. • • Trauma: o Neck or spinal cord trauma. o Jugular venipuncture or other trauma to the vagosympathetic trunk. • Infections: o Guttural pouch infections. • Obstructive Disorders: o Obstructive esophageal disorders. • Thoracic Disorders: o Cranial thoracic disorders. • Neoplasia: o Metastatic neoplasia (e.g., melanoma, squamous cell carcinoma). • Surgical Procedures: o Carotid artery catheterization. o Cervicothoracic ganglion blocks. • Central Nervous System Disease: o Neoplasia. o Equine protozoal myeloencephalitis. o Traumatic brain injury. Signup and view all the answers

Flashcards

Pupillary Functions

Regulate light entering the eye, increase depth of focus, minimize lens aberrations.

Iris Constrictor Muscle

Encircles the pupil, derived from neural ectoderm, innervated by oculomotor nerve (CN III).