<PAGE>   1


                                                                  EXHIBIT 10.1


SECOND ADDENDUM attached to and made a part of that certain Lease dated
September 10, 1991, between FIRST INDUSTRIAL MORTGAGE PARTNERSHIP, L.P.,
successor in interest to WS Development Company, as Landlord, and SOMANETICS
CORPORATION, as Tenant, covering premises at 1653 E. Maple Road, Troy, Michigan.

NOTWITHSTANDING anything to the contrary contained in the Lease, Agreement,
Addendum to Lease, Termination of Agreement, and Extension of Lease to which
this Second Addendum is attached to and made a part thereof, the Landlord and
Tenant agree as follows:        

1.  The Term of the Lease shall be extended two (2) years commencing January 1,
    1998 and terminating December 31, 1999.

2.  The minimum net rental for the extended two (2) year Term shall be Three
    Hundred Fifty-Two Thousand Eight Hundred Ninety-Seven and 78/100 Dollars 
    ($352,897.78) payable monthly in advance at the rate of Fourteen Thousand
    Seven Hundred Four and 07/100 Dollars ($14,704.07).

3.  Option to Extend Term:
    a)  Grant of Option:  Tenant shall have the right and option to extend the
        Term of this Lease for one (1) period of (1) year (hereinafter from
        time to time referred to as the "Extension Period"), at the rental
        rate and upon the other terms and conditions set forth herein.  Tenant
        shall not be entitled to so extend the Term of the Lease if then in
        default or if during the year immediately preceding the date for
        exercise of the option in questions, Tenant shall have been in default
        under this Lease for any prior consecutive period of two (2) months, or
        any non-consecutive period totaling four (4) months.

    b)  Exercise of Option:  The option to extend the Term granted herein shall
        be exercised by written notice to Landlord given not less than one
        hundred eighty (180) days prior to December 31, 1999.


    c)  Minimum Net Rental:  Tenant's possession of the Premises during the
        Extension Period shall be under and subject to all the terms, covenants
        and conditions set forth in the Lease, including the payment of the
        minimum net rent at the rate of One Hundred Seventy-Six Thousand Five
        Hundred Forty-Eight and 89/100 Dollars ($176,448.89) per year payable in
        monthly installments in advance of Fourteen Thousand Seven Hundred Four
        and 07/100 Dollars ($14,704.07). 

    d)  Option Personal to Tenant:  The option to extend Term of this Lease
        granted in this Second Addendum is personal to Tenant and may not be
        assigned in whole or in part in any manner whatsoever and can only be
        exercised if Tenant is occupying and utilizing the Premises for the uses
        set forth in Section 7 of this Lease.

4.  Subordination; Notices to Superior Lessors and Mortgagees; Attornment:

    4.1   Subordination of Lease.  This Lease, and all rights of Tenant
hereunder, are subject and subordinate to all ground leases of the Premises now
or hereafter existing and to all mortgages or trust deeds (all of which are
hereafter referred to collectively as "Mortgages"), that may now or hereafter
affect or encumber all or any portion of the Premises.  This subordination
shall apply to each and every advance made, or to be made, under such
Mortgages; to all renewals, modifications, replacements and extensions of such
Mortgages; and to "spreaders" and consolidations of such Mortgages.  This
Section 4.1 shall be self-operative and no further instrument of subordination
shall be required; however, in confirmation of such subordination, Tenant shall
from time to time promptly execute, acknowledge and deliver any instrument that
Landlord may from time to time reasonably require in order to evidence or
confirm such subordination.


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     Tenant acknowledges that this Lease has been (and, in the future, may be)
     assigned by Landlord to a Superior Mortgagee, (defined below) as additional
     collateral security for the loans secured by the Superior Mortgagee
     (defined below) held by such Superior Mortgagee.  Any ground lease to
     which this Lease is subject and subordinate is hereinafter referred to as
     a "Superior Lease," and the lessor of a Superior Lease is hereinafter
     referred to as a "Superior Lessor; and the Lessee thereunder, a "Superior
     Lessee"; and any Mortgage to which this Lease is subject and subordinate
     is hereinafter referred to as a "Superior Mortgage," and the holder of a
     Superior Mortgage is hereinafter referred to as a "Superior Mortgagee." 
     Notwithstanding the foregoing, at Landlord's election, this Lease may be
     made senior to the lien of any Superior Mortgage, if and only if the
     Superior Mortgagee, thereunder so requests.

     4.2  Notice in the Event of Default.  If any act or omission of Landlord or
     Agent would give Tenant the right to cancel or terminate this Lease, or to
     claim a partial or total eviction, Tenant shall not exercise such right (a)
     until it has given, by registered or certified mail, return receipt
     requested, written notice of such act or omission to Landlord and to each
     Superior Mortgagee and Superior Lessor whose name and address shall
     previously have been furnished to Tenant, and (b) until a thirty (30)-day
     period for remedying such act or omission shall have elapsed following the
     giving of such notice; provided, however, that said thirty (30)-day cure
     period shall be automatically extended in the event that the act or
     omission cannot, by its nature, be cured within thirty (30) days and one or
     more of Landlord, the Superior Mortgagee or the Superior Lessor is
     diligently proceeding to cure said default.  See Exhibit A as integral
     part of this "Second Addendum."

