The lack of a tail defines the Manx cat -- even though some Manx are born with nearly normal tails or little stumps. That lack of a tail is also the primary health issue in Manx cats. The potential spinal defects are part and parcel of the gene that produces no tail in the breed.
It's unlikely your cat will suffer from Manx syndrome, because most breeders don't allow kittens to leave before they are 4 months old. By that time, most affected kittens have shown signs of Manx syndrome, and likely have been euthanized. While sometimes the spinal issues are obvious in small kittens, often the signs don't appear until after the first month or more -- up to that four-month period. Kittens with Manx syndrome may have trouble walking, but more importantly, lack bladder and bowel control or suffer from colon defects.
Spinal bifida occurs when the spinal cord's neural tube doesn't completely close. It's part of Manx syndrome, but some cats aren't so severely affected that euthanasia is necessary. Less affected cats may move with an odd, hopping gait, or not possess full use of the rear legs. Others may use more of the hind legs to move than a normal cat, walking on his hocks rather than his feet.
Manx Tail Arthritis
Even if your Manx was born with a long tail, it's likely the breeder had it docked when he was just a few days old. That's not purely for cosmetic purposes. As Manx cats with long tails age, the vertebrae calcify, becoming arthritic and quite painful, often requiring amputation. It's another side effect of Manx genetics. It's much easier for a newborn kitten to have the tail removed than to have the operation performed on a middle-aged feline.
Female Manx usually have smaller litters than the typical cat. That's because approximately 25 percent of kittens conceived when two purebred Manx mate die in utero.