James Montgomery, DVMDecember 29, 2008

Boomer 9 year old, MC, Portuguese water dog 11 month history of left front lameness

Intermittent non-weight bearing Non-responsive to NSAIDs or

acupuncture Survey radiographs – WNL Left shoulder arthrogram (Acc

#107751) - WNL

Biceps brachii tendon is smooth and regularly marginated.  No filling defects are identified within the joint space.

The infraspinatus tendon is also outlined by contrast medium; it appears smoothly marginated. 

1 cm tubular mass caudomedial to the left shoulder joint

Extends dorsomedially and appears contiguous with thickened 7th and 8th cervical spinal nerves

Apparent extension into the vertebral canal on the left at C7 – T1

Findings consistent with malignant peripheral nerve sheath tumor

Have been referred to as schwannomas, neurinomas, neurilemmomas, neurofibromas, and neurofibromas

Most canine PNST are poorly differentiated Cell origin is difficult to identify

May arise from any peripheral nerve 80% - brachial plexus or its nerve roots 20% - pelvic plexus, thoracolumbar nerve

roots, and cranial nerves (trigeminal most commonly affected CN)

Typically slow growing Extend by local invasion along peripheral

nerve roots and its branches

Rarely invade surrounding tissues Metastasis rare

Neurologic signs due to compression of peripheral axons, spinal cord, or brain stem

Signalment: Mature dogs, rarely seen in cats No breed or sex predilection

Clinical signs: Chronic, progressive forelimb lameness Muscle atrophy Pain in the axillary area Palpable mass Horner’s syndrome, with loss of ipsilateral

panniculus – loss of the first 2 thoracic nerve roots (contain preganglionic sympathetic fibers supplying the face, and give rise to the lateral thoracic nerve)

Clinical signs continued: LMN signs to affected limb May be hemiparetic

Early signs very much like orthopedic disease Differentiating between orthopedic and

neurologic disease can be difficult▪ May result in a long period before a diagnosis is

made▪ Between 4.8 and 6.1 months in one report

Imaging Survey radiographs – usually normal▪ Rule out orthopedic disease▪ May show enlarged intervertebral foramen or

occasionally vertebral body lysis Myelography▪ May see intradural extramedullary mass if nerve

roots have been invaded Ultrasound▪ Good screening tool but lack of tumor identification

doesn’t rule out presence of a tumor With severe muscle atrophy and no

orthopedic disease, CT or MRI should be performed

Hypoechoic tubular axillary mass with no blood flow

Limitations Difficulty in detecting

extent of tumor Measurements do not

correlate with size of tumor

Axillary lymph nodes – false positive diagnosis

Rim enhancement noted frequently, often with central hypoattenuating areas

Masses as small as 1.0 cm can be identified on contrast enhanced scans

Smaller mass-like lesions may not be specifically associated with neuronal structures

Relies on noticeable soft tissue asymmetry or contrast enhancement

Can be difficult to assess nerve structures oriented obliquely to the transverse plane

MRI superior to CT for detecting brachial plexus tumors Excellent contrast resolution Able to distinguish nerve bundles from vessels

Majority – hyperintense to muscle on T2W images and isointense on T1W images

Most only minimally or heterogeneously contrast enhancing Contrast enhancement critical to detecting

subtle diffuse nerve sheath involvement or small isointense nodules

Transverse plane images including both axillae and the vertebral canal allow in-slice comparison to detect lesions by asymmetry

Higher resolution, smaller field of view, multiplaner examination of the cervicothoracic spine important for appreciating nerve root and foraminal involvement

Findings: ~1/2 had diffuse thickening of brachial

plexus nerves (smooth or nodular) ~3/4 with diffuse thickening had

extension into the vertebral canal

Findings (continued): Nerve root thickening associated with

widening of the intervertebral foramen Contrast enhancement was only mild to

moderate, and usually non-uniform Ipsilateral muscle atrophy with

hyperintense muscle bundles – T2, STIR, and pre-contrast T1▪ Neurogenic atrophy, edema, fatty infiltrate,

and fibrosis

Transverse precontrast (A) and postcontrast (B) T1-weighted images

Mass is isointense to muscle, and is identifiable by absence of a similar structure in slice images of the contralateral axilla.

Postcontrast -minimal heterogeneous contrast enhancement

PD sequences did not provide unique info T2W – lesions hyperintense to muscle but

less intense than axillary fat T2W – STIR – suppresses fat making

hyperintense neoplastic lesions more noticeable

Small or diffuse lesions particularly difficult to see due to similar intensity to fat on T2W images, and to muscle on T1W images (pre-contrast)

As with people, visible contrast enhancement on T1W images is critical to detecting canine peripheral nerve sheath tumors

Early diagnosis and surgical removal key to good outcome

Canine PNST have a more infiltrative and malignant behavior than the human counterpart

Tumor margins difficult to identify grossly Aggressive surgery recommended Tumor recurrence common▪ Median survival▪ Brachial plexus tumor – 12 months▪ Nerve roots – 5 months

If nerve root and vertebral canal involvement, poor outcome

Inzana KD. Peripheral Nerve Disorders. In Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 6th ed (St. Louis, MO: Elsevier Saunders, 2005) pp. 896-7.

Kraft S, et al. MRI characteristics of peripheral nerve sheath tumors of the canine brachial plexus in 18 dogs. Vet Radiol Ultrasound 48(1):1-7;2007.

Platt SR, et al. MRI and ultrasonography in the diagnosis of a malignant peripheral nerve sheath tumor in a dog. Vet Radiol Ultrasound 40(4):367-71;1999.

Rose S, et al. Ultrasonographic evaluation of brachial plexus tumors in 5 dogs. Vet Radiol Ultrasound 46(6):514-7;2005

Rudich SR, et al. CT of masses of the brachial plexus and contributing nerve roots in dogs. Vet Radiol Ultrasound 45(1):46-50;2004.