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  INTERNUCLEAR ORGANIZATION: GAZE CENTERS

  
  
  
  
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    Horizontal gaze

    
    
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    Vertical gaze

  
  
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  SUPRANUCLEAR PATHWAYS AND DISORDERS

  
  
  
  
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    Saccades

    
    
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    Pursuit

    
    
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    Vestibulo-ocular reflex

    
    
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    Optokinetic reflex

    
    
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    Vergence

    
    
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    Fixation reflex

  
  
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  NYSTAGMUS

  
  
  
  
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    Classification and terminology

    
    
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    Physiological nystagmus

    
    
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    Other forms of nonpathological nystagmus

    
    
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    Congenital nystagmus and nystagmus in children

    
    
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    Recognizable forms of acquired nystagmus

    
    
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    Other nystagmus-like oscillations

    
    
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    Nystagmus treatment

  
  
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  KEY POINTS

Six systems coordinate and stabilize eye movements. The systems are termed supranuclear because they are higher in the chain of command than the ocular motor nuclei. Internuclear pathways connect the ocular motor nuclei to coordinate conjugate movement of yoke muscles and provide a common pathway for supranuclear systems. Disorders of supranuclear or internuclear pathways can cause conjugate gaze palsies or ocular misalignment. Supranuclear disorders can also cause nystagmus or nystagmus-like oscillations, which are unwanted eye movements that can degrade vision or cause oscillopsia.

Gaze centers are premotor nuclei that organize and relay supranuclear commands to the appropriate individual motor nuclei of yoked muscles to move the two eyes together in the same direction. Separate systems exist for horizontal and vertical eye movements.

HORIZONTAL GAZE

To achieve horizontal gaze, motor neurons innervating the lateral rectus and the contralateral medial rectus subnucleus need to receive equal and simultaneous activation.

Organization

The paramedian pontine reticular formation (PPRF) is the horizontal gaze center. This nucleus is adjacent to the abducens (cranial nerve [CN] VI) nucleus in the pons (Figure 10–1). The PPRF activates the abducens nucleus in response to supranuclear gaze commands. As discussed in Chapter 9, the abducens nucleus contains two sets of neurons: (1) motor neurons whose axons innervate the ipsilateral lateral rectus muscle, and (2) internuclear neurons with axons that decussate to the contralateral medial longitudinal fasciculus (MLF) and travel to the medial rectus subnucleus (Figure 10–2). Thus, activation of the PPRF in turn activates the two populations of cells in the CN VI nucleus and produces conjugate gaze (to the ipsilateral side).

Disorders

Internuclear Ophthalmoplegia Lesions of the MLF interrupt the pathway from the abducens nucleus to the medial rectus subnucleus. Thus, a lesion of the left MLF causes a left internuclear ophthalmoplegia (INO): an isolated adduction deficit of the left eye on attempted right gaze, with normal left gaze (Figure 10–3). In many patients, the eye on the affected side does not adduct past midline. In less severe instances, the adduction deficiency is seen only by observing the horizontal saccades: The adducting eye moves more slowly as it slides into position, lagging behind the faster-moving abducting eye (Figure 10–4). The abducting eye commonly displays a dissociated horizontal jerk nystagmus. Adduction with convergence is spared if the MLF lesion is in the pons, but may be affected if the lesion is anterior in the MLF (in the midbrain) where convergence input to the medial rectus subnucleus may be interrupted as well.

Common causes of INO include multiple sclerosis (MS) (in patients 50 years old or younger) or brainstem vascular disease (in patients older than 50 years). An INO can be differentiated from a CN III palsy by the lack of ptosis, anisocoria, or other CN III–mediated motility disturbances, as well as the preservation of adduction with convergence (in most cases). Myasthenia gravis can precisely mimic the clinical findings of an INO (including the abducting nystagmus).

A bilateral INO can cause a large exotropia referred to as WEBINO syndrome (“wall-eyed” bilateral INO). This condition likely results from a lesion that involves the MLF on both sides (Figure 10–5). As discussed for unilateral INO, adduction with convergence is spared unless the lesion is anterior, also involving the medial rectus subnuclei.

