Fig. 5.1
Head tilt (torticollis) in a boy with posterior fossa tumor (chordoma)
5.1.6 Cerebellar Mutism
This sign is mostly seen after neurosurgical operations; however, it is also rarely seen in patients with posterior fossa tumors preoperatively. A tumor-related mutism in an adult subject has been described [6]. A 7-year-old boy was reported to have cerebellar mutism due to a hemorrhage of a neoplastic lesion (pilocytic astrocytoma) [7].
5.1.7 Eye Abnormalities
5.1.7.1 Abducens Nerve Palsy
The paired abducens nucleus is located in the dorsal lower portion of the pons, separated from the floor of the fourth ventricle by the genu of the facial nerve (facial colliculus). The abducens motoneurons are intermixed with internuclear neurons that send their axons across the midline to the opposite medial longitudinal fasciculus (MLF), where they ascend through the pons and midbrain to end in the third nerve nucleus. Thus, the abducens nuclear complex coordinates the action of both eyes to produce a horizontal gaze [2].
Localization of Lesions
Lesions of abducens nucleus early in life can cause Möbius syndrome or Duane’s retraction syndrome. In addition to horizontal gaze disturbances, patients with Möbius syndrome have facial diplegia and may have other cranial nerve abnormalities. Duane’s retraction syndrome is characterized by a narrowing of the palpebral fissure and occasionally globe retraction on adduction. Although Duane’s retraction syndrome is predominantly congenital, and is thought to be due to anomalous innervation of the lateral rectus muscle by the inferior division of the oculomotor nerve, acquired Duane’s syndrome has been described in patients with pontine glioma, with rheumatoid arthritis, following trigeminal rhizotomy, and after removal of an orbital cavernous hemangioma by lateral orbitotomy [8, 9].
Paralysis of the abducens nerve: Lesions of the abducens nerve cause impaired ipsilateral lateral gaze (Fig. 5.2). Therefore, patients with unilateral abducens palsy complain of horizontal diplopia, worst in the direction of the paretic lateral rectus muscle. Unlike a peripheral CN VI lesion, a nuclear CN VI lesion impairs ipsilateral gaze of both eyes. This is due to the fact that the abducens nuclear complex contains interneurons projecting via the medial longitudinal fasciculus (MLF) to the contralateral oculomotor nucleus (innervating the contralateral medial rectus muscle). Brainstem lesions (abducens nuclear lesions) produce a conjugate (both eyes involved) horizontal gaze palsy toward the side of the lesion, often associated with other neurologic signs of injury to the pons (usually ipsilateral peripheral CN VII palsy) [10]. In cerebellar tumors, abducens palsy with resultant diplopia is usually the result of increased intracranial pressure rather than direct brainstem compression by the cerebellar tumor. Compression of the dorsolateral pons can produce a facial asymmetry. Actual extension of cerebellar tumor into the brainstem can produce other cranial palsies [4].
Fig. 5.2
A boy with right lateral abducens palsy secondary to a posterior fossa tumor (ependymoma)
B-nystagmus is an instability of gaze characterized by continuous movement of the eyes in any plane. When both arcs of the movement are of equal amplitude and the frequency is slow, this instability of gaze is referred to as pendular nystagmus. In most circumstances, pendular nystagmus is indicative of diminished visual acuity. If the amplitude of the movements is unequal and the frequency is rapid, the abnormality is referred to as jerk nystagmus. Abnormalities involving the vestibular system result in jerk nystagmus. Gaze-evoked jerk nystagmus indicates vestibular abnormalities, whereas vertical nystagmus is indicative of brainstem dysfunction. If gaze-evoked nystagmus is unilateral, it may indicate ipsilateral cerebellar or brainstem pathology; if bilateral it is of limited localizing value [11].
In downbeat nystagmus, the most common cause is type I Chiari malformation and other surgically treatable pathology involving craniocervical junction must be excluded. It can also be seen vascular, demyelinating or neoplastic lesions in the same area [12]. Upbeat nystagmus is much less common and is usually due to a focal gray matter lesion either at the pontomesencephalic or pontomedullary junction [13]. Up-beating vertical nystagmus shows a high correlation with lesions of the cerebellar vermis and is often seen in children with medulloblastomas [12].
Torsional nystagmus is usually due to a lesion of the lateral medulla involving the vestibular nucleus although other brainstem sites of pathology may also be responsible. It beats away from the side of the lesion, increases on lateral gaze, and can be associated with skew deviation. Seesaw nystagmus consists of elevation of one eye with intorsion, accompanied by contralateral depression and extorsion, followed by reversal of the cycle. It may be asymmetric and is usually associated with midline mesodiencephalic or hypothalamic tumor or other pathology in this region. Chiasmal glioma in childhood may present this way, and there may be an associated bitemporal hemianopia [14].
Periodic alternating nystagmus is a primary position, horizontal nystagmus that changes direction, usually every 90 s in a crescendo-decrescendo fashion, often with a short-lived null period. It can occur in a large variety of clinical contexts, usually involving diffuse bilateral brainstem pathology such as MS [11].
How to Examine the Patient with Nystagmus
When nystagmus occurs in posterior fossa tumors, it is usually most marked on lateral gaze and is coarser to the side of a hemispheric. When nystagmus is absent, the patient is rapidly brought to a full sitting position from the supine. Nystagmus induced by this maneuver can occur in vermian tumors of the cerebellum and in intrinsic brainstem tumors. Spontaneous vertical nystagmus can occur in anterior vermian tumors and in intrinsic brainstem tumors [4].
