NA Lab 8
Plate 15 - Pons @ Level of deep Cerebellar Nuclei
4 = 4th Ventricle
Plate 22 - Middle Diencephalon Coronal Section through Mammillary Bodies
CC = Corpus Callosum IC = Internal Capsule 3 = Third Ventricle L = Lateral Ventricle
What is the major function of the Basal Ganglia?
Participates in complex neuronal networks that influence the initiation and cessation of movement. There are four main "channels" or circuitry loops through the basal ganglia: motor, oculomotor, cognitive, and limbic.
Plate 19 - Junction of Midbrain and Diencephalon
Red Circle = Cerebral Aqueduct
A 72-year-old man with a history of hypertension and hypercholesterolemia was watching TV one night and suddenly saw two faces on the screen, diagonally displaced. This diplopia went away when he covered one eye. In order not to alarm his wife, he quietly made his way to bed, but the next morning the diplopia was unchanged, and he also noticed gait unsteadiness with staggering to the right. He tried using a friend's walker, but then also noticed that his right hand was clumsy. For example, he had difficulty picking up a credit card from the table with his right hand. On exam, his left eye would elevate by only 1 mm, adduct by only 2 mm, and depress by only 3 mm (see diagram below). Left eye abduction was normal. He had diagonal diplopia, and there was a left ptosis, with the palpebral fissure measuring 4 mm on the left and 9 mm on the right (see diagram below). The left pupil had a slightly irregular shape, but reacted normally to light. The patient also had mild right dysmetria on finger-to-nose and heel-to-shin testing, and an unsteady gait, tending to list to the right. The remainder of the exam was normal, except for the right plantar response, which was equivocal. Impaired innervation of which of the following most likely accounts for the ptosis in this patient? a. Levator palpebrae superioris muscle b. Palpebral portion of orbicularis oculi muscle c. Superior rectus muscle d. Superior tarsal muscle
a. Levator palpebrae superioris muscle CN III innervates levator palpebrae superioris which opens the lid.
Large-amplitude, rotatory or flinging movements of proximal limb muscles are referred to as which of the following? a. Athetosis b. Ballismus (ballism) c. Cogwheel rigidity d. Dystonia e. Tremor
b. Ballismus (ballism) Athetosis refers to writhing, twisting movements of the limbs, face, and trunk, whereas tremor refers to rhythmic or semirhythmic oscillating movements, in which both agonist and antagonist muscles are activated. Cogwheel rigidity is characteristic of Parkinson's Disease, and in dystonia the patient assumes abnormal, often distorted positions of the limbs, trunk, or face that are more sustained than in athetosis. The patient in this case exhibits hemiballismus (unilateral ballism), which refers to large amplitude rotary or flinging movements of the extremities that manifest contralateral to a lesion in the basal ganglia.
A 72-year-old man with a history of hypertension and hypercholesterolemia was watching TV one night and suddenly saw two faces on the screen, diagonally displaced. This diplopia went away when he covered one eye. In order not to alarm his wife, he quietly made his way to bed, but the next morning the diplopia was unchanged, and he also noticed gait unsteadiness with staggering to the right. He tried using a friend's walker, but then also noticed that his right hand was clumsy. For example, he had difficulty picking up a credit card from the table with his right hand. On exam, his left eye would elevate by only 1 mm, adduct by only 2 mm, and depress by only 3 mm. Left eye abduction was normal. He had diagonal diplopia, and there was a left ptosis, with the palpebral fissure measuring 4 mm on the left and 9 mm on the right. The left pupil had a slightly irregular shape, but reacted normally to light. The patient also had mild right dysmetria on finger-to-nose and heel-to-shin testing, and an unsteady gait, tending to list to the right. The remainder of the exam was normal, except for the right plantar response, which was equivocal. Involvement of which of the following cranial nerves best accounts for the patient's eye movement abnormalities? a. CN II b. CN III c. CN IV d. CN VI e. CN VIII
b. CN III The ptosis and eye movement abnormalities in this patient indicate involvement of CN III, most likely with the nerve fascicles in the midbrain, rather than the entire oculomotor complex. The pupil is not dilated on the left side, but does have an irregular shape (corectopia) that may be seen with some midbrain lesions. A technique called the red glass test can be used when examining patients with diplopia; a transparent piece of red glass or plastic is held over the right eye, and a small white light held directly in front of the patient. The patient is asked to follow the light as it is moved to nine different positions of gaze, and to report the position of the red (right eye) and white (left eye) images. The red and white images are fused (superimposed) in all positions of gaze for an unimpaired patient; in a patient with diplopia, the abnormal patterns of red glass test results can indicate which cranial nerve is affected. In the case of this patient, red glass testing was consistent with diagonal diplopia and CN III involvement.
