Monoparesis

The term monoparesis denotes the presence of neurological deficits in one limb. However, monoparetic animals are frequently presented to the veterinary surgeon with the main complaint being lameness. True paresis and lameness of orthopaedic origin can be difficult to differentiate so complete and careful neurological and orthopaedic examinations are mandatory. A lame dog or cat without an obvious orthopaedic cause may well have a neurological lesion which, in many cases, could be resolved if detected early in the course of the disease, whereas a delayed diagnosis (i.e. nerve root neoplasia) can have devastating consequences.

Definitions of clinical terms summarizes the definitions of the more commonly used clinical terms in this site.

Definitions of clinical terms

Clinical term Definition
Monoparesis Decreased voluntary motor function of one limb
Monoplegia Absent voluntary motor function of one limb
Mononeuropathy Dysfunction of a single peripheral nerve
Nerve root signature Pain manifested as lameness due to nerve root irritation/compression
Horner’s syndrome Miosis, ptosis, enophthalmos and protrusion of the third eyelid
Paraesthesia Abnormal sensation ― tingling or itching ― due to denervation, usually in the distal part of limbs. Can lead to self-mutilation

Clinical signs

The monoparetic animal shows motordysfunction (usually manifested as weakness) in one limb and frequently sensory dysfunction (manifested as conscious proprioceptive deficits and areas of hypoaesthesia or anaesthesia) in the same limb.

Monoparesis is most commonly caused by dysfunction of the lower motor neurons (LMNs) innervating the affected limb (). The lesion responsible for the dysfunction may affect the motor neuron cell body in the ventral horn of the spinal cord grey matter, its axon (ventral nerve root, spinal nerve, peripheral nerve) or the neuromuscular junction (). A lesion affecting either a single nerve or several nerves that lie in close proximity (i.e. in the brachial or lumbosacral plexi) can result in clinical monoparesis.

Thoracic limb monoparesis can present in conjunction with ipsilateral Homer’s syndrome and/or a decrease or loss of the ipsilateral cutaneous trunci reflex ().

  • Horner’s syndrome is caused by decrease or loss of sympathetic innervation to the eye. Lesions affecting the T1 and T2 ventral nerve roots can injure the preganglionic sympathetic nerves exiting the spinal canal at this level and cause miosis (partial Horner’s syndrome) of the ipsilateral pupil. Complete Horner’s syndrome (miosis, ptosis, enophthalmos, protrusion of the third eyelid) rarely occurs with lesions in this location.
  • Decrease or loss of the ipsilateral cutaneous trunci reflex occurs when the C8-T1 motor neuron cell bodies or ventral nerve roots forming the lateral thoracic nerve are injured, thus causing subsequent decrease or loss of innervation of the ipsilateral cutaneous trunci muscle ().
  • Lateralized disc protrusions or extrusions localized to the caudal cervical spine will commonly cause cervical pain in addition to motor dysfunction of one thoracic limb (monoparesis) or the ipsilateral thoracic and pelvic limbs (hemiparesis). Lateralized disc extrusions or protrusions localized to the caudal lumbar spine or lumbosacral junction can cause ipsilateral pelvic limb monoparesis with neurological deficits localizing to the L4-S2 nerve roots (femoral and sciatic nerves).

Lesion localization

Lesions causing monoparesis of the thoracic limbs can commonly be located in the grey matter of spinal cord segments C6-T2, or at any anatomical region of the peripheral nerves forming the brachial plexus (Origins and functions of the peripheral nerves of the brachial plexus). Lesions affecting the spinal cord at this level can also cause ipsilateral hemiparesis if they affect the upper motor neurons (UMNs) to the ipsilateral pelvic limb. Unilateral spinal cord lesions (Clinical signs caused by unilateral spinal cord lesions) at C1-C5 will more commonly cause ipsilateral UMN hemiparesis. UMN monoparesis of the pelvic limbs can be caused by unilateral T3-L3 spinal cord lesions, whereas lesions causing LMN monoparesis of the pelvic limbs will be located in the L4-S2 spinal cord segments, or will affect the peripheral nerves of the lumbosacral plexus (Origins and functions of the peripheral nerves of the lumbosacral plexus).

Origins and functions of the peripheral nerves of the brachial plexus

Nerve Spinal cord segments Muscles innervated Reflexes affected Muscle function loss Cutaneous sensation Signs of dysfunction
Suprascapular C6-C7 Supraspinatus; infraspinatus Shoulder extension Shoulder Little/limited gait abnormality ± shoulder abduction
Musculocutaneous C6-C8 Biceps brachii; brachialis Biceps; withdrawal (flexor) Elbow flexion Medial antebrachium and first digit Little/limited gait abnormality, weak elbow flexion
Radial C7-T2 Triceps brachii; extensor carpi radialis; digital extensors Triceps, extensor carpi radialis Elbow extension; carpus extension; digit extension Cranial antebrachium and foot Loss of weight bearing, knuckling
Median and ulnar C8-T2 Superficial and deep digital flexors; carpal flexors Withdrawal (flexor) Carpus flexion

