Explain clinical manifestation of diseases of the nervous system in domestic animals and enlist the methods of Neurological Examination.

Clinical Manifestations of Diseases of the Nervous System in Domestic Animals

Diseases of the nervous system in domestic animals can result from various etiologies including trauma, infections, inflammatory conditions, metabolic disorders, toxicities, congenital defects, and degenerative processes. The clinical signs observed are highly dependent on the specific anatomical location of the lesion within the nervous system – broadly categorized into the brain, spinal cord, and peripheral nervous system.

1. Manifestations of Brain Diseases

Disorders affecting the brain, including the cerebrum, cerebellum, and brainstem, often present with a wide array of symptoms due to the brain's diverse functions:

• Changes in Mentation and Behavior: Animals may appear confused, disoriented, excessively dull (obtunded, stuporous, comatose), or agitated. Behavioral changes like aggression, anxiety, head pressing, compulsive pacing or circling, and inappropriate vocalization are common. For instance, Bovine Spongiform Encephalopathy (BSE) in cattle is characterized by abnormal behavior, aggression, anxiety, and changes in temperament.
• Seizures: These are sudden, uncontrolled electrical disturbances in the brain, leading to episodes of abnormal physical activity, loss of consciousness, or altered behavior. They can range from focal (twitching of a limb) to generalized (grand mal seizures with full-body convulsions). Epilepsy is a common cause of recurrent seizures in dogs.
• Balance and Coordination Issues (Ataxia):
  • Cerebellar Ataxia: Characterized by an uncoordinated gait, exaggerated limb movements (dysmetria), head tremors, and truncal sway. Animals may appear to be "goose-stepping."
  • Vestibular Ataxia: Often involves a head tilt, circling to one side, leaning, falling, nystagmus (uncontrolled jerky eye movements), and loss of balance.
• Cranial Nerve Deficits: Specific cranial nerve dysfunctions can lead to localized signs such as:
  • Blindness or impaired vision (Optic nerve - CN II).
  • Abnormal pupil size or light reflexes (Oculomotor - CN III).
  • Facial paralysis, lip droop, or ear droop (Facial nerve - CN VII).
  • Difficulty chewing or masticatory muscle atrophy (Trigeminal - CN V).
  • Swallowing difficulties (Glossopharyngeal - CN IX, Vagus - CN X).
  • Head tilt or loss of balance (Vestibulocochlear - CN VIII).
• Weakness or Paralysis: While often associated with spinal cord issues, severe brainstem lesions can also cause generalized weakness (paresis) or paralysis.

2. Manifestations of Spinal Cord Diseases

Spinal cord lesions typically result in signs affecting motor function and sensation caudal to the injury site. The severity depends on the location and extent of the damage:

• Paresis (Weakness) or Paralysis:
  • Monoparesis/Monoplegia: Affecting one limb.
  • Paraparesis/Paraplegia: Affecting both pelvic limbs.
  • Tetraparesis/Tetraplegia: Affecting all four limbs.
• Ataxia: Uncoordinated gait, often described as wobbly or drunken. Animals may knuckle over their paws, scuff their feet, or drag their limbs.
• Spinal Pain: Animals may exhibit reluctance to move, cry out when touched along the spine, have an arched back, or develop muscle spasms in the affected region. Intervertebral Disc Disease (IVDD) in dogs is a classic example causing severe back pain and neurological deficits.
• Loss of Proprioception: Reduced awareness of body and limb position, leading to abnormal foot placement.
• Incontinence: Loss of bladder or bowel control due to disruption of nerve pathways regulating these functions.
• Abnormal Reflexes: Exaggerated, diminished, or absent spinal reflexes, depending on whether the lesion affects upper motor neurons or lower motor neurons.

3. Manifestations of Peripheral Nervous System Diseases

Diseases affecting peripheral nerves or muscles typically manifest as weakness, muscle atrophy, and altered reflexes in the affected limbs or body parts:

• Focal Weakness or Paralysis: Affecting specific muscles or limbs, e.g., facial paralysis due to damage to the facial nerve.
• Muscle Tremors, Twitching, or Fasciculations: Involuntary muscle movements.
• Muscle Atrophy: Wasting away of muscles due to denervation.
• Loss of Sensation: Numbness or reduced pain perception in specific areas.
• Changes in Voice or Swallowing: If nerves supplying the larynx or pharynx are affected.
• Lameness: Often presenting as a limp or inability to bear weight.

