Motor Problems

• Neuroanatomy of upper and lower motor neurons

  Upper motor neurons (UMN):

  • Cell bodies are in the cortex – mainly pyramidal cells in the motor cortex – or brainstem – extrapyramidal neurons. Carry information to the lower motor neurons via the pyramidal tracts (corticospinal and corticobulbar) and extrapyramidal tracts.
  • An UMN lesion is a motor pathway lesion above the level of the anterior horn cell, and typically refers to pyramidal lesions.
  • The lateral corticospinal tract (90% of the corticospinal tract) decussates in the medulla, and controls fine muscle movement, mainly of the limbs. Cortical lesions thus cause contralateral deficits, while spinal cord lesions cause ipsilateral deficits. The anterior corticospinal tract decussates where it exits the spinal cord, and mainly controls trunk muscles and posture.
  • UMN tracts usually synapse with interneurons, and less commonly with the lower motor neurons themselves.

  Lower motor neurons (LMN):

  • Cell bodies are in the anterior horn of the spinal cord or the motor nuclei of the cranial nerves. When the term 'motor neuron' is used without a qualifier, it typically refers to these.
  • Skeletal muscle is controlled by α motor neurons. γ motor neurons keep muscle spindles taught, to help detect muscle movement.
  • A LMN lesion is a motor pathway lesion of the anterior horn cell or below, including the ventral nerve root which emerges from the spinal cord, the peripheral nerve fibres outside of the spinal cord, the neuromuscular junction (NMJ), or the muscle.
  • This is slightly different from the 'peripheral nervous system', as some lower motor neuron elements are in the central nervous system: anterior horn cell, nerve root, cauda equina, cranial nerve bodies, and cranial nerve peripheral fibres.

• Upper motor neuron lesions

  Clinical features

  • Early UMN lesions are like LMN lesions, with reduced tone and reflexes.
  • Established lesions: normal bulk, ↑tone, ↑reflexes (including Babinski and clonus), ↓power, no fasciculations.

  Pyramdial and extrapyramidal motor symptoms

  • Pyramidal lesions: arm extensors weak, leading to flexed arm, but leg flexors weak (including plantarflexion), leading to leg extension. Spasticity: velocity-dependant ↑tone.
  • Extrapyramidal lesions: rigidity, ↑tone throughout passive movement.

  Presentation by location

  Cortical lesions:

  • Contralateral symptoms.

  Brainstem lesions may cause:

  • Contralateral UMN lesions (if above medulla).
  • Ipsilateral LMN cranial nerve lesions.
  • May also affect sensory tracts.

  Spinal cord lesions (aka myelopathy):

  • UMN symptoms below the level of the lesion.
  • Symptoms often bilateral, and may cause paraplegia (legs only) or quadriplegia (all limbs; aka tetraplegia).
  • There may be LMN symptoms at the level of the lesion, due to LMN cell body or root damage.
  • Causes: spinal cord compression, transverse myelitis, multiple sclerosis, spinal artery thrombosis, cord vasculitis, ↓B12, syphilis.

• Lower motor neuron lesions

  General presentation

  Established lesions: ↓bulk, ↓tone, ↓reflexes, ↓power, fasciculations.

  Presentation by location

  • Lower motor neuron: fasciculations, wasting.
  • Nerve root (radiculopathy): mainly sensory symptoms. Conditions: radiculopathy, cauda equina syndrome.
  • Peripheral nerves (neuropathy): weak, hyporeflexia, peripheral neuropathy.
  • NMJ: fatigable weakness and normal reflexes (myasthenia gravis), or weak and reduced reflexes that improve with repetition (Lambert-Eaton).
  • Muscle (myopathy): dystrophy leads to wasting, ↓tone, and possible additional symptoms like myotonia (slow relaxation after contraction). Reflexes usually preserved.

• Mixed upper-lower motor neuron lesions

  Causes

  • Motor neuron disease.
  • Disc degeneration which compresses the cord and a root.
  • Spinal cord lesions in which there is anterior horn cell and/or root damage.
  • Conus medullaris syndrome.
  • Multifocal motor neuropathy.
  • Vitamin B12 deficiency.

• Myotomes and reflexes

  Upper limb

  • C5: shoulder abduction, elbow flexion. Test with biceps reflex (C5-6).
  • C6: elbow flexion, forearm supination. Test with supinator reflex.
  • C7: finger and elbow extension. Test with triceps reflex (C7-8).
  • C8: finger flexion.
  • T1: small muscles of hand.

  Lower limb

  • L1-L2: hip flexion.
  • L3-L4: knee and foot extension (i.e. dorsiflexion of foot). Test with knee reflex.
  • L5-S1: knee flexion, hip extension, ankle eversion, hallux extension/dorsiflexion (L5), ankle (plantar)flexion (S1-2). Ankle reflex tests S1-2.

• Proximal weakness

  Weakness of the hips and shoulders. 'Global weakness' if there is distal limb weakness too.

