The Vestibular System In Health and Disease

The Vestibular System in Health and Disease

What is the Vestibular System?
The vestibular system is the primary sensory system that maintains the animal's balance or its normal orientation relative to the gravitational field of the earth. This orientation is maintained in the face of linear or rotatory acceleration and tilting of the animal. The vestibular system is responsible for maintaining the position of the eyes, trunk, and the limbs in reference to the position or movement of the head at any time. Alexander DeLahunta, 1977

The Anatomy of the Vestibular System - begin from the outside and work in.

Bony and membranous labyrinth

Semi-circular canals.

These canals are paired right and left lateral duct, right posterior and left anterior ducts, and right anterior and left posterior ducts. These behave as synergistic pairs. While one is activated, the other is inhibited (less active). The receptors do not respond to constant velocity, but acceleration and deceleration. Each canal contains a material called endolymph and each canal terminates on an area (Ampulla) on neuroendothelium and gelatinous material called the Cristae Ampillaris. Rotation of the head on any plane causes endolymph flow and activation of the neuroepithelium (hair cells and the supporting cells).
  • The semicircular canals, each opening to the utricle.
  • Crista Ampillaris - Ampulla - end of the semicircular duct. Cupula- gelatinous material across the lumen of the ampula. Hair cells (stereocillia and kinocillium) and supporting cells.
  • The Neurons of the vestibulcochlear nerve are derived from the ectoderm in a synaptic relationship to the hair cells.
  • Mechanism of function: Movement of the endolymph bends the stereocillia. The vestibular neurons are always activated and the direction of the hair cell deflection will then influence whether the neurons are activated or inhibited.
  • The macula (2) - receptors that are present within the Utriculus and the Saccule (within the bony vestibule). Each receptor is very similar to the Crista Ampullaris. The labyrinth is thickened in the shape of an oval plaque, the macula. The macula is covered by a neuroepithelium that is then covered by the otolithic membrane with statoconia on the surface. Movement of the statoconia away from the cells causes stimulation of the vestibular neurons. The macule in the saccule is orientated in a vertical direction and in the utricle is orientated on a horizontal plane. The macula are responsible for the static sensation of head position and linear acceleration and deceleration.
  • Macula of the utricle is responsible for the sensation associated with a change of posture to the head.
  • Macula of the saccule is responsible for vibrational sense and loud sounds.

The Vestibulochoclear Nerve

  • Synapses on the hair cells of the Crista Ampullaris and the Macula, then enter the cranial vault through the internal acoustic meatus, forms the vestibular ganglion within the petrosal bone, course along the lateral surface of the rostral medulla at the cerebellomedullary angle. At the level of the cerebellomedullary angel the VIII cranial nerve is formed. The VIII cranial nerve is the only cranial nerve that does not exit the skull. The axons enter the brain stem at the trapezoid body and the caudal cerebellar peduncle. The neurons then either terminate at the vestibular nuclei of the medulla and the remainder move through the caudal cerebellar peduncle and create the vestibulocerebellar tract.

The Vestibular Nuclei

  • There are four vestibular nuclei along the dorsal aspect of the medulla, adjacent to the ventrolateral wall to the fourth ventricle. The nuclei are grouped as the rostral, medial, lateral and caudal vestibular nuclei. The nuclei lay near the descending facial neurons, spinal tract of the trigeminal nerve. The vestibular nuclei receive the afferents of the vestibular portion of the vestibulocochlear nerve. Axons from these nuclei then project to the spinal cord and the medial longitudinal fasciculus (MLF)

Spinal Tracts

  • The Vestibulospinal Tract - descends along the ipsilateral, ventral funiculus. It terminates on the interneurons in the ventral grey column. These interneurons are fascilitatory to the ipsilateral extensor muscles and inhibitory to the ipsilateral flexors. Some interneurons cross to the contralateral ventral grey column and are inhibitory to the extensors on that side. Mainly influence extensor tone.
  • The Medial Longitudinal Fasciculus (MLF) -
    • travels rostrally to influence eye position and caudally within the spinal cord. The extension of the MLF is the medial vestibulospinal tract. The rostral extension terminates on the nuclei of cranial nerves III, IV, and VI. This interaction then influences the position of the eyes in space and the occulocephalic reflex.
    • The medial vestibulospinal tract is responsible for the maintenance of the position of the body and the limbs relative to the head.

