Neural circuitry of eye movement
- Rostral interstitial nucleus: This is a group of cells in the prerubral field of the mesencephalic reticular formation that are part of the riMLF. The riMLF is considered the location of vertical presaccadic burst neurons (12).
- Paramedian pontine reticular formation: This group of cells within the pontine segment of the riMLF is the location of both horizontal and vertical gaze control.
- Input circuits: The supranuclear control is provided by signals from frontal, parietal and occipital cortex, cerebellum and superior colliculi.
- Output circuits: The efferent pathways for upward and downward gaze are separated. The one for the upward gaze leaves the riMLF dorsolaterally and decussates through the posterior commissure to the contralateral riMLF before projecting bilaterally to the oculomotor nuclei. This long circuit explains the selective damage to this pathway (12). The decussation of the downgaze fibers is probably lower in the midbrain. Other midbrain structures involved are the interstitial nucleus of Cajal and the nucleus of Darkschewitsch (12).
Aqueductal syndrome (aka Koerber-Salus-Elshnig, pretectal or dorsal midbrain syndrome)
- Eye movement abnormalities: Upward gaze paralysis (Parinaud’s syndrome), convergence spasm, nystagmus retractorius (1).
- Pupillary abnormalities: Reaction to accommodative stimuli but not to light (1).
- Collier’s sign: Retraction of the upper eyelids. Also called the ‘tucked lid” sign, or the posterior fossa stare, it is invariable associated with a Parinaud’s syndrome. It is seen when the patient looks either straight ahead or upwards. Sustained upward gaze worsens the lid retraction. The retraction resolves with downward gaze (1, 6,15).
- Sun-setting sign: Collier’s sign associated with upward gaze paralysis corresponds to the “setting sun sign” described in infant patients. The transtentorial pressure gradient, accountable for severe deformation of the periaqueductal region and severe compression of the periaqueductal gray matter ventral to the aqueduct, is probably responsible for this syndrome, together with the compression of tectum and posterior commissure between the dilated suprapineal recess of the third ventricle above and the dilated rostral aqueduct below (18, 66).
The impairment of different neuronal networks at the level of the striatum and midbrain can explain the parkinsonian signs (26). The degree of reversibility of symptoms in response to treatment of hydrocephalus should reflect the extent of permanent neuronal damage. Direct or indirect mass effect and ischemia can explain the damage in these circuits (26, 111).
- Substantia nigra: The network most frequently involved is the nigro-striatal dopaminergic system, because of torsion or direct compression of the midbrain or its striatal efferents by the expanding third ventricle and rostral aqueduct. These alterations explain the levodopa-responsive parkinsonism that develops.
- Extrapyramidal pathways: Occasionally, the pallidal efferents to thalamus and the connection of the thalamus to the supplementary motor cortex in the periventricular matter are implicated. They are accountable for the levodopa resistant parkinsonism characteristic of normal pressure hydrocephalus.
- Rhythmic contractions of eyelids: Other symptoms may be secondary to damage of the nigro-striatal dopaminergic pathway, such as blepharoclonus (39), inappropriate brief rhythmic contractions of the orbicularis oculi, and akinetic mutism. This is a condition characterized by unresponsiveness with the appearance of alertness (4).
- Extreme parkinsonism: Curran and Lang (26) placed akinetic mutism at the extreme end of the hydrocephalus-parkinsonian spectrum identifying the pathogenetic mechanism in a combination of severe bradykinesia/akinesia plus compromise of the ascending reticular activating system, such as the monoaminergic projection fibers from the brainstem in regions adjacent to the third ventricle that may be impaired when rapid ventricle dilatation occurs.
- Extinguished after multiple shunt failures: Multiple shunt failures may ease this phenomenon by the repeated bouts of ventricle dilation and relaxation that induced alterations of the physical characteristics of the brain parenchyma, so that expansion at a lower pressure and more rapid dilation are permitted. Usually dopamine agonists, such as levodopa and bromocriptine, are effective (4).