Gaze palsy Lesions that affect the PPRF produce a gaze palsy to the ipsilateral side. Lesions that affect the abducens (CN VI) nucleus also produce a gaze palsy (not just an abduction deficit) because the nucleus activates both the ipsilateral lateral rectus muscle and the contralateral medial rectus subnucleus. The resulting gaze palsy may not be easily differentiated from lesions in cortical areas of supranuclear gaze initiation, such as the frontal eye fields. Vestibular input occurs at the level of the CN VI nucleus, so the vestibulo-ocular reflex is unaffected by PPRF or supranuclear lesions. Therefore, oculocephalic or caloric tests should be normal in PPRF or supranuclear palsies, but will be abnormal if the abducens nucleus or more distal CN VI pathway is affected.

One-and-a-half syndrome Lesions that involve the PPRF or CN VI nucleus can also involve the adjacent ipsilateral MLF. The condition produces a gaze palsy to the ipsilateral side and an INO in contralateral gaze. The gaze palsy to the ipsilateral side is the “one” and the INO in contralateral gaze is the “half.” The only normal movement remaining in the horizontal plane is abduction of the contralateral eye (Figures 10–6 and 10–7). Similar to INO, demyelinating disease and brainstem infarction are common causes of one-and-a-half syndrome. Wernicke encephalopathy is another brainstem process that can cause a variety of horizontal gaze disturbances (Box 10–1).

VERTICAL GAZE

Organization

The rostral interstitial nucleus of the MLF (riMLF) is the vertical gaze center. These paired nuclei are located in the pretectum anterior to the mesencephalon near the CN III complex (see Figure 10–1). The lateral portion of the riMLF mediates upgaze; outflow crosses to the contralateral side and communicates with both inferior oblique and superior rectus subnuclei. These motor nuclei control yoked muscles in opposite eyes (remember that the superior rectus fascicles decussate) that elevate the eyes (Figure 10–8A). The medial portion of each riMLF mediates downgaze; outflow travels down the MLF to the ipsilateral inferior rectus subnucleus and fourth nerve (CN IV) nucleus (see Figure 10–8B). These motor nuclei control yoked muscles in opposite eyes that are depressors (remember that CN IV decussates). Vertical gaze is initiated by bilateral activation of the medial or lateral portions of the riMLF.

Disorders

Dorsal midbrain syndrome (also known as Parinaud syndrome, sylvian aqueduct syndrome, or dorsal mesencephalic syndrome) is a constellation of signs and symptoms caused by lesions of the dorsal midbrain that affect the vertical gaze centers and the CN III nuclear complex, their interconnections, and midbrain pupillary circuits (Table 10–1). Upgaze disturbances are the hallmark, with convergence-retraction nystagmus on attempted upgaze, and a light-near dissociation of the pupils is frequently present (discussed in Chapter 11). Convergence-retraction nystagmus is a nystagmus-like oscillation that occurs from co-contraction of the extraocular muscles innervated by CN III on attempted upgaze, without inhibition of the remaining muscles (Figure 10–9). Attempted upward saccades cause retraction of the globes into the orbits, best seen when viewing the patient’s globes from the side. At the same time, the eyes make a convergence movement. A downward-moving optokinetic stimulus produces repeated upward saccades, allowing the best opportunity to observe the phenomenon.

Lesions in the dorsal midbrain include caudal aqueductal stenosis, stroke, MS, arteriovenous malformations, trauma, and compression from tumor. Pinealomas cause the dorsal midbrain syndrome by extrinsic compression of the dorsal mesencephalon, usually in young patients.

