Neurology: menace reflex in Horses (Equis) | Vetlexicon
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Neurology: menace reflex

ISSN 2398-2977

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Introduction

  • The menace reflex is a complex learned reflex.
  • Afferent pathway:
    • Optic nerve   →   crossover at optic chiasm   →   lateral geniculate nucleus   →    visual cortex.
  • Efferent pathway:
    • Visual cortex   →   projections to the motor cortex and cerebellum via the pontine nucleus   →    ipsilateral facial nucleus   →   auriculo-palpebral nerve   →   orbicularis oculi muscle.

Uses

Advantages

  • Easy to perform.
  • No equipment required.

Disadvantages

  • Interpretation may be difficult as response can be variable, but in general horses have a brisk menace reflex.
  • Not useful in young foals due to variable development of reflex in first two weeks of life as this is a learned response.
  • Can be affected by a number of neurologic disorders such as facial paralysis and cerebellar diseases   Cerebellum: hypoplasia  .
  • Stoic, depressed or excited animals may not blink to menace reflex.

Time required

Preparation

  • None.

Procedure

  • 5 min.

Decision taking

Criteria for choosing test

Requirements

Materials required

Minimum equipment

  • Blindfold.

Ideal equipment

  • Glass or perspex screen - some people recommend carrying out the hand movements behind a screen to decrease air movements to the eye.

Other requirements

  • Handler and long rein.
  • Loose box.

Preparation

Pre-medication

  • None.

Other preparation

  • Horse needs to be calm and relaxed in a well lit loose box with minimal head restraint.

Restraint

  • Minimal handling on long rein in a well lit loose stall.

Technique

Core procedure

 

Step 1 - Threaten horse

 
  • Threaten horse with hand or object and observe reaction.

Step 2 - Normal reaction

 
  • Normally the animal blinks and withdraws its head.

Do not touch the eye or produce air currents which may be felt by cornea.

Step 3 - Foal

 
  • Absent at birth.
  • Most foals have a positive menace reflex by 9 days post-partum.
  • A partial 'blink' may be elicited about a day before a full closure occurs.
  • There may be an unequal response between eyes.
  • Although the menace reflex is absent at birth some vision is assumed. The protective presence of the mare seems to prevent ocular globe trauma to the foal in the absence of a well-developed menace response.

Exit

 

Step 1 - Interpret results

 
  • In the menace response the normal central visual pathway is the afferent arm and the efferent arm requires an intact motor cortex, cerebellum and facial nerve .
  • Vision tested in one eye by the menace reflex is being perceived in the opposite visual cortex of the cerebellum.
  • It is important to distinguish loss of vision from facial nerve dysfunction.

Step 2 - Partial visual deficits

 
  • Partial visual deficits unilaterally may best be detected by performing the test relatively rapidly and alternately in each eye.

Step 3 - Medial and lateral visual fields

 
  • Assessing medial and lateral visual fields in each eye is possible but difficult practically.

Aftercare

Outcomes

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Enzerik E (1998) The menace response and pupillary light reflex in neonatal foals. Equine Vet J 30, 546-548 PubMed.