Nerve root tumours
Tumours of the peripheral nerve sheath such as schwannomas are slow-growing and typically present as a chronic persistent forelimb lameness lasting months for which no orthopaedic explanation can be found. They are common in middle-aged dogs and more than 80% affect the brachial plexus or its nerve roots.
There was a 6-week duration of right forelimb lameness. The limb had recently been giving way causing the dog to stumble when running or turning. There had been no history of trauma. The signs had not improved with NSAID therapy.
There was a generalized atrophy of the right forelimb. Hopping was decreased in the right fore and also in the right hind. Both the right fore and hind paws were quickly returned to position during proprioceptive testing but they were replaced in an abducted position which is abnormal. Placing in the right fore was slightly reduced. The right fore withdrawal reflex was reduced but pain perception in the limb was present. No Horner’s syndrome was found. The cutaneous trunci reflex was normal bilaterally.
The first sign was lameness, suggesting nerve root involvement, possibly compression by a lateralized IVD extrusion which had since worsened to cause right hind deficits. Peripheral nerve sheath tumours, meningioma, lymphoma were possible.
Early diagnosis of a nerve root tumour requires an index of suspicion. The caudal cervical nerve roots C6–8 are most commonly affected, weakening both flexion and extension. When the T1 and T2 nerve roots are involved ipsilateral Horner’s syndrome and cutaneous trunci reflex loss occurs. Successful treatment requires forelimb amputation with clean tissue margins in the severed nerve roots. If the dog is an unsuitable candidate for a forelimb amputation then one could argue that further diagnostics are not warranted.
Electromyography is a useful tool. It detects an unstable muscle membrane caused by denervation and aids differentiation of orthopaedic from neurogenic causes of lameness. Lumbar CSF should be collected even in the absence of ipsilateral hindlimb signs. A compressive spinal cord lesion often results in elevated total protein levels and its presence would signal that invasion of the spinal canal has taken place. Enlargement of the intervertebral foramen by an encroaching nerve root tumour can be detected on non-contrast radiography. Myelography has been used to rule out extension of the tumour to the spinal canal. Oblique views are required. An intradural-extramedullary pattern (‘golf tee’) compression is seen. MRI and CT have replaced myelography to a large extent.
MRI has proved extremely useful in locating nerve root tumours. Most have some degree of contrast-enhancement. Multiple planes through both axillae and the spinal canal are required. Metastases are rare.