In immature animals, the annulus fibrosus is vascularized, and the semiliquid nucleus pulposus could be injected in the annulus fibrosus blood vessels when the intradiscal pressure suddenly increases during exercise or minor trauma [6]. The vertebral growth-plate cartilage (which is vascularized) could also be the source of the fibrocartilaginous material in immature dogs [7]. Another hypothesis on the source of the fibrocartilaginous material suggests that it may arise from metaplasia of the vascular endothelium, which later ruptures into the lumen and embolizes within the intrinsic spinal cord vasculature [1]. The vascular anatomy of the spinal cord is illustrated in Figure 9.1.

The ischemic injury caused by the arterial obstruction initiates a series of biochemical and metabolic events (similar to the secondary injury phenomenon described in Chapters 15 and 25), which result in neuronal and glial cell death. The gray matter, due to its greater metabolic demand, is generally affected more severely than the white matter.

Clinical presentation

FCEM has been reported in dogs more commonly than in cats. At the time of writing, there are several studies including 24 or more dogs with FCEM (Table 9.1). The literature on feline FCEM is limited to case reports, and three case series for a total of 38 cats are reported in the literature (Table 9.2).

FCEM has been reported in large and giant breed dogs more commonly than in small- to medium-sized dogs [5, 8–13], and it is most common in nonchondrodystrophoid canine breeds. The reported male to female ratio in dogs ranges from 1:1 [8] to approximately 2.5:1 [9, 12]. The age at diagnosis in dogs ranges from 2 months [6] to 13 years and 5 months [13], with a median of 4–6 years in most studies [5, 8, 10, 12, 13].

Combining the data available on all the cats reported with FCEM in the literature, the domestic short hair is the most represented breed, the male to female ratio is 1.3:1, and the age at diagnosis ranges from 2 to 16 years (median, 10 years) [3, 14–22].

The typical clinical presentation is characterized by peracute (<6 h) onset of nonprogressive and nonpainful (after the first 24 h) and often lateralized neurological deficits consistent with the location, extent, and severity of the spinal cord infarction [12]. Rarely, neurologic dysfunction can progress for longer than 24 h, possibly as a result of additional embolizations or secondary spinal cord injury. In up to 80% of dogs diagnosed with FCEM, the dog was performing some type of physical activity, such as running, jumping, or playing, at onset of neurologic signs [8]. Signs of sudden and transient hyperalgesia (e.g., yelping as in pain) are observed by the owners at the onset of neurologic signs in up to 61.5% of affected dogs [8]. This transient hyperalgesia may be the result of stimulation of nociceptors in the endothelium of embolized spinal blood vessels [23]. This focal spinal hyperalgesia generally resolves in a few minutes to hours. Lateralization of neurologic signs has been reported in 52.8–86.5% of dogs with FCEM [5, 8, 10, 12, 13]. The asymmetry of signs can be very marked and often result in severely paretic or plegic limb/s on one side, while the contralateral limb/s has/have minimal deficits. This marked lateralization of signs is uncommon with other myelopathies and results from asymmetric branching of the intrinsic spinal cord vasculature, particularly the central branches of the ventral spinal artery, which have an asymmetric distribution in several spinal cord segments (Figure 9.1). The central arteries supply most of the gray matter and part of the lateral and ventral white matter of the spinal cord, sending branches to each side, or alternatively and irregularly to the left or right side of the spinal cord. The mean percentage of central arteries that send branches unilaterally is 79% in the cervical region, 58% in the thoracic region, and 37% in the lumbar region [24]. FCEM can also result in bilateral and symmetrical neurologic deficits. FCEM most commonly affects the L4–S3 and C6–T2 spinal cord segments in dogs with a histologic diagnosis [8, 10] and L4–S3 and T3–L3 in dogs with an antemortem diagnosis (Table 9.1) [5, 8, 10, 12, 13]. Neurologic deficits usually refer to the affected spinal cord segment (C1–C5, C6–T2, T3–L3, or L4–S3). However, in the acute stage of the disease, a decreased withdrawal reflex has been reported in severely paraparetic and paraplegic dogs with magnetic resonance imaging (MRI) changes consistent with FCEM within the T3–L3 spinal cord segment [12]. The majority of these dogs had an interruption of the cutaneous trunci reflex consistent with the site of FCEM on MRI [12]. It has been suggested that the transient decrease of the pelvic limb withdrawal reflex in dogs with acute thoracolumbar spinal cord injury is due to sudden interruption of descending supraspinal input on motor neurons and interneurons, fusimotor depression, and increased segmental inhibition [25]. This phenomenon is clinically relevant, as it may lead to an incorrect neuroanatomic localization, site of diagnostic investigation, and prognosis.