     4.3  Successor Landlord.  If any Superior Lessor or Superior Mortgagee
     shall succeed to the rights of Landlord hereunder, then, at the request of
     such party (hereinafter referred to as "Successor Landlord"), Tenant shall
     attorn to and recognize each Successor Landlord as Tenant's landlord under
     this Lease and shall promptly execute and deliver any instrument such
     Successor Landlord may reasonably request to further evidence such
     attornment.  Tenant hereby acknowledges that in the event of such
     succession, then from and after the date on which the Successor Landlord
     acquires Landlord's rights and interest under this Lease (the "Succession
     Date"), the rights and remedies available to Tenant under this Lease
     against Successor Landlord shall be limited to the equity interest of the
     Successor Landlord in the Premises; and the Successor Landlord shall not
     (a) be liable for any act, omission or default of Landlord or other prior
     lessor under this Lease; (b) be required to make or complete any tenant
     improvements or capital improvements, or to repair, restore, rebuild or
     replace the Premises or any part thereof in the event of damage, casualty
     or condemnation; or (c) be required to pay any amounts to Tenant that are
     due and payable, under the express terms of this Lease, prior to the
     Succession Date. Additionally, from and after the Succession Date, Tenant's
     obligation to pay Rent shall not be subject to any abatement, deduction,
     set-off or counterclaim against the Successor Landlord that arises as a
     result of, or due to, a default of Landlord or any other lessor that occurs
     prior to the Succession Date in (a) (b) and (c) above.  Moreover, no
     Successor Landlord shall be bound by any advance payments of Rent made
     prior to the calendar month in which the Succession Date occurs, nor by any
     security deposit that is not actually delivered to, and received by, the
     Successor Landlord.

5.   Notices.

     Any notice required to be given by either party pursuant to this Lease,
     shall be in writing and shall be deemed to have been properly given,
     rendered or made only if personally delivered, if sent by Federal Express
     or other comparable overnight delivery service, or if sent by registered or
     certified mail, return receipt requested, postage prepaid, addressed to the
     other party at the addresses set forth below (or to such other address as
     Landlord or Tenant may designate to each other from time to time by written
     notice), and shall be deemed to have been given, rendered or made on the
     day so delivered or three (3) business days after having been mailed:   
<PAGE>   3



If to Landlord:         First Industrial, L.P.
                        150 North Wacker Drive, Suite 150
                        Chicago, Illinois  60606
                        Attn:  Michael W. Brennan

With a copy to:         Barack, Ferrazzano, Kirschbaum & Perlman
                        333 West Wacker Drive
                        Suite 2700
                        Chicago, Illinois  60606
                        Attn:  Howard Nagelberg and Suzanne Bessette-Smith

If to Tenant:           ____________________________________________________

                        ____________________________________________________

                        ____________________________________________________

                        ____________________________________________________

6.  Landlord, at its sole cost and expense, will undertake the following work
    in the Premises upon execution of this Second Addendum by the Landlord and
    Tenant:

    a)  Construct offices as shown on the attached Exhibit 1 (Smith & Schurman
        Associates, Inc. drawing dated April 8, 1997) using building standard
        materials.  Consistent with tenant finish.

    b)  Remove and replace carpet and base in high traffic areas and enclosed
        offices with building standard carpet and base in colors selected by
        Tenant.    

    c)  Repair roof leaks.

All other terms and conditions of said Lease, Agreement, Addendum to Lease,
Termination of Agreement and Extension of Lease to remain in full force and
effect unless in conflict with the terms and conditions of this Second Addendum
in which event the terms and conditions of this Second Addendum shall prevail
and control.

6(a), (b) and (c) to be completed by May 31, 1997.

                                LANDLORD:
                                FIRST INDUSTRIAL MORTGAGE
                                PARTNERSHIP, L.P., a Delaware Limited
                                Partnership, successor in interest to WS
                                Development Company
                                By:   First Industrial Mortgage Corporation, a
                                      Maryland Corporation
                                Its:  General Partner



                                By:   /s/ David P. Draft
                                     --------------------------
                                      David P. Draft
                                Its:  Signing Officer    

                                TENANT:

                                SOMANETICS CORPORATION, a Michigan
                                Corporation

                                
                                By:    /s/ Raymond W. Gunn
                                     --------------------------
                                
                                Its:   EVP & CFO
                                     --------------------------    



Dated:  April 14, 1997
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                                   EXHIBIT 1





                             SOMANETICS CORPORATION

                              SCHEMATIC FLOOR PLAN
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                                                                      EXHIBIT A


In the event of foreclosure of the Mortgage or conveyance in lieu of
foreclosure, which foreclosure or conveyance occurs prior to the expiration date
of the lease now provided thereunder, and so long as Tenant is not in default
under any terms, covenants and conditions of the Lease beyond any applicable
grace period, Mortgagee agrees on behalf of itself, its successors and assigns,
and on behalf of any purchaser at such foreclosure ("Purchaser") that Tenant
shall not be disturbed in the quiet, peaceful possession of the premises
demised under the Lease.





                                                Somanetics Corporation


                                                By: /s/ Raymond W. Gunn
                                                    -------------------------
                                                     Raymond W. Gunn

                                                Its:  EVP & CFO
                                                     ------------------------