Lameness in horses can be difficult to diagnose.  Sometimes, the lameness is so subtle that it is only felt by the rider and when lameness is that subtle, nerve blocks to numb the potential affected area are not helpful.  Also, it is possible the lameness in a leg is not because of a leg problem at all, but the problem is in the neck.  Arthritis in the neck and associated nerve impairment can cause lameness in horses that usually appears in the forelegs but can be severe enough to affect the gait of the rear legs as well if the intervertebral joint is involved and the spinal cord is compressed.  Dr. Gwenola Touzot-Jourde is an anesthesiologist in France and she mentions that sometimes you cannot find a problem in the limbs because the problem is not in there but in the neck. 

She says that arthritis commonly occurs at the junction of the C6 and C7 vertebrae at the lower neck, and nerve compression is also common in this area.  This vertebrae in this area have two articular process joints on either side of the vertebrae as well as the intervertebral joint.  This area receives a lot of rotational movement and stress in ridden horses, and can compress the seventh cervical nerve that is associated with shoulder and foreleg sensation.  This specific syndrome is different from spinal cord compression, which can lead to hind limb weakness that can also occur in the neck.  The researchers examined the compression of the seventh cervical nerve by anesthetizing the nerve in normal horses and found these horses had shortened gaits and general lack of shoulder tone.  However, there was no stumbling or falling, which was expected.  So if your horse is off on the forelimbs and your veterinarian cannot find a problem in the leg, the neck could be the source of the problem.