FCEM most commonly affects the cervical spinal cord in cats. Lesion distribution in the 38 cats reported in the literature is C1–C5 in 11 cats, C6–T2 in 14 cats, T3–L3 in 7 cats, and L4–S3 in 7 cats. Lateralization of neurologic signs has been reported in 26 of 38 (68%) cats in which this information is available (Table 9.2).

Differential diagnoses

The main differential diagnoses for peracute, nonprogressive, lateralized myelopathy include ischemic myelopathy due to obstruction of the intrinsic spinal vasculature by material other than fibrocartilage, such as thrombi, or bacterial, parasitic, neoplastic, or fat emboli and acute noncompressive nucleus pulposus extrusion (ANNPE) (see Chapter 13). Underlying medical conditions that may predispose to embolization or thrombosis, including cardiomyopathy, endocarditis, hypothyroidism, hyperthyroidism, hyperadrenocorticism, chronic renal failure, and hypertension, should be considered and investigated, particularly in cats with peracute onset nonpainful and nonprogressive myelopathy. ANNPE occurs when nondegenerated nucleus pulposus extrudes during strenuous exercise or trauma, contuses the spinal cord, and dissipates within the epidural space, causing minimal to no spinal cord compression. The clinical presentation of dogs with ANNPE is often very similar to one of dogs with FCEM. Lateralization of neurological deficits has been reported in 62% of dogs with ANNPE [26]. The main clinical difference between ANNPE and FCEM is discomfort or hyperesthesia during palpation of the affected spinal segments, which has been reported in 57% of dogs with ANNPE, while it is uncommon in dogs with FCEM [12, 26]. High-field MRI and experience in neuroimaging also may help differentiate these two diseases [2, 12, 26].

Other differential diagnoses include compressive intervertebral disc extrusion, infectious and noninfectious meningomyelitis, neoplasia, hemorrhage (e.g., secondary to coagulopathy), and vertebral fracture, subluxation, or luxation. The history, onset, and progression of signs and the presence/absence of spinal hyperesthesia on clinical examination help to expand or narrow the list of differential diagnoses. Animals with FCEM may show discomfort on palpation of the affected spinal segment when examined within a few hours of onset of neurological dysfunction or anytime after onset of signs in case of concurrent spinal pathology such as spondylosis deformans. Alternatively, animals with spinal disorders commonly associated with some degree of spinal hyperesthesia (such as intervertebral disc extrusion) may show no obvious signs of hyperesthesia on palpation of the affected spinal segment because they are very stoic or due to recent administration of anti-inflammatory and/or analgesic medications. Pharmacological treatment may also interfere with progression of signs resulting in clinical stabilization or improvement of animals with disorders that would be progressive if untreated. Therefore, diagnostic investigations are needed to support the clinical suspicion of FCEM.

Diagnosis

The antemortem diagnosis of FCEM is based on history, clinical findings, exclusion of other etiologies of myelopathy (through diagnostic imaging and cerebrospinal fluid (CSF) analysis), and visualization of a lesion compatible with spinal cord ischemia on MRI. The definitive diagnosis of FCEM can only be reached through histological examination of the affected spinal cord segments.