Chapter 029. Disorders of the Eye (Part 22) ppsx

Disorders of the Eye Part 22 Horizontal Gaze Descending cortical inputs mediating horizontal gaze ultimately converge at the level of the pons.. Neurons in the paramedian pontine retic

Chapter 029 Disorders of the Eye

(Part 22)

Horizontal Gaze

Descending cortical inputs mediating horizontal gaze ultimately converge

at the level of the pons Neurons in the paramedian pontine reticular formation are responsible for controlling conjugate gaze toward the same side They project directly to the ipsilateral abducens nucleus A lesion of either the paramedian pontine reticular formation or the abducens nucleus causes an ipsilateral conjugate gaze palsy Lesions at either locus produce nearly identical clinical syndromes, with the following exception: vestibular stimulation (oculocephalic maneuver or caloric irrigation) will succeed in driving the eyes conjugately to the side in a patient with a lesion of the paramedian pontine reticular formation, but not in a patient with a lesion of the abducens nucleus

Internuclear Ophthalmoplegia

This results from damage to the medial longitudinal fasciculus ascending from the abducens nucleus in the pons to the oculomotor nucleus in the midbrain (hence, "internuclear") Damage to fibers carrying the conjugate signal from abducens interneurons to the contralateral medial rectus motoneurons results in a failure of adduction on attempted lateral gaze For example, a patient with a left internuclear ophthalmoplegia will have slowed or absent adducting movements of the left eye (Fig 29-19) A patient with bilateral injury to the medial longitudinal fasciculus will have bilateral internuclear ophthalmoplegia Multiple sclerosis is the most common cause, although tumor, stroke, trauma, or any brainstem process

may be responsible One-and-a-half syndrome is due to a combined lesion of the

medial longitudinal fasciculus and the abducens nucleus on the same side The patient's only horizontal eye movement is abduction of the eye on the other side

Figure 29-19

Left internuclear ophthalmoplegia (INO) A In primary position of gaze the eyes appear normal B Horizontal gaze to the left is intact C On attempted

horizontal gaze to the right, the left eye fails to adduct In mildly affected patients the eye may adduct partially, or more slowly than normal Nystagmus is usually

present in the abducted eye D T2-weighted axial MRI image through the pons

showing a demyelinating plaque in the left medial longitudinal fasciculus (arrow)

Vertical Gaze

This is controlled at the level of the midbrain The neuronal circuits affected in disorders of vertical gaze are not fully elucidated, but lesions of the rostral interstitial nucleus of the medial longitudinal fasciculus and the interstitial

nucleus of Cajal cause supranuclear paresis of upgaze, downgaze, or all vertical

eye movements Distal basilar artery ischemia is the most common etiology Skew deviation refers to a vertical misalignment of the eyes, usually constant in all

positions of gaze The finding has poor localizing value because skew deviation has been reported after lesions in widespread regions of the brainstem and cerebellum