Methods of Neurological Examination

A comprehensive neurological examination is a systematic process designed to identify the presence, severity, and anatomical localization of neurological disease. It typically involves a "hands-off" observation followed by "hands-on" specific tests.

The neurological examination can be divided into the following key components:

1. Mentation and Behavior Assessment:
   • Observation: Assess the animal's level of consciousness (alert, dull, stuporous, comatose) and general demeanor. Note any abnormal behaviors such as aggression, disorientation, head pressing, or compulsive circling.
   • Interaction: Observe response to environmental stimuli and interaction with the examiner.
2. Posture and Gait Evaluation:
   • Observation: Allow the animal to walk freely in an enclosed space. Assess for symmetry, presence of lameness, weakness (paresis), or incoordination (ataxia). Note head tilt, abnormal limb placement, scuffing of paws, or dragging of limbs. Evaluate different gaits (walk, trot, run) and maneuvers (circling, backing up).
   • Specific Tests:
     • Wheelbarrowing: Supporting the hind limbs and observing the forelimb movement.
     • Hopping Test: Supporting three limbs and forcing the animal to hop on one to assess proprioception and strength.
     • Hemi-walking: Lifting both limbs on one side and observing movement on the other side.
3. Postural Reactions (Proprioception):
   • Conscious Proprioception (Paw Placement Test): Each paw is knuckled over, and the time taken for the animal to correct its position is observed. Delayed or absent correction indicates proprioceptive deficits.
   • Placing Reactions (Visual and Tactile): Assessing the animal's ability to place its foot correctly when it makes contact with a surface or when it sees a surface.
4. Cranial Nerve Examination:

   Each of the 12 cranial nerves (CN I-XII) is assessed for its specific function. Key tests include:

   • Menace Response (CN II, VII): Waving a hand towards the eye to elicit a blink.
   • Pupillary Light Reflex (PLR) (CN II, III): Shining a light into the eye to observe pupil constriction (direct and consensual).
   • Palpebral Reflex (CN V, VII): Touching the medial and lateral canthus of the eye to elicit a blink.
   • Corneal Reflex (CN V, VII): Touching the cornea to elicit a blink.
   • Oculocephalic Reflex (CN III, IV, VI, VIII): Turning the head and observing eye movement (physiological nystagmus).
   • Facial Sensation (CN V): Touching the muzzle and nasal mucosa to elicit a response.
   • Masticatory Muscle Symmetry (CN V): Palpating the temporal and masseter muscles for atrophy.
   • Facial Symmetry (CN VII): Observing for lip droop, ear droop, or eyelid asymmetry.
   • Tongue Tone and Movement (CN XII): Assessing tongue strength and symmetry.
5. Spinal Reflexes:

   Tests specific segmental reflexes to evaluate the integrity of nerve pathways within the spinal cord and peripheral nerves. Key reflexes include:

   • Patellar Reflex (L4-L6): Tapping the patellar ligament to elicit quadriceps contraction and stifle extension.
   • Withdrawal (Flexor) Reflex (Thoracic limbs: C6-T2; Pelvic limbs: L6-S1): Pinching a digit to elicit withdrawal of the limb.
   • Perineal Reflex (S1-S3): Touching the perineal region to elicit anal sphincter contraction and tail flexion.
   • Cutaneous Trunci (Panniculus) Reflex (C8-T1): Pinching the skin along the back to elicit twitching of the trunk muscles. This helps localize spinal cord lesions.
6. Spinal Palpation and Pain Assessment:
   • Palpation: Systematically palpate the entire vertebral column, from the head to the tail, to identify areas of pain, muscle spasms, asymmetry, or swelling.
   • Range of Motion: Gently flex and extend the neck and back to assess for pain or restriction.
   • Nociception (Deep Pain Sensation): In paralytic animals, deep pain sensation can be assessed by firmly pinching the toe to elicit a conscious behavioral response (e.g., vocalization, head turn), distinct from a simple withdrawal reflex. This is crucial for prognosis.
7. Muscle Tone and Atrophy:

   Palpate limb muscles for tone and symmetry, noting any signs of atrophy.

The neurological examination, when performed meticulously, helps the clinician to localize the lesion within the nervous system (e.g., forebrain, cerebellum, brainstem, C1-C5 spinal cord, C6-T2 spinal cord, T3-L3 spinal cord, L4-S3 spinal cord, or peripheral neuromuscular system). This neuroanatomical localization is the cornerstone for formulating a differential diagnosis list and guiding further diagnostic imaging and laboratory tests, such as MRI, CT scans, cerebrospinal fluid analysis, and electrophysiology.