Digit flexion

Caudal antebrachium and foot, lateral aspect of 5lh digit Little/limited gait abnormality; mild carpus hyperextension
Lateral thoracic C8-T1 Cutaneous trunci Cutaneous trunci Cutaneous trunci
Sympathetic nerves to head and neck T1-T3 Dilator of pupil Pupillary light Pupil dilation Miosis (partial Homer’s syndrome); ipsilateral peripheral vasodilation causing elevated skin temperature

Clinical signs caused by unilateral spinal cord lesions

Spinal cord lesion localization Signs in thoracic limbs Signs in pelvic limbs Other signs
C1-C5 UMN signs UMN signs
C6-T2 LMN signs UMN signs Homer’s syndrome []

Loss of ipsilateral cutaneous trunci reflex

Possible self-mutilation of ipsilateral limb

T3-L3 Normal UMN signs
L4-S2 Normal LMN signs Incontinence []

Flaccid tail

Flaccid anus

Possible self-mutilation of ipsilateral limb

Origins and functions of the peripheral nerves of the lumbosacral plexus

Nerve Spinal cord segments Muscles innervated Reflexes affected Muscle function loss Cutaneous sensation Signs of dysfunction
Obturator L4-L6 Pectineus; gracilis Hip adduction Little/limited gait abnormality
Femoral L4-L6 Quadriceps group; psoas group Patellar Stifle extension, hip flexion Medial surface of limb and first digit Loss of weight bearing
Sciatic L6-S2 Biceps femoris; semimembranosus; semitendinosus; cranial tibial; gastrocnemius Withdrawal (flexor); cranial tibial; gastrocnemius Hip extension; stifle flexion; hock flexion and extension; digits flexion and extension Entire limb, except medial aspect and first digit Knuckling of paws but weight bearing present
Peroneal L6-S2 Cranial tibial Cranial tibial Hock flexion digit extension Craniolateral surface of limb, distal to stifle Hyperextended hock; knuckled paw
Tibial L6-S2 Gastrocnemius Gastrocnemius Hock extension; digit flexion Caudal surface of limb, distal to stifle Dropped hock

Ipsilateral partial Homer’s syndrome and loss of the ipsilateral cutaneous trunci reflex indicate lesions affecting the C8-T2 spinal cord segments or their respective nerve roots.

In order to localize a lesion causing monoparesis accurately, it is important to know the motor and sensory innervation of the thoracic and pelvic limbs (). The cutaneous area innervated by a particular nerve is called the dermatome or cutaneous zone of that nerve. This area includes a peripheral zone (where there is overlapping of several cutaneous zones) and a central autonomous zone innervated exclusively by that nerve (). Sensory dysfunction (decrease or loss of cutaneous sensation) in these specific autonomous zones can localize the lesion to one or more specific peripheral nerves or spinal cord segments ().

Pathophysiology

Peripheral nerve injuries can be classified based on the degree of injury and physical and functional integrity of the nerve trunk.

  • Neurapraxia refers to interruption of nerve conduction without physical disruption of the axon. This type of injury is more commonly caused by transient loss of blood supply, blunt trauma or compression, which can sometimes cause demyelination without axonal discontinuity. Recovery is usually spontaneous and complete, and occurs within 1-2 weeks. If demyelination has occurred recovery may take a little longer (5-6 weeks).
  • Axonotmesis refers to physical interruption of the axon, with separation of the axon from the neuronal cell body, which results in Wallerian-like degeneration and loss of conduction distal to the injury. The endoneurium and the Schwann sheath remain intact. Recovery, if possible, depends on the regrowth of axons, usually at 1 mm/day ().
  • Neurotmesis implies complete severance of the nerve trunk (axons, Schwann cells and supporting connective tissue). This is the most severe type of injury. Successful regeneration to the correct target is unlikely to occur and may result in neuroma formation.

Differential diagnoses

The main disease conditions causing acute non-progressive and chronic progressive monoparesis are listed in Differential diagnoses for clinical monoparesis.

Differential diagnoses for clinical monoparesis

Acute non-progressive

Trauma []:

Fibrocartilaginous embolic myelopathy []

Arterial thromboembolism []

Chronic progressive

Foraminal stenosis:

  • Disc disease []

Neoplasia:

  • Nerve sheath tumours []
  • Spinal tumours []

Inflammatory disease:

  • Myelitis/meningomyelitis []
  • Plexus neuritis []

Peripheral neuropathy []

Neurodiagnostic investigation

All the specific diagnostic tests to assess a monoparetic animal need to be performed under general anaesthesia (). A minimum database of complete blood count (CBC), serum biochemistry profile, urinalysis, thoracic radiographs and abdominal ultra-sonography is indicated in any neurological patient. These tests are required to assess the general health status of the patient and rule out metastatic neoplasia or systemic disease. In addition, a comprehensive coagulation profile (activated clotting time (ACT), prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrin degradation products (FDPs)) should be performed when vascular (haemorrhage, infarction) disease is suspected.

