The saccadic system rapidly shifts the fovea to targets of interest. Saccades are ballistic, rapid movements that generally cannot be altered once initiated. The speed of saccades correlates with the extent of eye movement; larger-amplitude saccades are faster than smaller-amplitude saccades. This relationship is referred to as the main sequence. Saccadic velocity may exceed 500° per second, a speed that allows the eyes to move from primary position to the farthest extent of the temporal visual field in only 0.2 second. Saccadic latency (the interval between appearance of a target and onset of a saccade) is approximately 200 milliseconds, and saccadic duration is generally less than 100 milliseconds.
Saccades may be volitional or reflexive (as in the reflexive quick phases of optokinetic and vestibular nystagmus). Volitional saccades are controlled by several areas of the cerebral cortex, including premotor zones that project to the frontal eye fields (FEFs) (see Chapter 1). Descending pathways from the FEFs also communicate with several intermediate structures, including the basal ganglia and superior colliculi. Reflexive saccades are controlled primarily by the superior colliculi. Activation of these pathways generates conjugate, contralateral saccades by innervating the contralateral PPRF. The PPRF contains 2 populations of neurons: (1) burst cells that activate the neighboring CN VI nucleus and (2) omnipause cells that inhibit the burst cells. The FEFs and superior colliculi generate a saccade by stimulating the burst cells and inhibiting the omnipause cells within the contralateral PPRF. The PPRF burst cells project to the adjacent CN VI nucleus that innervates the ipsilateral lateral rectus muscle and, via the medial longitudinal fasciculus (MLF), the contralateral medial rectus muscle.
The PPRF generates saccades via 2 innervational components: (1) a pulse and (2) a step (Fig 8-2). The pulse initiates the saccade with a burst of high-frequency, phasic innervational activity that enables the globe to overcome inertia and the viscous drag of the orbit (gaze initiation). The step consists of tonic innervational activity that continues after the saccadic eye movement is complete in order to hold the eye in its new eccentric position against elastic forces in the orbit (gaze holding). The pulse determines saccade velocity, and the step maintains the position of the eye after the saccade. For the saccade to be accurate, the velocity of the saccade (pulse innervation) must match the final ocular position maintained after the saccade is complete (step innervation). The process of matching the pulse and step components of a saccade is referred to as neural integration. For horizontal movements, the neural integrator consists of the medial CN VIII nucleus and the accessory nucleus of CN XII (also known as the nucleus prepositus hypoglossi [NPH]) in the medulla. The PPRF contains neurons that project directly to the CN VI nucleus as well as neurons that project to the neural integrator prior to innervating the CN VI nucleus, enabling pulse–step integration of horizontal saccades.
The FEFs and the superior colliculi generate volitional and reflexive vertical saccades, respectively, via bilateral descending pathways that project to the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). The riMLF, which is located in the midbrain, is the supranuclear brainstem center for control of conjugate vertical and torsional saccadic eye movements. Neural integration of vertical saccades occurs in the nearby interstitial nucleus of Cajal (INC), which matches phasic pulse innervation (gaze initiation) with tonic step innervation (gaze holding) in order to hold the eyes in the proper vertical eccentric position after completion of the saccade; note that this structure is the homologue of the NPH, which performs the neural integration for control of horizontal eye movements. Thus, the riMLF contains burst-cell neurons that project directly to the CN III and CN IV nuclei as well as neurons that project to the INC prior to innervating these nuclei, enabling pulse–step integration of vertical saccades.
Saccadic eye movements are fine-tuned by the cerebellum, which receives neural input from the pons and supplies innervation to the PPRF and riMLF. The cerebellum influences saccadic speed and accuracy and assists in neural integration (pulse–step matching).
Volitional saccades can be tested by having the patient rapidly shift gaze between 2 targets, such as the extended index fingers of the clinician’s outstretched hands, which are held to the left and right of the patient. Reflexive saccades are tested by observing the quick phases of OKN (elicited by patient fixation on a rotating OKN drum) and vestibular nystagmus (elicited by patient rotation in an examination chair). The latency (duration from stimulus to movement), accuracy (arrival of the eyes on target), velocity, and conjugacy (degree to which the 2 eyes move together) of the movements should be monitored. A hypometric saccade falls short of the intended target; 1–2 small catch-up saccades may be within normal limits. A hypermetric saccade overshoots the target and is usually pathologic.