Brain Stem

  • Axons move through the MLF and the reticular formation to influence the nuclei of the cranial nerves III,IV,and VI, Provides coordinated, conjugate eyeball movement.
  • Axons course to the reticular formation of the medulla. Afferents move to the vomiting center.
  • Projections to the temporal lobe provide conscious awareness of the body's position in space.

Cerebellum

  • Projections from the caudal cerebellar peduncle to the flocculonudular lobe and the fastigial nucleus. Functions to coordinate movement of the eyes, trunk and head. Maintains equalibrium both during movement and at rest.

Clinical Presentation of Dysfunction

  • Signs of vestibular disease are usually presented as a unilateral or asymmetric ataxia. With peripheral vestibular disease there is a preservation of strength. Attempt to evaluate the patient in a calm, well lit environment. As always it is best to observe the neurologic patient rather than immediately manipulate them. Often you can observe the patient, give them time to orient themselves as you take a complete history. Your observations and the history provided you are the two most important contributors to your diagnosis.
  • The neurological examination - areas of particular interest when evaluating the vestibular system.
    • Nystagmus - a rhythmic, involuntary movement of the eyes. An eyeball oscillation either with equal movements (pendular) or quick and slow phases (jerk).
      • The Oculocephalic Reflex or Physiologic Nystagmus - A reflex movement of the eye relative to the position of the head in an attempt to maintain normal eye position and optimal visual acuity. Rotation of the head triggers the extraocular muscles opposite the direction of rotation to contract and move the eyes back to the center of the visual field. Next, the eyes are quickly released and move back in the direction of the rotation. Slow phase is opposite the side of rotation. The quick phase is in the direction of the rotation of the head. The slow component is due to activation of the labyrinth. The fast phase is a function of the brain stem. Jerk nystagmus' direction is described by the direction of the fast phase.
      • Pendular Nystagmus - Usually seen in otherwise normal Siamese and Himalyans. In other instances it is observed in patients with congenital visual deficits.
      • Pathologic Nystagmus
        • Damage to the vestibular system leads to imbalance in the activity of the vestibular nuclei. The normal side continues to supply a constant signal. The imbalance is then interpreted as a rotation. The nystagmus then has a slow phase toward the lesion (opposite the rotation) and a fast phase away from the lesion (toward the rotation or the active vestibular nuclei).
        • Spontaneous Nystagmus - Nystagmus present when patient is normally positioned.
        • Positional Nystagmus - Present only when the patient's head is placed in an abnormal position.
        • Direction of the nystagmus is dictated by the semicircular canals affected. Usually with peripheral vestibular disease a combination of horizontal and rotatory nystagmus is appreciated.
  • Eye position
    • The vestibular system and its association with cranial nerves III, IV and VI serve to help maintain normal eye position within the palpebral fissure. Dysfunction=strabismus
    • Strabismus - abnormal eye position.
      • Usually a ventrolateral, unilateral eye deviation. Ipilateral to the side of the lesion. May be positional or spontaneous (always there). Spontaneous strabismus is not seen related to vestibular disease.
  • Coordination of limbs and position of the body
    • The peripheral vestibular system along with it's central components, in particular the vestibulospinal tract, allow orientation of the limbs and body and coordination of movement to maintain an appropriate posture. Dysfunction=ataxia, rolling, leaning
  • Coordination of the head and maintaining a normal static head position
    • Again, the normal function of both the peripheral central components of the vestibular system allows appropriate orientation of the head. Dysfunction= Head tilt
  • Vomiting/Motion sickness
    • Due to direct connections of the vestibular nuclei on the reticular formation of the medulla.

