Download Crossed corticospinal or `pyramidal` tracts

Crossed corticospinal or 'pyramidal' tracts
Clinical characteristics of pyramidal lesions
(see Table 18.13)
It is important to understand that the signs of a pyramidal lesion may be minimal. Weakness, spasticity or
changes in superficial reflexes may predominate and the absence of one group of signs does not exclude a
UMN lesion.
Drift of the upper limb
In the normal individual the outstretched upper limbs are held symmetrically, even when the eyes are
closed. In a pyramidal (UMN) lesion, when both upper limbs are held outstretched, palms uppermost, the
affected limb drifts downwards and medially. The forearm tends to pronate and the hand flex slightly at the
fingers. This important sign is often the first to occur, sometimes before weakness or reflex changes are
evident.
Weakness and loss of skilled movement
A pyramidal (UMN) lesion above the decussation in the medulla (e.g. an infarct in the internal capsule),
causes weakness of the opposite limbs; this is a contralateral hemiparesis. In such an acute lesion, this
weakness will be immediate and dense – a hemiplegia (see below) – but in partial progressive lesions (e.g.
a glioma) there is a characteristic pattern of increasing weakness in the hemiparetic limbs. In the upper limb
the flexors remain stronger than the extensors, while in the lower limb the extensors remain stronger than
the flexors. In the upper limb the weaker movements are thus shoulder abduction and elbow extension; in
the forearm and hand, the wrist and finger extensors and abductors are weaker than their antagonists.
In the lower limb the weaker movements are flexion and abduction of the hip, flexion of the knee, and
dorsiflexion and eversion of the ankle.
In addition to weakness, there is loss of skilled movement. For example, fine finger and toe control
diminishes.
When the UMN lesion is below the decussation of the pyramids, the hemiparesis is on the same side as the
lesion. This situation is unusual.
Increase in tone (spasticity)
An acute lesion of one pyramidal tract (e.g. the internal capsule infarct mentioned above) causes initially a
flaccid paralysis, and areflexia. An increase in tone follows within several days owing to loss of the
inhibitory effect of the corticospinal pathway and an increase in spinal reflex activity. This increase in tone
affects all muscle groups on the affected side but is most easily detected in the stronger muscles. It is
characterized by changing resistance to passive movement; the change is sudden – the clasp-knife effect.
The tendon reflexes are exaggerated and clonus is often evident.
Changes in superficial reflexes
The normal flexor plantar response becomes extensor. In a severe lesion (e.g. the internal capsule infarct)
this extensor response can be elicited from a wide area of the affected limb. As recovery occurs, the area
that is sensitive diminishes until only the posterior third of the lateral aspect of the sole is receptive. The
stimulus should be unpleasant (an orange-stick is the correct instrument). An extensor plantar is certain
when the dorsiflexion of the great toe is accompanied by fanning (abduction) of the other toes.
The abdominal (and cremasteric reflexes) are abolished on the affected side.
Muscle wasting (except from disuse) is not a feature of pyramidal lesions. Muscles remain normally
excitable electrically.
Clinical patterns of UMN disorders
Two main patterns of UMN (pyramidal) lesions are recognizable: hemiparesis and paraparesis.
Hemiparesis means weakness of the limbs of one side; it is usually (but not always) caused by a lesion
within the brain. Paraparesis means weakness of both lower limbs and is characteristically diagnostic (but
again, not always) of a spinal cord lesion.
The terms hemiplegia and paraplegia strictly indicate total paralysis, but the terms are often used loosely to
describe severe weakness.
Hemiparesis
The level within the corticospinal tract is recognized by various accompanying features.
Motor cortex. Weakness localized to one contralateral limb (monoplegia) or part of a limb (e.g. a weak
hand) is characteristic of an isolated lesion of the motor cortex (e.g. a secondary neoplasm). There may be a
defect in higher cortical function (e.g. aphasia if the speech area is affected). Focal epilepsy may be present.
Internal capsule. Since all corticospinal fibres are tightly packed in the internal capsule, occupying about 1
cm2, a small lesion causes a large deficit. For example, an infarct of a small branch of the middle cerebral
artery (see p. 1049) causes a sudden, dense, contralateral hemiplegia that includes the face.
Pons. A pontine lesion (e.g. a plaque of multiple sclerosis) is rarely confined only to the corticospinal tract.
As adjacent structures such as the sixth and seventh nuclei, MLF and PPRF (see p. 1018) are involved,
there are other localizing signs – VI and VII nerve palsies, intranuclear nuclear ophthalmoplegia (INO) or a
lateral gaze palsy, with the contralateral hemiparesis.
Spinal cord. An isolated lesion of a single lateral corticospinal tract within the cord (which is unusual)
causes an ipsilateral UMN lesion, the level of which is indicated by a reflex level (e.g. absent biceps jerk),
the presence of a Brown–Séquard syndrome or muscle wasting at the level of the lesion (see p. 1034).
Paraparesis
(see Table 18.14)
Paraparesis (and tetraparesis, when the four limbs are involved) indicates bilateral damage to the
corticospinal tracts. Spinal cord compression (see p. 1085) or other cord disease is the usual cause, but
cerebral lesions occasionally can produce paraparesis. Paraparesis, including here tetraparesis, is a feature
of many neurological conditions which are recognizable by their clinical features, making this differential
diagnosis one of pivotal importance in neurology.
The extrapyramidal system
The extrapyramidal system is a general term for the basal ganglia. In disorders of this system, either or both
of two features become apparent in the limbs and axial muscles:
• reduction in speed, known as bradykinesia (slow movement) or akinesia (no movement), with muscle
rigidity
• involuntary movements (tremor, chorea, dystonia, hemiballismus, athetosis).
The most common extrapyramidal disorder is Parkinson’s disease.