There are two basic types of eye movements: (1) fast eye movements to reposition the eyes, in which afferent visual information is suppressed, and (2) slow eye movements to keep the eyes on an object of regard despite movement of the world view or observer. Six systems of eye movement control the fast or slow eye movements to coordinate the eyes. Four of the systems coordinate types of conjugate gaze: saccade, smooth pursuit, vestibulo-ocular, and optokinetic systems. The vergence system controls the disconjugate movement of convergence and divergence for far and near binocular fixation. The fixation system keeps the eyes relatively still and continuously trained on objects of interest. Table 10–2 is an overview and summary of the systems and pathways, each of which is discussed in the following sections.

SACCADES

Saccades are phasic fast eye movements (rotational velocities of 300 to 500° per second) that redirect the eyes to a new fixation object. Saccades occur voluntarily, but they can be involuntary: a normal reflexive response to the sudden appearance of a new visual, auditory, or tactile stimulus; a part of the optokinetic or vestibulo-ocular reflex; or may be intrusive and unwanted in certain disease states. Saccades can occur in any direction, but the pathways for saccades in the horizontal plane are better understood than vertical saccades.

Synthesis of a Saccade

The production of a horizontal saccade involves more than just activating the PPRF for horizontal gaze. To generate a horizontal saccade, a strong pulse is needed to overcome orbital viscous forces and get the eyes started moving, followed by a step up in the baseline firing rate to the muscles to keep the eye in its new position. Specialized cells in the PPRF and other adjacent areas produce the key components of the pulse and step for horizontal saccades. Burst cells in the PPRF provide the strong (high-frequency) pulse signal to initiate the saccade (Figure 10–10). The strength of the signal is proportional to the size of the intended saccade. Pause cells (in nucleus raphe interpositus) hold the burst cells in check, discharging continuously except immediately before and during a saccade to allow burst cells to fire. Neural integrator cells in the nucleus propositus hypoglossi (NPH) and medial vestibular nucleus (see Figure 10–1) also receive the burst cell pulse signal and generate a proportional step signal to hold the eye at the new position. The larger the saccade, the larger the pulse signal, and the larger the step (firing rate) required to hold the eyes in a new eccentric position. Thus, the pulse portion of the saccadic signal is generated by the PPRF, and the step portion is calculated by the neural integrator in the NPH. The combined (pulse-step) signal drives the two populations of cells in the abducens nucleus: motor neurons for ipsilateral CN VI and intercalated neurons that connect to the contralateral medial rectus subnucleus. The result is a coordinated saccadic movement (and holding) of both eyes toward the side of the activating PPRF and abducens nucleus.

Pathways for Horizontal Saccades

The frontal eye field and superior colliculus, on the side contralateral to the direction of gaze, are the major supranuclear initiators of horizontal saccadic movement. These areas have direct connections to the contralateral PPRF (Figure 10–11). Other contributors to saccadic control include the supplementary eye fields, dorsolateral prefrontal cortex, areas of the parietal lobe, and the ipsilateral frontal eye fields.

Pathways for Vertical Saccades

Supranuclear pathways for vertical saccades originate from either both frontal eye fields or both superior colliculi. These areas provide their input to the riMLF, which generates the vertical pulse, similar to the role of the PPRF in producing horizontal gaze (Figure 10–12). The interstitial nucleus of Cajal (INC) is the vertical step integrator that is analogous to the NPH for horizontal gaze (Table 10–3).

Saccadic Disorders

Abnormalities of the saccadic system include the inability to produce voluntary saccades (ocular motor apraxia), saccades that are too slow, saccades that overshoot (hypermetric saccades) or undershoot (hypometric saccades), or unwanted saccades (saccadic intrusions).

Frontal lobe lesions Injury to the frontal eye fields can cause difficulty in generating horizontal saccades to the contralateral side, creating a preferential gaze to the ipsilateral side. Pursuit, optokinetic, and vestibulo-ocular (oculocephalic or caloric) responses are intact, demonstrating the supranuclear nature of the lesion. Over several weeks, the patient regains the ability to generate bilateral saccades, even after permanent frontal lobe injury. This recovery results from activation of secondary projections from the intact contralateral frontal eye field to the PPRF on the same side (ipsilateral projection), allowing the remaining frontal lobe to initiate saccades to both sides.