A 72-year-old man with a history of hypertension and hypercholesterolemia was watching TV one night and suddenly saw two faces on the screen, diagonally displaced. This diplopia went away when he covered one eye. In order not to alarm his wife, he quietly made his way to bed, but the next morning the diplopia was unchanged, and he also noticed gait unsteadiness with staggering to the right. He tried using a friend's walker, but then also noticed that his right hand was clumsy. For example, he had difficulty picking up a credit card from the table with his right hand. On exam, his left eye would elevate by only 1 mm, adduct by only 2 mm, and depress by only 3 mm (see diagram below). Left eye abduction was normal. He had diagonal diplopia, and there was a left ptosis, with the palpebral fissure measuring 4 mm on the left and 9 mm on the right (see diagram below). The left pupil had a slightly irregular shape, but reacted normally to light. The patient also had mild right dysmetria on finger-to-nose and heel-to-shin testing, and an unsteady gait, tending to list to the right. The remainder of the exam was normal, except for the right plantar response, which was equivocal. Which is the most likely localization for this patient's lesion? a. Left pons tegmentum b. Right pons tegmentum c. Left midbrain tegmentum d. Right midbrain tegmentum e. Right cerebellar hemisphere
c. Left midbrain tegmentum The most likely clinical localization for the lesion in this case is the left midbrain tegmentum. The patient is a man in his seventies with sudden onset of symptoms, and a history of hypertension and elevated cholesterol; thus it is likely that the infarct was caused by occlusion of penetrating vessels at the rostral end of the basilar artery and proximal left posterior cerebral artery. The lesion involved oculomotor nerve fascicles and the dentatothalamic tract (continuation of the superior cerebellar peduncle to the thalamus) in the left midbrain tegmentum (Claude's syndrome), and possibly part of the left cerebral peduncle (subtle right plantar abnormality). A larger midbrain lesion that includes oculomotor nerve fascicles, dentatothalamic tract, and cerebral peduncle on one side (resulting in a combination of the findings from Cases 8 and 10) results in ipsilateral CN III palsy, contralateral hemiparesis, and contralateral ataxia (Benedikt's syndrome). The Table on the following pages summarizes three lesion types in the midbrain that involve oculomotor nerve fibers exiting the brainstem.
A 65-year-old HIV-positive man began having involuntary flinging movements of the right arm and leg, which became progressively worse over the course of 1 month, making gait and use of the right hand difficult. On exam, he had continuous wild, uncontrollable flapping and circular movements of the right arm and occasional jerky movements of the right leg, with an unsteady gait, and falling to the right. The remainder of the exam was unremarkable. An alternative localization of the lesion in this patient, also consistent with the clinical presentation, might be which of the following? a. Direct pathway neurons in the right striatum b. Direct pathway neurons in the left striatum c. Indirect pathway neurons in the right striatum d. Indirect pathway neurons in the left striatum
d. Indirect pathway neurons in the left striatum
A 65-year-old HIV-positive man began having *involuntary flinging movements of the right arm and leg*, which became progressively worse over the course of 1 month, making gait and use of the right hand difficult. On exam, he had continuous wild, uncontrollable flapping and circular movements of the right arm and occasional jerky movements of the right leg, with an unsteady gait, and falling to the right. The remainder of the exam was unremarkable. Which is the most likely localization for this patient's lesion? a. Left cerebellar hemisphere b. Right cerebellar hemisphere c. Right subthalamic nucleus d. Left subthalamic nucleus e. Substantia nigra pars compacta
d. Left subthalamic nucleus The diagram below is a reminder that the indirect pathway of the motor loop, which suppresses movement, includes the subthalamic nucleus in the diencephalon. When the indirect pathway predominates (e.g. when we want to suppress movement of a limb), the contralateral subthalamic nucleus is "disinhibited," and the neurons of this structure release excitatory neurotransmitter (glutamate) at their terminals in GPi, which in turn inhibits the neurons of the VL nucleus of the thalamus. Transmission to the SMA and primary motor area of the cerebral cortex is reduced, and movement on the side contralateral to these structures is suppressed. For the patient in this case, a lesion in the subthalamic nucleus on the left side would reduce or destroy the ability of the indirect pathway to suppress movement of the upper and lower limbs on the right side, resulting in the uncontrollable, large-amplitude flinging and rotary movements characteristic of hemiballismus. A lesion affecting the indirect pathway neurons in the left striatum (caudate or putamen) might also cause hemiballismus on the right side.
Which of the following clinical findings is most consistent with a lesion that involves cerebellar pathways? a. Diagonal diplopia b. Irregular left pupil (corectopia) c. Left ptosis d. Mildly abnormal right plantar response e. Right-sided dysmetria on finger-to-nose and heel-to-shin
e. Right-sided dysmetria on finger-to-nose and heel-to-shin Superior Cerebellar Peduncle/Dentatothalamic Tract Involvement. This patient has mild right ataxia on finger-to-nose and heel-to-shin testing, with an unsteady gait and tendency to list to the right; all of these findings are consistent cerebellar involvement. However, since the oculomotor nerve impairment indicates that the lesion is in the midbrain, we need to consider cerebellar pathways that traverse this region of the mesencephalon. Recall that the major efferent pathways from the cerebellum exit by way of the superior cerebellar peduncles; included in the SCP are the dentatothalamic and interpositothalamic tracts, which will terminate in the VL nucleus of the thalamus on the contralateral side. All of these efferent axons cross the midline at the decussation of the superior cerebellar peduncles, in the caudal midbrain ("Russian man" section); therefore, dentatothalamic axons that originated in the right dentate nucleus will terminate in the left VL nucleus of the thalamus. Thalamocortical axons from the VL nucleus terminate in the left primary motor cortex, which is the location of UMNs for control of movement on the right side of the body. The diagram below shows the location of the dentatothalamic tract in the rostral midbrain.
In addition to a lacunar infarction or small hemorrhage, which of the following intracranial lesion types or disorders should be considered, due to the HIV status of the patient? a. Glioblastoma b. Lyme disease c. Meningioma d. Non-communicating hydrocephalus e. Toxoplasmosis
e. Toxoplasmosis Because the patient in this case is 65 years old, a lacunar infarction or small hemorrhage in the left subthalamic nucleus would be a possible cause of the lesion. However, because of his HIV-positive history, and because of the gradual onset and progression of symptoms over a month (unusual for an infarct or hemorrhage), an intracranial mass lesion such as toxoplasmosis or primary CNS lymphoma should be considered. In this case, a brain MRI with gadolinium revealed a ring-enhancing lesion in the region of the left subthalamic nucleus, and both serum and cerebrospinal fluid Toxoplasma titers were positive.