Peripheral vs. Central vestibular disease


Peripheral Vestibular Disease -
  • Loss of equilibrium with the maintenance of strength, i.e. no paresis. Usually signs are asymmetric.
    • Imbalance demonstrated as a head tilt with the most ventral ear directed towards the side of the lesion.
    • Trunk tips, falls and rolls in the direction of the lesion (rolling is typically seen more with central disease). This is due to vestibulospinal dysfunction, the ipsilateral tract is dysfunctional, contralateral still functional. Dysfunction of the tract results in increased flexor tone on the ipsilateral side and increased extensor tone on the contralateral side. Therefore the patient will be leaning, falling or rolling towards the side of the lesion.
    • Circles toward the side of the lesion. Usually, short, tight circles. Larger circles more often associated with a forebrain lesion.
    • Patient has difficulty righting itself with an exaggerated response toward the side of the lesion. Again, due to the patient's tendency to roll/fall (see above)
    • **** Hypertonia and hyperreflexia on the side of the body opposite (contralateral) to the lesion.
    • Patient uses vision to orientate himself/herself and therefore compensates for their deficit by using vision. Blind folding the patient or placing them in a darkened room will accentuate their deficit.
    • Nystagmus - is always horizontal or rotatory and always spontaneous. The fast phase is away from the side of the lesion.
      • Rotatory nystagmus is characterized by the direction of the fast component as it moves from the 12 o'clock position
 
  • Strabismus - a deviation of the eyeball
    • Tonic extension of the neck causes ventral deviation of the eyeball. Normally the eye remains centered within the palpebral fissure. This is referred to as a ventral strabismus. A resting strabismus (lower motor neuron strabismus) may also be appreciated. If the patient may be induced to 'correct' the strabismus by altering their gaze, it is referred to a vestibular strabismus. When a strabismus results from vestibular disease, there is no paralysis of the extraocular muscles. The ventrally deviated eyeball is on the side of the lesion. 
  • Vomiting 
    • Associated cranial nerve deficits - VII, (remember parasympathetic component of VII) and Horner's syndrome, disruption of the sympathetic innervation of the eye, (miosis, ptosis, enophthalmus, and protrusion of the 3rd eyelid).

Central Vestibular Disease

  • An asymmetric ataxia occurs as well (loss of equilibrium), but there may be an associated paresis or proprioceptive ataxia (due to lesions associated with descending motor pathways or ascending spinocerebellar pathways) 
  • Nystagmus May be horizontal, vertical or rotatory.
    • Positional nystagmus- usually can be appreciated with altering the position of the head, dorsal extension.
    • Fast phase may be toward or away from the side of the lesion.
    • Tends to be a more persistent deficit with central disease. 
  • Strabismus
    • Dysconjugate strabismus is usually seen with central disease.
  • May appreciate associated deficits of cranial nerve V, VI, VII and other cranial nerve deficits. 
  • Paradoxical Vestibular Disease
    • Lesions involving the caudal cerebellar peduncle and flocculonodular lobe of the cerebellum and the middle and rostral vestibular nuclei.
    • Head tilt is away from the side of the lesion.
    • Fast phase of nystagmus is toward the lesion.

 

  • Altered Mental Status 

 

  • Paresis
    • Assessment is technically challenging. Discuss the difficulty of checking proprioception on an ataxic patient.

 

  • Cerebellar signs 

 

  • No Horner's syndrome 


Diseases Affecting the Vestibular system

  • Peripheral vestibular diseases
    • Congenital
      • English Cocker Spaniel, G. Shephard, Tibetin Terriers, Burmese kittens. Demonstrates between birth and 4 months of age. May be static, progressive or regress.
      • Dobermans, Beagles, Akita and Siamese. Birth to 10 weeks, early deafness and vestibular signs. Neuroepithelial degeneration of the chochlea and the hair cells. An autosomal recessive (in Dobermans). Good prognosis for clinical improvement, may have relapses of vestibular signs.
      • Pendular nystagmus without other vestibular signs
        • Siamese cats, Burmese cats, Belgian sheepdogs (may resolve).
  • Otitis Media and Otitis Interna
    • Middle ear
      • An airspace in the temporal bone containing the auditory ossicles. An area between the tympanum and the oval window containing the malleus, incus and the stapes
    • Inner ear
      • The vestibule, cochlea and the semicircular canals.
    • Etiology of infection – extension across the tympanic membrane, hematogenous spread, ascending infection from the nasopharynx. Most common agents: Staphylococcus spp., Streptococcus spp., Proteus spp., Pseudomonas spp., Enterococcus spp., and E. coli. Also consider foreign bodies, polyps (papillary adenomas or inflammatory polyps), and yeast Pityrosporon spp. and Candida spp.
    • Cholesteatoma (epidermoid cyst) - layers of keratin resting on a fibrous stroma of granulation tissue. Usually associated with or attached to the tympanum.
    • Reason for concern- infection may spread along the meninges and lead to encephalitis.
    • Diagnostic Testing to be considered
      • Radiographs of the osseous bulla, otoscopy, myringotomy, appropriate culture and sensitivities, thyroid testing, referral to a dermatologist?, CT/MRI

