MySQL can create spatial indexes using syntax similar to that for
creating regular indexes, but extended with the SPATIAL keyword.
Currently, columns in spatial indexes must be declared NOT NULL. The
following examples demonstrate how to create spatial indexes:

o With CREATE TABLE:

CREATE TABLE geom (g GEOMETRY NOT NULL, SPATIAL INDEX(g));

o With ALTER TABLE:

ALTER TABLE geom ADD SPATIAL INDEX(g);

o With CREATE INDEX:

CREATE SPATIAL INDEX sp_index ON geom (g);

For MyISAM tables, SPATIAL INDEX creates an R-tree index. For storage
engines that support non-spatial indexing of spatial columns, the
engine creates a B-tree index. A B-tree index on spatial values will be
useful for exact-value lookups, but not for range scans.

For more information on indexing spatial columns, see [HELP CREATE
INDEX].

To drop spatial indexes, use ALTER TABLE or DROP INDEX:

o With ALTER TABLE:

ALTER TABLE geom DROP INDEX g;

o With DROP INDEX:

DROP INDEX sp_index ON geom;

Example: Suppose that a table geom contains more than 32,000
geometries, which are stored in the column g of type GEOMETRY. The
table also has an AUTO_INCREMENT column fid for storing object ID
values.

URL: http://dev.mysql.com/doc/refman/5.1/en/creating-spatial-indexes.html

