Chapter 17 Diagnosis of and Treatment Options for Disorders of the Spine 447Involvement of the dorsal roots of the femoral
nerve may inhibit sensory impulses from
stretch receptors located in the quadriceps mus
cle. Flexor (withdrawal) reflexes may also be
normal or show crossed extension (sugges
tive of chronic UMN dysfunction). Often
within 12 months from time of disease onset,
patients progress to nonambulatory parapare
sis (Kanazono et al., 2013). Clients usually elect
euthanasia when their dogs can no longer
support weight in their pelvic limbs. Clients
can care for smaller dog breeds over a longer
time (Matthews & de Lahunta, 1985; Coates
et al., 2007). The median disease duration prior
to euthanasia in the Pembroke Welsh Corgi was
19 months (Coates et al., 2007).
Late disease (lower motor neuron signs)
If the DM‐affected patient is not euthanized
early, clinical signs will progress to LMN para
plegia and ascend to affect the thoracic limbs
within 18–24 months. LMN signs emerge as
hyporeflexia of the patellar and withdrawal
reflexes, flaccid paralysis, and widespread
muscle atrophy beginning in the pelvic limbs
as the patient becomes nonambulatory
(Matthews & de Lahunta, 1985; Awano et al.,
2009). The paresis becomes more symmet
rical and progresses to flaccid tetraplegia.
Widespread and severe loss of muscle mass
occurs in the axial and appendicular muscula
ture. Most previous reports attributed loss
of muscle mass to disuse, but flaccidity in
patients with protracted disease suggests den
ervation. Cranial nerve signs include dyspha
gia and inability to bark. The hypoventilation
as a result of respiratory muscle dysfunction
results in hypoxemia in later stages of DM
(Oyake et al., 2016). Onset of urinary and fecal
incontinence will vary with individual dogs
but frequently develops near the development
of nonambulatory paraparesis.
Diagnosis
Accurate antemortem diagnosis is based
on pattern recognition of the progression of
clinical signs supported by inclusionary and
exclusionary diagnostic testing. A careful neu
rological examination is fundamental to devel
oping a diagnostic approach. Lack of paraspinal
hyperesthesia is a key clinical feature of DM
that distinguishes it from compressive mye
lopathies. An antemortem diagnosis of canine
DM is based on ruling out spinal cord compres
sive diseases. A presumptive diagnosis of DM
often is made based on lack of clinically rele
vant compressive myelopathy as determined
by MRI. If MRI is unavailable, CT/myelogra
phy also can be performed. Often, imaging
reveals disc protrusions that can confound a
diagnosis of DM. Ultimately, the clinician must
be guided by clinical experience to evaluate for
rapidity of disease progression, presence of
paraspinal hyperesthesia, and amount of spinal
cord compression to account for the signifi
cance of the compressive myelopathy.
Electrodiagnostic testing is useful for
detecting evidence of neuromuscular disease.
Early in the progression of DM, when UMN
signs predominate, electromyography (EMG)
and nerve conduction studies are within nor
mal limits. Later in the disease course with the
emergence of LMN signs, EMG reveals multi
focal spontaneous activity, fibrillation poten
tials, and positive sharp waves in the
appendicular musculature. Recordings of
compound muscle action potentials (M
waves) from stimulation of mixed nerves
have shown decreases in amplitudes consist
ent with axonopathy, and temporal dispersion
and decreased motor nerve conduction veloc
ities that also signify demyelination (Awano
et al., 2009).
A DNA test based on the SOD1 mutation is
commercially available and can assist in the
diagnosis of DM. Dogs homozygous for the
mutation are at risk for developing DM and
will contribute one chromosome with the
mutant allele to all of their offspring. The hete
rozygotes are DM carriers and are less likely to
develop clinical DM but could pass on a chro
mosome with the mutant allele to half of their
offspring. The normal homozygotes are
unlikely to develop DM and will provide all of
their offspring with a protective normal allele.
A test result of “at risk” can support a presump
tive diagnosis of DM in light of typical clinical
signs and normal findings on neuroimaging
and cerebrospinal fluid analysis. The SOD1
DNA test is of potential use to dog breeders
wishing to reduce the incidence of DM in the
breed or line.