Saccadic disorders may produce involuntary saccades, reduced speed of saccades, poor accuracy of saccades (ie, hypometria or hypermetria), delayed latency to initiate saccades, or the inability to maintain eccentric gaze after completion of a saccade. The specific saccadic disorder relates to the pattern of neural activity delivered to the ocular motor nuclei. Excessive firing of burst cells (or excessive inhibition of omnipause cells) in the PPRF will cause unwanted, involuntary saccadic intrusions that disrupt fixation (see the section “Ocular fixation dysfunction” earlier in this chapter, and Chapter 9).
When a patient appears unable to initiate saccades, the doll’s head maneuver can determine whether this inability is the result of a supranuclear or nuclear/infranuclear lesion. In a patient with a supranuclear lesion that causes a horizontal saccadic gaze deficit, the doll’s head maneuver will activate the intact vestibular-ocular system, bypassing the PPRF and directly stimulating the CN VI nucleus to induce horizontal gaze. Conversely, a CN VI nuclear lesion will impair horizontal gaze in response to the doll’s head maneuver as well as attempted volitional saccades.
Slow saccades can result from central or peripheral lesions (including nuclear and infranuclear lesions). Peripheral lesions almost always produce slowed saccadic movements that are hypometric. In contrast, slow saccades with normal amplitude are typically due to CNS lesions, especially lesions of the cerebral hemispheres or basal ganglia. Irrespective of the location of a CNS lesion, slowed saccades ultimately result from decreased discharge frequency of burst-cell neurons causing a deficit in phasic innervation (the pulse component) of the saccade. Saccadic slowness confined to the horizontal plane suggests dysfunction of the PPRF in the pons, whereas saccadic slowness confined to the vertical plane suggests dysfunction of the riMLF in the midbrain. Patients with PSP have slow volitional saccades, especially in the vertical plane, but their reflexive saccades (the fast phases of OKN) often are initially normal. Other CNS disorders that cause slow saccades include cerebellar degeneration, Huntington disease, Wilson disease, and Whipple disease. Most of these cases typically include a prolonged latency to initiate saccades. Myasthenia gravis may produce faster-than-normal (“lightning-like”) saccades that suddenly stop short of the intended target (reduced saccadic amplitude).
Dysmetric saccades result from peripheral lesions or from abnormal discharge duration of the burst-cell neurons responsible for phasic innervation (the pulse component) of the saccade. Hypometric saccades can be observed with peripheral or central lesions, whereas hypermetric saccades are usually the result of disease of the cerebellum or its interconnections. The accuracy of saccadic eye movements can be difficult to assess in patients with significant bilateral loss of vision, especially those with large visual field defects (eg, homonymous hemianopia and bitemporal hemianopia). Thus, the clinician should exercise caution in describing the presence of hypometric saccades in these patients.
In other cases, saccades are initiated only after prolonged latency, which is a characteristic feature of ocular motor apraxia (discussed later in this chapter). Assessment of saccadic latency must take into account the patient’s age; a gradual increase in latency may occur with advancing age.
The inability to maintain eccentric gaze after completion of a saccade is a result of decreased tonic innervation (the step component of a saccade). Insufficient tonic innervation arises from infranuclear lesions, cerebellar disease, or malfunction of the neural integrator, often referred to as a leaky integrator (the neural integrator consists of the NPH for horizontal saccades and the INC for vertical saccades). After completion of the saccade, the deficient tonic innervation causes the eyes to drift slowly off of the target back to the central position, followed by a corrective saccade toward the target. This combination of eye movements is known as gaze-evoked nystagmus (see Chapter 9).