Electrodiagnostic tests

Electrodiagnostics provide more accurate information about the integrity and function of peripheral nerves ().

Electromyography(EMG) allows detection of spontaneous electrical activity (fibrillation potentials and positive sharp waves) in denervated muscles 7-10 days after the nerve injury has occurred. If regeneration is producing reinnervation, this can be detected by the presence of giant motor unit potentials on EMG, so repeated studies to monitor progress after acute injuries are useful.

Motor and sensory nerve conduction velocity studies (MNCV, SNCV) can be performed in specific peripheral nerves to assess nerve function and integrity, and to determine the severity of the lesion.

Cord dorsum potentials can also be recorded to assess dorsal nerve root function in cases of sensory nerve dysfunction ()

F wave studies allow assessment of ventral nerve root function in cases of proximal motor nerve injuries.

Imaging

Radiography

Survey radiographs of the vertebral column may show signs of intervertebral disc disease, enlarged inter-vertebral foramina in cases of peripheral nerve sheath tumours, or lytic changes indicative of other neoplasms.

Myelography

A myelogram can be helpful in cases of lateralized intervertebral disc extrusions and protrusions, as well as in cases of peripheral neoplasms that grow into the vertebral canal.

Computed tomography and magnetic resonance imaging

CT and MR imaging techniques can be useful additional diagnostic tests to detect and better determine lateralization and extension of nerve root tumours, intervertebral disc disease, fibrocartilaginous embolism and other inflammatory or neoplastic conditions affecting the spinal cord.

Diseases causing monoparesis

Monoparesis: Degenerative diseases

Anomalous diseases

Peripheral neuropathy

Peripheral nerve diseases can occasionally cause monoparesis if they affect a single nerve or the nerves of the brachial or lumbosacral plexuses. However, peripheral neuropathies more commonly affect more than one limb and are discussed further in site.

Monoparesis: Neoplastic diseases

Inflammatory diseases

Myelitis meningomyelitis

Focal forms of myelitis and meningomyelitis of infectious or immune-mediated origin can cause monoparesis of the thoracic or pelvic limbs when the inflammatory focus is unilateral and located in the brachial (C6-T2) or lumbosacral (L4-S2) intumescences of the spinal cord. These diseases are described and discussed in detail in site.

Plexus neuritis

This is an uncommon inflammatory condition that can affect the brachial plexus of dogs () and cats (). Reported cases describe an acute onset of thoracic limb paresis with decreased orabsentspinal reflexes. Asimilarcondition in humans (serum neuritis) has been described after the administration of certain vaccines () and associated with specific viral infections (). The pathogenic mechanism is believed to have an immunoallergic basis that causes severe shoulder and upper arm pain followed by upper arm weakness.

The sporadic cases described in dogs and cats have been related to a purely horsemeat diet (one dog) and to administration of modified-live rabies virus vaccines (one dog, one cat). In other animals a definite cause could not be found. Affected animals show diffuse EMG changes consistent with denervation in the thoracic limbs and neurogenic atrophy in all thoracic limb muscles. Cerebrospinal fluid (CSF) evaluation is usually normal. Pathological examination shows severe Wallerian degeneration of the brachial plexus nerves, most pronounced in the ventral nerve roots, together with a prominent inflammatory infiltration of mononuclear cells. Affected dogs may respond to corticosteroid treatment and/or a change to a poultry-based diet that contains no beef or horse products. Prognosis is guarded, since some animals may recover slowly (months) while others may remain non-ambulatory. The single feline case reported recovered spontaneously over a 3-week period.

Monoparesis: Traumatic diseases

Toxic diseases

Tetanus

Tetanus usually causes generalized signs of increased extensor rigidity and is described in full in site. However, signs can be limited to a single limb, causing monoparesis associated with extensor rigidity. A wound can usually be found on the distal extremity of the affected limb. This syndrome has been reported in people and occurs either when the patient has preexisting anti-tetanus antibodies, or an extremely small injury. Local tetanus has been reported most commonly in cats but can occur in dogs ().

Vascular diseases

Fibrocartilaginous embolism

Unilateral infarctions affecting the spinal cord ventral grey matter at the C6-T2 or the L4-S2 spinal cord segments, or affecting the white matter of the T3-L3 spinal cord segment may cause acute monoparesis of the thoracic (C6-T2) or pelvic limbs (T3-L3 and L4-S2). The paretic pelvic limb will show UMN deficits if the infarct occurs in the T3-L3 segment, and LMN deficits if the infarct occurs in the L4-S2 spinal cord segment. Specific clinical signs, diagnosis, treatment and prognosis of this type of injury are discussed in detail in site.

Arterial thromboembolism

The acute onset of monoparesis (thoracic or pelvic limbs) in cats can be associated with arterial thromboembolism. The long-term prognosis is very poor as many cats have a severe underlying cardiovascular disease. Further details on this condition can be found in site.

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