 

  • Feline Idiopathic Vestibular Disease
    • Affects 80% of cats between July and August
    • Acute onset with no progression of signs
    • Severe incapacitation, may require supportive care
    • Gradual improvement over
    • May leave mild residual deficits
    • Chance of recurrence - rare
    • Diagnosis - based on clinical signs and rate of onset, absence of any otic disease or associated cranial nerve deficits, and no other signs to support central disease
    • Treat the signs

 

  • Canine Idiopathic Vestibular Disease
    • Acute onset with no progression of signs
    • Often times vomit at the onset of signs
    • Severe incapacitation, may require supportive care
    • Diminished mobility in a, often times, large breed dog becomes a challenge for the owner and they may require continued hospitalization. Usually do not discharge until they are ambulatory.
    • Also, severely affected individuals may be very difficult to examine and a complete neurologic examination, with neuroanatomic localization, may not be accomplished until day 2-3.
    • Gradual improvement over 2-3 weeks
    • May leave mild residual deficits
    • Chance of recurrence - rare
    • Diagnosis - based on clinical signs and rate of onset, absence of any otic disease or associated cranial nerve deficits, and no other signs to support central disease.
    • Treat the signs

 

  • Polyneuropathy/neuritis – thyroid(?) or related to other inflammatory disease 
  • Injury aminoglycosides, streptomycin, loop diuretics 
  • Degeneration - usually more appreciable when considering hearing. 
  • Neoplasia - Neurfibrosarcoma (fibrosarcoma) of the VIII cranial nerve; osteosarcoma and chondrosarcoma of the petrous portion of the temporal bone. Squamous cell carcinoma (most common tumor of the middle ear in cats) or cerumineous gland adenocarcinoma. 

 

  • Diagnostics
    • Radiographs, MRI, biopsy

 

  • Central vestibular diseases
    • Inflammatory
    • GME (Granulomatouos Meningoencephalomyelitis, FIP, RMSF, ehrlichiosis, toxoplasmosis, Neospora Caninum, coccidiomycosis, cryptococosis(most common fungal organism affecting the nervous system of the dog or cat), Canine Distemper, parasitic migration.
    • Neoplasia
    • Meningioma, metastaic disease, astroctytoma, medulloblastoma, choroids plexus papilomas, intracranial subarachnoid cysts.
    • Toxicity
    • Metronidazole
    • After 7-12 days at a dose of greater than 60mg/kg/day. Requires 1-2 weeks to recover.
    • Injury
    • Vascular
    • Hemorrhage, infarction
    • Nutritional
    • Thiamine deficiency
    • Diagnostics to be considered
      • MRI, CSF analysis, serologies

 

  • Video cases

Medications, their actions and doses

 

  • Meclizine HCl (Bonine, Antivert)
    • An antihistamine with sedative and antiemetics effects. Contraindicated with bladder neck obstructions, prostatic enlargement, glaucoma, and severe heart failure.
    • Dose Dogs 25 mg q24h to 12.5 mg BID; Cats 12.5 mg daily.
  • Antibiotics commonly used for peripheral vestibular disease, i.e. otitis media/interna
    • Usually based on culture and sensitivity results. Chloamphenical and cephalexin are good choices, also enrofloacin (would not administer to cats).
    • Also consider the use of anti-inflammatory doses of prednisone with OM/OI cases
  • Antibiotics that cross the blood brain barrier, i.e. for central vestibular disease.
  • Steroids

Posted on July 14, 2014
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