The most common saccadic dysfunction—the conjugate limitation of upgaze characterized by eye movements that have reduced range but normal velocity—is part of normal aging. Abnormalities of saccadic function are relatively nonspecific with regard to etiology and site of the lesion, but notable exceptions occur in which saccadic abnormalities provide important clues to the diagnosis. These exceptions include
slowed saccades in a patient with extrapyramidal (ie, Parkinson-like) syndrome with imbalance and impaired cognition, which suggest a diagnosis of PSP
hypermetric saccades, which usually indicate disease of the cerebellum or its outflow pathways
unidirectional hypermetric saccades, ocular lateropulsion, and hypermetric pursuit movements, which are generally present as part of the lateral medullary syndrome (Wallenberg syndrome)
Ocular motor apraxia
An extreme instance of saccadic dysfunction is ocular motor apraxia. An apraxia is an inability to voluntarily initiate a movement that can be initiated by another means, which reveals that a paralysis is not present.
Ocular motor apraxia in children is characterized by increased latency or failure of saccadic initiation. Voluntary and reflexive saccades (in response to OKN and VOR) are affected. Horizontal OKN drum rotation induces lateral gaze in the direction of drum rotation, as slow-phase pursuit movements are relatively intact. However, the failure to generate saccades consistently results in the eyes getting “locked up” in lateral gaze. Saccades, once generated, are often hypometric.
Congenital ocular motor apraxia (as described by Cogan) is idiopathic and is characterized by increased latency and intermittent failure of horizontal saccadic initiation. Vertical eye movements are normal. Children with congenital ocular motor apraxia characteristically use horizontal head thrusts past the point of interest, employing the VOR to move the eyes into extreme contraversion until foveation on the target is possible; this maneuver is followed by slower head rotation in the opposite direction to primary position while the eyes maintain target fixation. Older patients use subtle head movements and blinking to generate saccades. Infants with congenital ocular motor apraxia may be misperceived as having poor vision because they cannot initiate saccades to fixate objects, and they do not yet have the head control to utilize head thrusts.
The location of the lesion that causes congenital ocular motor apraxia is not known. Saccades, once initiated, are of normal velocity and often hypometric; however, normal-velocity, large-amplitude saccades are occasionally generated, indicating that the burst-cell neurons are intact and that the lesion is therefore proximal to the PPRF. Patients with this condition may have other neurologic abnormalities, including mild developmental delay. Neuroimaging may be normal, although multiple abnormalities of uncertain significance, most commonly cerebellar vermis hypoplasia, have been described. Most cases are sporadic, although familial inheritance (autosomal dominant and recessive) has been reported.
Ocular motor apraxia in children is associated with several diseases, including ataxia telangiectasia, Joubert syndrome, Pelizaeus-Merzbacher disease, Niemann-Pick disease type C, Gaucher disease, Tay-Sachs disease, abetalipoproteinemia (which causes vitamin E deficiency), and Wilson disease. In these cases, both vertical and horizontal saccades are affected. Head thrusting is present in approximately half of these cases. Other signs that should prompt a search for an associated disease include nystagmus, skew deviation, retinal pigment epithelial changes, ataxia, seizures, and progressive developmental delay. If these signs are present, a workup should include neuroimaging, an electroretinogram, and genetic evaluation.
Acquired ocular motor apraxia in adults results from bilateral lesions of the supra-nuclear gaze pathways of the frontal and parietal lobes, usually caused by bilateral strokes, often as part of an anoxic encephalopathy following either cardiac arrest or coronary artery bypass grafting. This condition has also been observed after thoracic aortic aneurysm repair. Patients are unable to initiate voluntary saccades (horizontal and vertical) and often blink to break the fixation and then turn their head toward a new point of interest. Reflexive saccades are intact. Bilateral lesions at the parieto-occipital junction may impair the guidance of volitional saccades. Such inaccurate saccades, together with optic ataxia (inaccurate arm pointing; eg, when a patient misdirects his or her hand when attempting to shake yours, despite being able to see your hand) and simultanagnosia (disordered visual attention that makes it difficult for a patient to perceive all the major features of a scene at once) are known as Balint syndrome; this syndrome is often associated with cognitive dysfunction (see Chapter 6).
Cogan DG. Congenital ocular motor apraxia. Can J Ophthalmol. 1966;1(4):253–260.
Harris CM, Shawkat F, Russell-Eggitt I, Wilson J, Taylor D. Intermittent horizontal saccade failure (‘ocular motor apraxia’) in children. Br J Ophthalmol. 1996;80:151–158.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.