World Journal of Veterinary Science, 2018, 6, 19-22 19
E-ISSN: 2310-0796/18 © 2018 Synergy Publishe rs
Sleep Attacks with Associated Sleep Terrors in a Six Year Old
Knockadoon Lodge, 17 Keymer Street, Ascot, 6104, Western Australia
Abstract: A six year old thoroughbred gelding presented with a two year history of somnolence and altered behavior of
two years duration. In the three months immediately preceding presentation episodes of sleep attacks and
accompanying slee p ter rors were observed. There had been no evidence of recumbent or paradoxical sleep during this
period. An examination of the upp er airways revealed evid ence of palatal instability (PI). An oral pal atopharyngoplasty
(OPP) was performed to reduce this instability (PI). Clinical observati ons in the eight months following surgery suggested
a possib le link between PI, upper airways obstruction (UAO) and a h orse’s ability to achieve adequate paradoxical sleep.
Keywords: Horse, oral palatopharyngoplasty, palatal instability, sleep terrors.
Normal sleep patterns in equids had been
previously investigated [1-3]. More recent studies into
sleep attacks [4,5] and narcolepsy  often associated
with recumbent sleep deprivation had been
undertaken. The six year old gelding in this study
presented with symptoms consistent with sleep attacks
[4,6] and sleep terrors [7,8] but not narcolepsy as the
horse would attempt to support itself during periods of
collapse . In this case there was an absence of
physical, environmental or social factors  that had
previously been associated with recumbent sleep
deprivation. The possibility that PI , which occurred
with a breakdown of the oropharyngeal seal (OPS)
[2,10] and which could precipitate upper airways
obstruction (UAO), may have contributed to a reduction
in recumbent and thence paradoxical sleep, was
In July 2013 a rising 4 year old thoroughbred
gelding was purchased to be trained for dressage. It
was said to be unraced due to its under development,
poor body condition and failure to cope with race
In October the horse was sent to a breaker to be re
schooled as its aberrant behaviour suggested that its
original education may have been inadequate. On the
ground the horse appeared to have a split personality.
It was extremely lethargic and frequently seen yawning,
was prone to tripping and stumbling when walked in
hand and yet could also become aggressive, biting and
*Address correspondence to this author at the Knockadoon Lodge, 17 Keymer
Street, Ascot, 6104, West ern Aus tralia; E-mail: ahernvet @hotmail.com
lashing out when being geared up. The lethargy and
inattentive stumbling was also apparent when being
ridden but at this stage there were no aggressive traits
exhibited when under saddle.
One other unusual behaviour was its daily routine of
moving to lateral recumbency for extended periods of
up to two hours at a time. This most often occurred
around late morning. This habit had unnerved many
with concerns being voiced with regard to its health.
Several subsequent veterinary examinations of both a
medical and orthopedic nature revealed no significant
or incriminating abnormalities. Despite frequent
observations, only one report on the state of its eyes
during these prolonged recumbencies was available.
On this occasion the eyes appeared to be in a state of
rapid eye movement (REM) but remained open. At one
point the lids did close momentarily before opening
Several months later a professional trainer of some
thirty years experience was employed to take over the
horse’s education as it appeared to have little respect
for its owner and others who were involved with its
handling. One example of what was perceived to be a
lack of respect was when, on a number of occasions
and with different attendants, as the lead rope was
being attached, the horse would suddenly lurch forward
and walk straight over the handler. At other times the
horse would firstly roll its eyes into the back of its head
as occurs in an oculogyric crisis (OGC). It would then
take several short steps forward and launch itself into
the air. On landing, it appeared to return to a normal
state. This did not occur under saddle. In the months
that followed these incidents the handler adopted the
tact of using physical and audible stimulation to arouse
the horse when altered behaviors were anticipated.
There appeared to be some reduction in these,
20 World Jou rnal o f Veterinary S cience, 2018, Vol. 6 T. Ahern
however the lethargy, stumbling and daily
In August 2014 the horse was taken to a training
camp and yarded overnight without company. The
following day the horse was readied for its lesson and
mounted. The rider then urged it forward. It took two
steps and then exploded. Those present stated that in
what seemed to be only a split second the horse was
forty meters further down the arena. A period of
uncoordinated bucking and kicking then followed which
lasted for several minutes. This behaviour from a horse
previously described by its owners as lethargic and
rarely seen exercising. The rider found herself on the
arena floor with a handful of mane in one hand.
As time passed, the episodes of unpredictable
behavior continued. On several occasions whilst
standing in the wash bay an oculogyric crisis (OGC)
would be evident. The horse would then launch itself
into the air, halt, charge forward, launch, halt and
continue this for up to a minute. On one occasion when
the horse had just been saddled, it suddenly exploded
crashing into walls, seemingly oblivious to the presence
of obstacles or people. It then leapt out of the barn and
charged over two twenty meter sand piles and
eventually careered into a ditch. These were obstacles
that in normal circumstances this horse would avoid.
In May 2015 the daily periods of recumbency
appeared to cease. There had been no significant
changes in environment or herd status as the horse
was on the same property and still paddocked next to a
dominant mare. After this the next major incident was
when the property owner was awakened at 2 AM by
the sounds of what she described as ‘blood curdling
screams’ coming from the barn. The horse was found
staggering, screaming, lashing out and throwing itself
on the walls with such force that plaster was being
dislodged in the next room. There appeared to be little
coordinated or conscious control over these actions.
