Proceedings of a Workshop on - The Havemeyer Foundation

Equine Recurrent Laryngeal Neuropathy
COMPARISON OF RECURRENT LARYNGEAL
NEUROPATHY WITH OTHER NEUROPATHIES IN
HORSES
I. G. Mayhew
Neuromuscular Diagnostic Laboratory, Royal (Dick) School <strong>of</strong> Veterinary Studies, The University <strong>of</strong>
Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG, UK
INTRODUCTION
This paper gives an overview <strong>of</strong> the known and
suspected neuropathies in horses, apart from
recurrent laryngeal neuropathy (RLN). It will not
discuss the primary traumatic neuropathies,
polyneuritis equi or the neuronopathy known as
equine motor neuron disease.
STRINGHALT
Stringhalt, also known as springhalt and
Hahnentritt, is an disease recorded from ancient
times that is characterised by a sudden, apparently
involuntary, exaggerated flexion <strong>of</strong> one or both
hind limbs during attempted movement. The hind
limb motion may be as mild as a slightly excessive
flexion to violent movements during which the
fetlock or toe will contact the abdomen, thorax and
occasionally the elbow with attempted steps
leading to a peculiar ‘bunny hopping’ and
plunging type gait. Ultimately marked atrophy <strong>of</strong>
muscles <strong>of</strong> the hind limb(s), particularly distally,
will occur.
Three forms <strong>of</strong> stringhalt are recognised
(Huntington et al. 1989). Firstly, the sporadic form
usually affects one limb and occurs worldwide.
The onset can be preceded by a history <strong>of</strong>, or
evidence <strong>of</strong>, trauma to the dorsal tarsal region or
the dorsoproximal metatarsus. The second form
that usually occurs as outbreaks is referred to as
Australian stringhalt, but is also seen in New
Zealand (Cahill et al. 1985), United States (Gay et
al. 1993), Chile (Araya et al. 1998) and Japan
(Takahashi et al. 2002). Usually there is
symmetrical or <strong>of</strong>ten asymmetrical involvement <strong>of</strong>
the pelvic limbs, with prominent distal muscle
atrophy in severe cases. This form has been
associated with exposure to several plants notably
Hypochoeris radicata, Taraxicom <strong>of</strong>ficinal and
Malva parviflora (Huntington et al. 1889, 1991;
Seddon and Belschner 1926; Cahill and Goulden
1985; Gay et al. 1993). These are related species
<strong>of</strong> flat weed, Taraxicom <strong>of</strong>ficinal being the
common dandelion. Finally, there is an atypical
form <strong>of</strong> stringhalt where the thoracic limbs are
also affected. There is knuckling <strong>of</strong> the forelimb
fetlocks with prominent extension <strong>of</strong> more
proximal joints and atrophy <strong>of</strong> the distal
musculature, in association with prominent
stringhalt in both hind limbs. It is possible that
atypical stringhalt is simply a more severe form <strong>of</strong>
Australian stringhalt. Experimental sweet-pea
poisoning (lathrysim) causes such a syndrome.
Huntington et al. (1989) have graded the clinical
severity <strong>of</strong> stringhalt from 1 to 5.
Several authors have treated sporadic
stringhalt by lateral digital extensor
myotenectomy, with allegedly good results.
However, Crabbel et al. (1994) retrospectively
studied 10 sporadic stringhalt cases where trauma
to the dorsoproximal metatarsal region had been
confirmed. Four were conservatively treated with
controlled exercise and turnout and 6 were treated
with extensive myotenectomy. Although the
numbers were very small, there appeared to be no
real difference in the follow up outcomes <strong>of</strong> the 2
groups.
Mephensein (Dixon and Stewart 1969),
bacl<strong>of</strong>en (Cahill and Goulden 1992) and
particularly phenytoin (Huntington et al. 1991)
appear to be quite useful in the symptomatic
treatment <strong>of</strong> Australian stringhalt cases. Whether
phenytoin is useful in the sporadic form has still to
be determined. However, at doses <strong>of</strong> 7–15 mg/kg,
sid to tid, several authors (Huntington et al. 1991;
Wijnberg et al. 2000; Takahashi et al. 2002) have
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