The higher the brainstem disorder, the greater the incidence of neurosurgical mass lesions because brainstem infarction most commonly involves the medulla oblongata, somewhat less often involves the pons, and occasionally is mesencephalic [4].
5.1.8 Midbrain Syndromes Associated with Tumors
Midbrain compression is usually due to a supratentorial-transtentorial process (e.g., uncal herniation), rather than due to a posterior fossa mass.
1.
Internuclear ophthalmoplegia – paralysis of the adducting eye on attempted lateral gaze but preservation of convergence; horizontal nystagmus most marked in the abducting eye usually with multiple ocular deficits including skew deviation, paralysis of vertical gaze, and ptosis. Usually bilateral and often associated vertical nystagmus (MLF syndrome) – indicates an intra-brainstem lesion [4].
2.
Intracranial hypertension with vertical gaze palsy, frequent convergence palsy, and occasional pupillary palsy (Parinaud’s or sylvian aqueduct syndrome) – indicates a pineal tumor with aqueductal obstruction and dorsal midbrain compression [4]. The syndrome consists of a paralysis of conjugate upward movement of the eyes in the absence of paralysis of convergence. As the tumor grows into the tegmentum of the midbrain, there will be additional nuclear paresis of III with reflex pupilloplegia and paresis of IV. Compression of the aqueduct causes hypertensive hydrocephalus of lateral and third ventricles. Damage to the inferior colliculi produces hearing loss. There is a tendency to fall backward and to the opposite side [15].
5.1.9 Pontine Syndromes Associated with Tumors
The pons is critically important as a nuclear center and as a bridge between the forebrain and the hindbrain. In addition to a careful neuro-ophthalmologic evaluation, evidence for dysfunction of the pontine cranial nerve nuclei, long tracts, and cerebellar pathways is sought. Lesions in the basis pontis cause a paralysis which eventually shows hypertonus due to interruption of the corticospinal tracts, ataxia due to damage of the pontocerebellar tracts, and frequently paralysis of facial and lateral rectus muscles due to interruption of the facial and abducens tracts. In the tegmentum, the nuclei of the facial and abducens nerves may be damaged. Additionally the pontine gaze centers and medial longitudinal fasciculus (MLF) can be affected [16].
When tumors infiltrate the tegmentum, they damage both nuclei and tracts – an abducens and an ipsilateral gaze palsy will usually occur together. Findings which implicate only the ventral pons strongly suggest a mass, e.g., an infiltrating tumor, abscess, vascular malformation, large basilar aneurysm, or a clivus tumor [17].
Several classical pontine syndromes have been described. In dorsolateral pontine syndrome (Foville’s syndrome), there is an ipsilateral gaze palsy and a contralateral hemiplegia; this results from involvement of both the basis and the tegmentum of the pons on one side [16]. The syndrome of Millard-Gubler or the ventral pontine syndrome is manifested by an ipsilateral facial and abducens palsy and a contralateral hemiplegia [16].
5.2 Symptoms Associated with Posterior Fossa Tumors
5.2.1 Headache
This is the most common symptom in patients with posterior fossa tumors. Headache is insidious and intermittent. It is most severe in the morning or after a nap because of increased intracranial pressure from recumbency and hypoventilation during sleep. Associated neck pain, stiffness, or head tilt suggest tonsillar herniation into the foramen magnum. Headache manifests in children as irritability and difficulty to be handled. In a study of children with posterior fossa tumors, headache was present in 63.6 % of patients [18].
5.2.2 Nausea and Vomiting
They may be due to generalized intracranial hypertension or irritation of the vagal nuclei in the medulla oblongata or area postrema of the fourth ventricle. Vomiting (including projectile vomiting) may occur, usually in the morning. Vomiting sometimes relieves headache. In the same study as above, nausea and vomiting were present in 75.8 % if children with posterior fossa tumors [18].
5.2.3 Strabismus
It is secondary to sixth nerve palsies from intracranial hypertension. Third nerve palsies may also occur (Figs. 5.3 and 5.4).
Fig. 5.3
Left oculomotor nerve palsy (pupillary dilatation, left eye down and out) in a 5-year-old girl with metastatic brainstem tumor
Fig.5.4
Strabismus secondary to the right abducens nerve palsy, this patient also has right facial nerve palsy and corneal changes on that side due to poor eye closure
5.2.4 Lethargy
Lethargy is a common symptom and it was present in 28.8 % of children in a study [18]. Young children may not complain of headache or diplopia; loss of normal childhood energy and vomiting may be the only symptoms due to pressure.
5.2.5 Hydrocephalus and Macrocephaly
In a study of children with posterior fossa tumors, 80.3 % demonstrated radiological evidence of hydrocephalus on their initial scan [18]. Children with posterior fossa tumors tend to present with a shorter duration of symptoms than supratentorial tumors secondary to early obstruction of cerebrospinal fluid (CSF) pathways [19]. In the first 6 months of life, the most common presenting signs are the signs of increased intracranial pressure and hydrocephalus [20, 21]. These infants are more likely to present with macrocephaly because their immature cranial vaults have the ability to accommodate the increased volume caused by the tumor, hydrocephalus, or a combination of both [20]. The intracranial hypertension and hydrocephalus may also contribute to the development of nystagmus, sundowning, and misalignment of the eyes. In addition, less specific complaints of emesis, lethargy, irritability, and poor feeding were more common than focal findings and seizures. The decreased incidence of focal neurologic deficits can be attributed to the relative immaturity of the neonatal brain [22]. Increased head circumference, hyperreflexia, hypotonia, stridor, pooling of secretions, abnormal gag reflex, nystagmus, dysconjugate eye movements, and facial palsy were reported in newborns with congenital diffuse intrinsic brainstem tumor [15].
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