Case reveal

Findings:

• At the level of the hypoglosal canals, the right and left hypoglosal nerves are, respectively, markedly and mildly diffusely thickened, T2 hyperintense, mildy T1 hypointense, and moderately diffusely contrast enhancing (blue arrows). 

• The mandibular branch of the right trigeminal nerve is mildly diffusely thickened, T2/FLAIR heterogeneously  hyperintense, T1 hypointense and mildly diffusely heterogeneously contrast enhancing (pink arrow). At the level of the trigeminal ganglions bilaterally, within the ventral aspect of the brainstem, there a mild ill-defined heterogeneous T2 hyperintensity and contrast enhancement (green arrows).

• The right ocular musculature is moderately T2 and FLAIR hyperintense and contrast enhancing.

• The optic nerves are normal. 
• No other intercalvarial abnormalities are identified.
• The right temporal muscle is minimally decrease in size when compared with the contralateral side. 
• There is a relatively symmetrical, curved to lobulated contrast enhancement of the left and right cranial oblique capitis musculature – between the nuchal crest and the spinous process of the axis. 

Conclusions:

• Severe right and mild left hypoglossal and moderate right trigeminal nerve thickening. 
• Right ocular and bilateral cranial oblique capitis muscle contrast enhancement.
• Minimal neurogenic muscular atrophy of the right temporal muscle.

The reported clinical signs are attributed to the described findings with differential diagnoses that include neuritis (e.g. infectious/inflammatory) and less likely infiltrative neoplasia (e.g. lymphoma). The contrast enhancement of the right ocular and cranial oblique capitis musculature is likely secondary to myositis (inflammatory and/or infectious). 

Follow-up:

• CSF was obtained from C1-C2. Cytological analysis of the sample revealed no major abnormalities. WBC/µL: 0; RBC/µL: 4; CSF microprotein mg/dL: 50 (Normal <80); Leukocyte differential: 8 Lymphocytes, 5 Large mononuclear cells. The preparation was of low cellularity and adequate preservation with minimal blood contamination. The preparation was composed of small lymphocytes and non- to mildly vacuolated large mononuclear cells. No infectious agents or neoplastic cells were identified.
• WNV IgG/IgM, Lyme multiplex and Vitamin E levels were normal.
• Due to the progressive and continued decline of his condition, despite a 3 week course of tapering
• Dexamethasone and Vitamin E supplementation, the decision was made to humanely euthanize

Necropsy:

• Ganglioneuritis, lymphocytic-histiocytic to plasmacytic, subacute to chronic, multifocal, mild to moderate (trigeminal ganglia), trigeminal nerves and ganglia, peripheral cervical spinal nerves, sciatic nerve, and cauda equina.
• Hepatitis, random, neutrophilic-histicocytic, acute, multifocal, moderate, liver.
• Nephritis, tubulointerstitial, neutrophilic to lymphocytic-plasmacytic, subacute to chronic, multifocal, mild to moderate, kidneys.
• Axonal degeneration, subacute to chronic, multifocal, minimal to mild, spinal cord.
• Bronchopneumonia, histiocytic to lymphocytic-plasmactyic, chronic, locally extensive, moderate, lung.
• Muscle atrophy, chronic, mild, hindlimbs.
• Fatty infiltration, locally extensive, moderate with mild muscle atrophy, tongue.

The findings raise concerns for a manifestation of polyneuritis equi.

A little bit more:

• Polyneuritis equi is a rare neurological disorder, marked by gradual and progressive granulomatous inflammation of peripheral nerves. It is primarily marked by symptoms associated with chronic granulomatous inflammation of the cauda equina. However, other nerves, especially cranial nerves, can also be involved. The term polyneuritis equi is now preferred over the older term “neuritis of the cauda equina” to better reflect the broader nerve involvement (Hahn et al.).
• The cause of the progressive granulomatous inflammation affecting the cauda equina, cranial nerves, and occasionally peripheral nerves remains unknown. It is hypothesized that an infectious agent might trigger an immune-mediated response. Potential infectious agents, including EHV-1, equine adenovirus, Campylobacter spp., and streptococcal bacteria, have been suggested, but there has been limited research published to support this (Hahn et al.).
• Horses often have asymmetric cranial nerve dysfunction, which can sometimes be the primary reason for their presentation. The most frequently affected cranial nerves are CN V, VII, and VIII, leading to issues such as masticatory and facial muscle weakness, and vestibular symptoms like head tilt, eye drop, and vestibular nystagmus. Additionally, involvement of CN III (resulting in reduced pupillary light responses), CN IX and X (causing abnormal swallowing), and CN XII (leading to tongue paralysis) has been documented. Clinical observations indicate that horses with polyneuritis equi (PNE) presenting mainly with cranial nerve symptoms eventually also develop cauda equina deficits (Hahn et al.).
• The results of cerebrospinal fluid (CSF) cytological analysis in PNE vary and are nonspecific, ranging from mild mixed mononuclear to predominantly neutrophilic pleocytosis, with protein levels that can be normal or elevated (Aleman et al.).
• The MRI features of polyneuritis are not reported. Base on pathologic report, thickening of the extradural nerve roots and less commonly intradural nerve roots may be expected. This is due to various degrees of granulomatous inflammation and infiltration of lymphocytes, eosinophils, macrophages, giant cells, and plasma cells (Hahn et al.).
• The clinical signs are usually progressive and prognosis is poor.

References:

1. Aleman M, Katzman SA, Vaughan B, Hodges J, Crabbs TA, Christopher MM, Shelton GD, Higgins RJ. Antemortem Diagnosis of Polyneuritis Equi. Journal of veterinary internal medicine. 2009 May 1;23(3).
2. Hahn CN. Miscellaneous disorders of the equine nervous system: Horner’s syndrome and polyneuritis equi. Clinical Techniques in Equine Practice. 2006 Mar 1;5(1):43-8.