STRUCTURE

FUNCTION AND DYSFUNCTION
STRUCTURE
The corpus striatum, consisting of the caudate nucleus, globus pallidus and putamen (the latter two forming
the lentiform nucleus), lies close to the substantia nigra, thalami and subthalamic nuclei. There are
interconnections between these structures and the cerebral cortex, the cerebellum and the reticular
formation, the cranial nerve nuclei (particularly the vestibular nerve) and the spinal cord.
FUNCTION AND DYSFUNCTION
The overall function of this complex system is the initiation and modulation of movement. The system
modulates cortical motor activity by a series of servo loops, between the cortex and the various structures
within the basal ganglia.
It is now clear that in many involuntary movement disorders there are substantial and specific changes in
neurotransmitter profile rather than discrete anatomical lesions. As an introduction to this difficult field,
neurotransmitter changes in two diseases are considered (Table 18.15).
Therapeutic alteration of the neurotransmitter profile causes characteristic clinical changes. For example:
• In patients with Parkinson’s disease, an increase in dopamine activity due to levodopa therapy or
dopaminergic agonists such as bromocriptine relieves rigidity. However, in excess (in both normal people
and those with Parkinson’s disease), levodopa therapy causes chorea.
• In normal subjects, an increase in acetylcholine activity or a decrease in dopamine activity causes rigidity
and bradykinesia (parkinsonism). Reserpine (an obsolete hypotensive drug which depletes neurones of
dopamine) and phenothiazines or butyrophenones (which block dopaminergic neurones) cause or
exacerbate parkinsonism.
Extrapyramidal disorders are classified broadly on clinical grounds into the akinetic-rigid syndromes (see
p. 1062) in which poverty of movement predominates, and the dyskinesias, in which there are a variety of
excessive involuntary movements (p. 1065).
The cerebellum
The third system of motor control is involved with coordination, rather than speed. The cerebellum receives
afferent fibres from:
• proprioceptive organs in joints and muscles
• vestibular nuclei
• basal ganglia
• the corticospinal system
• olivary nuclei.
Efferent fibres pass from the cerebellum to:
• each red nucleus
• vestibular nuclei
• basal ganglia
• the corticospinal system.
Each lateral lobe of the cerebellum coordinates movement of the ipsilateral limb. The vermis (a midline
structure) is concerned with maintenance of axial (midline) posture and balance.

Cerebellar lesions

Lateral cerebellar lobes

Midline cerebellar lesions
Cerebellar lesions
Expanding mass lesions within the cerebellum obstruct the aqueduct to produce hydrocephalus, causing
severe pressure headaches, vomiting and papilloedema. Coning of the cerebellar tonsils through the
foramen magnum and respiratory arrest occur, often within hours. Rarely tonic seizures of the limbs occur
with cerebellar masses.
Lateral cerebellar lobes
A lesion within one cerebellar lobe (e.g. a tumour or infarction) causes disruption of the normal sequence
of movements (dyssynergia) on the side of the lesion. A collection of specific signs develop.
Posture and gait. The outstretched arm is held still in the early stages of a cerebellar lesion, but there is
rebound upward overshoot when the limb is pressed downwards by the examiner and released. Gait is
ataxic with a broad base; the patient falters towards the side of the lesion.
Tremor and ataxia. Movement is imprecise in direction, in force and in distance (dysmetria). Rapid
alternating movements (tapping, clapping or rotary movements of the hand) are clumsy and disorganized
(dysdiadochokinesis). Intention tremor (action tremor, with past-pointing) is seen when the finger-nosefinger and heel-shin tests are performed.
Nystagmus. Coarse horizontal nystagmus (see p. 1024) appears with lateral cerebellar lobe lesions. Its
direction is towards the side of the lesion.
Dysarthria. Speech is affected (usually with bilateral lesions). A halting, jerking dysarthria results – the
scanning speech of cerebellar lesions.
Other signs. Titubation – rhythmic tremor of the head in either to and fro (yes–yes) movements or rotary
(no–no) movements – also occurs, mainly when cerebellar connections are involved (e.g. in essential
tremor and MS).
Hypotonia (floppy limbs) and depression of reflexes are also sometimes seen with cerebellar disease, but
are usually of little value as localizing signs. Pendular (i.e. slow) reflexes also occur
Midline cerebellar lesions
Lesions of the cerebellar vermis have a dramatic effect on the equilibrium of the trunk and axial
musculature. This truncal ataxia means difficulty in standing and sitting unsupported, with a rolling, broad,
ataxic gait.
Lesions of the flocculonodular region cause vertigo, vomiting and ataxia of gait if they extend to the roof of
the fourth ventricle.
Table 18.16 summarizes the main causes of cerebellar disease.