Several minutes passed and the events ceased. Two
further incidents occurred when the head collar was
being fitted prior to bringing the horse in from the
paddock. On the first occasion the horse lurched
forward knocking the handler to the ground before
collapsing. The second episode was more severe. As
the head collar was being placed the horse firstly
extended its head and neck and its body appeared to
stiffen. Rapid eye movements were evident before it
collapsed backwards over the fence. It then got to its
feet and careered down the fence line, staggering and
kicking out as though a dog was snapping at its heels.
The previously described screaming then began whilst
the horse continued cantering about the paddock.
Several minutes later this behaviour ceased and the
horse was able to be taken in hand. It was sweating
profusely, tachycardic with visible pulsations of the
On the 29th of August 2015 the horse presented at
the authors practice. At this time fourteen weeks had
elapsed since there had been any evidence of
recumbent sleep. There had been no significant
environmental or social changes during this period and
historically the horse was able to physically attain
recumbency . An upper airways examination
revealed significant ulceration and wear of the mid free
border of the soft palate. In the authors opinion this
was typical of the changes seen in cases of chronic PI
[9,11]. In the absence of any other etiological avenues,
the possibility that PI  or pharyngeal instability 
with resultant UAO may have been impacting
negatively on the horse’s ability to experience sufficient
paradoxical sleep was discussed with the owner. It was
then agreed that the horse would undergo an oral
palatopharyngoplasty (OPP)  procedure in an
attempt to reduce the incidence of this instability.
Following surgery the horse was rested for eight weeks
prior to resuming training. In the immediate
postoperative period the horse, which was already in
poor condition, lost considerably more weight.
In the eight months since resuming ridden exercise
the horse had steadily gained weight. There had been
no significant alterations to its diet. It was now
described by its owner as being in extremely good
condition with a normal healthy coat. The horse also
resumed regular recumbency although more often
during the evening rather than late morning which had
been its habit. There had been no further episodes of
collapsing, screaming or other untoward behaviours.
Stumbling was now a rarity and the horse was
described as being alert and keen to work. The
yawning had ceased.
Somnolence as a consequence of paradoxical sleep
deprivation had been recognised in horses [4,5].
Horses required 30 to 60 minutes of paradoxical or
deep sleep daily [1,2,5]. Investigations into paradoxical
Sleep Att acks with Associ ated Sleep Terrors in a Six Year Old World Journal of Vete rinary Science, 2018, Vol. 6 21
sleep deprivation in horses included studies on equine
narcolepsy  and sleep attacks [4,5].
Parasomnias such as sleep terrors [7,8] had not as
yet been formally researched in equids. In humans
sleep terrors share the same root causes as
sleepwalking. People experiencing sleep terrors were
usually described as being inconsolable and
unresponsive. Expressions of fear or panic often
preceded episodes of screaming. They could lash out
and often behave as though they were trying to flee
some form of physical threat . One causative group
was sleep disorders including obstructive sleep apnea
(OSA) , that occurred with repetitive collapse of the
upper airway. There were numerous treatments for
OSA including several surgical options which
attempted to reduce the incidence of UAO .
In equids there had been considerable research into
upper airways collapse during exercise  but non to
date on the possibility that similar obstructions could
occur during recumbent or deep sleep. Pharyngeal
instability or PI during exercise had been frequently
diagnosed in both racing  and sport horses  and
was often associated with periods of decreased
pharyngeal muscular tone or tension. During deep or
paradoxical sleep muscular tone including pharyngeal
was at its lowest . If nasopharyngeal compliance was
to yield to negative inspiratory pressures then logically
it would be at these times. If this was the case then
surgical procedures which aim to support the
nasopharyngeal airway during periods of increased
exercise demand may also be indicated where the
airways are prone to collapse during periods of deep or
In this case report the horse presented with
somnolence and sleep attacks which were
accompanied by symptoms of sleep terrors and other
behavioural anomalies which may or may not have
been related. The only ocular observation made during
the extended periods of recumbency suggested that
REM was occurring whilst the horse’s eyes were still
open. REM and paradoxical sleep normally occurred
with the lids closed. Episodes that resembled human
sleep terrors were those where the horse was found
screaming, lashing out as though threatened and at the
same time it was said to be disorientated and
In the absence of environmental or social changes
 that could precipitate sleep deprivation and thence
sleep attacks, and given the extensive history, it was
decided that an upper airways examination was
appropriate. Palatal instability (PI) were diagnosed and
an oral palatopharyngoplasty (OPP)  was
performed. Initial weight loss coincided with a rationed
fibre only diet in the two weeks following surgery. Also
contributing to this loss was the fact that the horse had
presented for surgery in both a fatigued state and in
very poor condition.
Once returned to training the previously apparent
somnolence, sleep attacks, sleep terrors and altered
behaviours dissipated. The exact time frames for each
were not known. However there were certainly no
further episodes of sleep attacks or sleep terrors
In this case it could be postulated that there was a
correlation between pharyngeal or palatal instability,
UAO and the horses ability to acquire adequate deep
or paradoxical sleep. Further case studies with more
rigorous pre and post operative assessment would be
required to confirm or refute any possible connection.
AUTHORS’ DECLARATION OF IN TERESTS
No affiliations and no conflicting interests have been
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Received on 29-07-2018 Accepted on 15-09-2018 Published on 04-10-2018
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