  Signs and symptoms

  • Weak hips: hard getting up stairs, from chairs, up hills.
  • Weak shoulders: hard reaching up to shelves, washing hair.

  Causes

  • Usually LMN muscle lesions.
  • Inflammatory myopathies e.g. dermatomyositis.
  • Metabolic and endocrine: ↓K+, ↑Ca2+, thyroid disease, Cushing's.
  • Autoimmune: myasthenia gravis, Guillain-Barré.
  • Drugs: statins, alcohol.

• Mononeuropathy

  • Damage to a single peripheral nerve. Usually due to trauma, compression, or stretching of the nerve.
  • It can also occur as part of mononeuritis multiplex, in which multiple, non-contiguous nerves are affected.
  • Most will eventually recover: some self-resolve, some need surgical decompression.
  • See Cranial nerve problems, Upper limb mononeuropathies, and Lower limb mononeuropathies for more.

• Mononeuritis multiplex

  Definition

  Multiple, non-contiguous peripheral nerve lesions.

  Causes

  • Diabetes
  • Vasculitis: PAN, GPA (Wegner's), EGPA (Churg-Strauss).
  • Inflammation: SLE, RA, sarcoidosis.
  • Infection: HIV, Lyme, leprosy.
  • Amyloidosis

• Polyneuropathy

  • Damage to many peripheral nerves.
  • Often synonymous with peripheral neuropathy.

  Causes

  Causes of mainly sensory polyneuropathies:

  • Diabetes
  • Alcohol
  • Vitamin B12 and folate deficiency.
  • CKD

  Causes of mainly motor polyneuropathies:

  • Acute (Guillain-Barré) and chronic inflammatory demyelinating polyneuropathy. Causes a patchy radiculopathy more than a distance-dependant polyneuropathy, so proximal weakness may be seen instead of the classic glove and stocking findings.
  • Lead poisoning.

  Signs and symptoms

  • Motor: usually a symmetrical distal weakness, leading to dropping things and/or falling. Flexors and extensors equally affected.
  • Sensory: usually glove and stocking distribution of numbness, pins and needles, and/or burning.
  • Symptoms all the way up to torso suggests a spinal cord lesion rather than a polyneuropathy.
  • May be accompanied by ANS symptoms.
  • Bowel and bladder are generally not affected, so if present it would suggest a CNS lesion (brain, spinal cord, or cauda equina).

• Autonomic neuropathy

  Causes

  • Diabetes
  • Amyloidosis
  • Autoimmune: Guillain Barré, SLE.
  • HIV
  • Lambert-Eaton myasthenic syndrome.

  CNS causes of ANS dysfunction present in a similar way (but are not a type of neuropathy):

  • Parkinson's
  • Multiple system atrophy.
  • MS

  Signs and symptoms

  • Urinary incontinence or retention.
  • Altered HR and orthostatic hypotension.
  • GI disturbances.

• Cerebellar disease

  Causes

  Acquired:

  • Stroke
  • Tumour. Cerebellar tumours may also obstruct CSF, causing hydrocephalus.
  • MS
  • Nutritional: ↓thiamine, ↓vitamin E, coeliac disease (gluten ataxia).
  • Infection: VZV, HSV1, HIV, leptospirosis.
  • Toxins and drugs: alcohol, heavy metals, phenytoin.

  Genetic or congenital:

  • Friedreich's ataxia: autosomal recessive mutation in the mitochondrial protein frataxin. Widespread central and peripheral nervous system disease, primarily affecting the spinal cord through degeneration and demyelination. Presents with limb and/or gait ataxia from cerebellar and spinocerebellar tract disease, as well as bulbar palsy, oculomotor symptoms, scoliosis, cardiomyopathy, and diabetes mellitus.
  • Spinocerebellar ataxia.
  • Ataxia telangiectasia.
  • Dandy-Walker syndrome: absent cerebellar vermis.
  • Arnold Chiari malformation: downward herniation of cerebellar tonsils through foramen magnum.

  Signs and symptoms

  • Signs are ipsilateral to the lesion.
  • Many are not unique to cerebellar disease.

  DANISH:

  • Dysdiadokokinesia
  • Ataxia: limb or truncal. If it's worse on eye closing, it's a dorsal column lesion ('sensory ataxia'), not cerebellar.
  • Nystagmus. Can be horizontal, vertical, or rotational. Fast phase towards lesion side.
  • Intention tremor and past-pointing (dysmetria).
  • Slurred speech – dysarthria – and/or scanning speech – slow with separated syllables.
  • Hypotonia.

  Brainstem or cerebellum?

  The afferent and efferent pathways to the cerebellum pass through the brainstem, so brainstem lesions will often produce cerebellar signs. In addition, you may also see:

  • Cranial nerve lesions.
  • Any range of motor and sensory problems in the body, as descending motor tracts and ascending sensory tracts all pass through.

  So think brainstem in any patient with a wide range of signs and symptoms. Common causes of lesions are MS, stroke, and tumour.